You will get more total boost than just adding the two pressures.  The reason being that the pressure ratios multiply, not add.

So if your turbo has a pressure ratio of 2.0 (14.7 psi boost), and your supercharger has a pressure ratio of 1.4 (5.9 psi boost) you get a total combined pressure ratio of 2.8 which is 26.5 psi, not 14.7 + 5.9 = 20.6 psi.

So expect to see 14.7 out of the turbo, and 26.5psi out of the supercharger. The supercharger will work A LOT HARDER with much denser air being forced into into it.

The best way to handle this is to just run your wastegate sensing line off the total combined boost, and select a wastegate spring for whatever total boost you finally decide to run. Boost will quickly rise to that pressure and stay there over a very wide Rpm range.

If your exhaust housing originally produced full boost at 4,000 Rpm with just the turbo, expect full boost at more like 2,000 Rpm with twincharging. I am not kidding.  You will definitely require a larger a/r exhaust housing, but try it first and see.

The whole thing will be far more responsive and have a much lower turbo boost threshold than you are probably expecting. But the top end power will still be there.

With boost pressure substantially higher than total exhaust back pressure, over a very wide Rpm range, it is going make some real horsepower, as well as having massive low end grunt.

thanks for the input...very interesting..can't wait to build her...

I would give some thought to charge temperature rise and strongly recommend charge cooling.  Ideally after each stage, but I understand packaging a cooler between the blower and the intake may be difficult.  An intercooler between the turbocompressor and the blower will help a lot though.

thanks..you are correct on heat..the current design has an intercooler for the turbo and alcohol injection after the blower....

It is likely that heat generation from this configuration should not be ignored. The blower will likely be making the most heat.

Diesels that run compounded use a fairly compact heat exchanger under the blower only.

 

True, heat is always the number one enemy.

But this has a couple of things going for it.  The supercharger pressure ratio is reasonably low at only 1.4, and if some fairly efficient intercooling is employed after the turbo, the temperature at the supercharger intake need not be very far above ambient.

The detonation problem that often plagues high boost turbo engines is usually more often due to intake charge heating by trapped exhaust residuals. The unavoidable high turbine back pressure, traps a lot of heat, especially where there is a low compression ratio.

With supercharging, or twincharging it is extremely easy to keep boost pressure well above total exhaust back pressure, and with sufficient valve overlap, the heat can escape due to much more efficient positive scavenging. With EFI, the injection point can be delayed until the exhaust valve has closed if a good fuel specific is required.

So the engine is going to have a far higher detonation threshold than can be had with a turbo.

High induction temperatures are never good, but at least with supercharging or twincharging, detonation should be far easier to avoid with some suitable (but not excessive) valve overlap.

ok, how about sizing for the twincharge or in my case tricharge, is there a methodology to do this..for example,

500 cu.in motor, 1400hp at flywheel desired...size of twin turbos...size of roots blower...i'm assuming we want to underdrive the roots... this is what i was thinking.. two g35's and an 8-71, but i'm using seat of the pants...i think i want somewhere around 20 pounds of boost...

I doubt if it is possible to design something like this entirely from first principles with just a calculator and a clean sheet of paper, and no previous twincharging experience.

But a fair start might be to size the turbo compressors to provide sufficient airflow at about half the total estimated final boost pressure. These days exhaust turbines are fairly well matched for speed and energy to the compressor, with typically three alternative sizes of exhaust a/r housing being offered.

The larger size of a/r would most likely end up being most appropriate. The compressors will be run at high flow but at a relatively modest pressure ratio. The turbines will also need to run at high flow and a similar low pressure ratio.

The required turbos will be very large by any standard. A 400 CID engine with a roots blower is going to look to the turbos more like a 700-800 Inch engine. The turbos don't "know" the engine is supercharged, they just see flow. So both compressors and turbines are going to be enormous.

GT35's will be far too small. Something like GT42's with 1.34 a/r may be more like it, even they would be a fairly conservative choice. I know of one three liter (183 CID) twin charged engine in a road car that runs this particular turbo,(GT4288) and he reaches full boost at 3,500 Rpm.

GT35's sound about right for a flexible 400 inch straight turbo engine. Many guys fit GT35 1.0 a/r to this same three liter engine, and it works well. But things change very significantly when a supercharger is added.

The 8-71 has plenty of displacement per revolution, certainly more than enough to provide a very good range of efficient boost pressure adjustment for a 400 CID engine.  But a suitable drive ratio for it will need to be selected experimentally.

Probably something that experience tells would "normally" produce around 4-5psi may be a good starting point. That will increase to something approaching double that, with the turbos feeding into it. So yes, I would expect a rather conservative underdrive ratio may be required to begin with.

Now how you wish to set all this up depends on the application and your preferences. The wastegates will limit boost to the 20psi set point, but that 20 psi will be made up of contributions from both supercharger and turbos.

Depending upon supercharger drive ratio, and turbine a/r the engine can behave quite differently with regard to boost profile, torque curve, and exhaust (turbine) back pressure.

Both supercharger and turbos could both be driven fairly hard to reach full boost below 1,500 Rpm, but that may not be what you want, as top end power would be compromised.

This is where it becomes tricky, it is fairly easy to change either the supercharger drive ratio, or the turbo exhaust housing a/r, or the wastegate boost set point. Not so easy to predict beforehand exactly what is going to happen with a particular untried combination.

There are three pressures that need to be monitored. Turbo compressor boost, combined total boost, and exhaust (turbine inlet)  pressure. They will give an excellent idea of what is actually going on, and what needs to be changed to get to where you wish to go. It is all extremely flexible, and the engine can be readily matched to the application.

One handy instrument I often use is a dual boost gauge from a twin engined aircraft. It has two pointers that sweep the same scale. I have painted one pointer red, the other blue.

It is easy to monitor two pressures, and see the exact difference between two pressures as the vehicle is driven. The only disadvantage is that these aircraft gauges are fairly heavily damped, and the pointer movement perhaps not as rapid as it could be. But it is excellent for watching for example, where boost and exhaust pressures cross over on an ordinary turbo engine. Or for measuring turbo and total boost simultaneously in a twincharge setup.

 
  

 

  

I respecfully disagree with an earlier post suggesting that a roots type air pump will work harder once the turbo gets up to a speed where its providing positive pressure. In an application where an engine is equipped with a belt driven roots pump and the drive belt is removed the engine will continue to run and the blower will simply "wind mill" as a result of the air flow going into the engine. If you add a exhaust driven supercharger into the mix it will simply spin faster once the turbo gets up to a pressure greater then atmospheric. My conclusion is that roots blower will behave like a restriction if the turbo is capable and allowed to achieve high boost levels.-----Phil

Yes it will certainly windmill freely without a drive belt, but there will also have to be a net pressure drop across the supercharger rotors required to do that.

A positive displacement supercharger when properly driven by the crank just gulps a fixed volume per revolution of whatever air density is there, and passes it to the output port. The boost pressure rises because the supercharger pumps a greater volume of air than the engine could otherwise inhale by itself. The supercharger can never be restrictive while a positive boost pressure is being produced.

If the incoming air density to the supercharger doubles, the supercharger mass flow will also double, and so will the torque required to drive the supercharger. Interestingly the supercharger pressure ratio stays about the same because the supercharger displacement and engine displacement (per revolution) maintain the same set drive belt ratio relationship. The flow through both supercharger and engine increases due to the turbocharger increasing the incoming air density to the supercharger.

There is a fairly widespread and seemingly immortal urban myth, that once the turbo winds up, the supercharger will become restrictive. That can never be the case if the boost pressure at the outlet of the supercharger is higher than at the supercharger inlet pressure. It can never be restricting the flow if that is the case.

As explained in an earlier post, the measured pressure increase across the supercharger actually increases fairly dramatically when the turbo begins producing more dense incoming air for the supercharger to work from. There is no question of there ever being a flow restriction by the supercharger.

If a supercharger operates at one atmosphere, or 14 psi absolute intake pressure, and has a pressure ratio of 1.5, the output pressure will one and a half atmospheres, or 21 psi absolute (creating 7psi boost)

But if the supercharger intake is operating forced up to two atmospheres, or 28 psi absolute, the pressure ratio of 1.5 still holds. That is the supercharger will have increased the incoming two atmospheres pressure up to three atmospheres or 42 psi absolute, or create a 14 psi boost increase, not the original extra 7 psi increase.

As the turbo winds up, the additional supercharger added boost pressure also increases as well. That can hardly be called restrictive.

  

 

This has been done several times before, but I will do it again.

A Roots blower is a positive displacement pump. For each turn of the blower it displaces a given volume of air irrespective of pressure.

Say it is attached to a 4 litre 4 stroke and is driven at 1:1 and displaces 4 litres per turn.

Putting aside variations in VE at different speed and the effects of cam timing, the 4 litre 4 stroke will displace 2 litres per turn of the engine.

Say atmospheric pressure is 15 psi, if the blower sucks in 4 litres, and pushes it into an engine that displaces 2 litres, the 4 litres of air will compress to 2 litres, but at twice the density, and putting aside temperature increase, it will also be twice the pressure.

This means the manifold pressure will increase from 15 psi to 30 psi (if there was no temperature increase) due to the compression from the blower.

If the Roots blower takes in air from a manifold fed by a turbocharger at say 30 psi, for each turn of the system, it will still take 4 litres into the blower and compress it to 2 litres as it passes through the engine, so 4 litres at 30 psi equals 2 litres at 60 psi.

The Roots blower does not know what pressure the air is, it just pumps 4 litres per turn irrespective, and the engine pumps 2 litres of air per turn irrespective of pressure. Due to this 2:1 volume overfeed, pressure must double.

In reality it will more than double as it will also be heated due to the compression.

Regards

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so what is the downside to twin charging?

Cost, weight complexity, space.

Regards

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Could you increase tunability, with a Waste gate before and after the Roots Blower?

That would also be adjustable so that the pressure against the roots could be controlled to keep the turbo from spinning heat at extreme pressures.

Also adjusting Fuel Pressure to compensate.

Cheers

I don't know anything but the people that do.

A wastegate will be essential. In would think that between the turbo and the Roots blower should be the pressure pick up point as you control the turbo directly by it's output.

Another thought is that by taking wastegate operating pressure from the manifold between the Roots blower and the head, you can allow the turbo to correct for lost efficiency of the roots blower.

If you need to change the second stage boost multiplication at the Roots blower, it is best changed by changing drive ratio, normally by pulley size change.

Regards

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ok..on a 454 cu. in. motor..if i run an 8-71(568 cu.in)..at 1 to 1...i get about 10 psi...that's about 1.67 - s/c to engine..568 into 227 .. so if i add 10psi of turbo boost to the intake of the s/c...then on paper i should get 10 * 1.67 = 16.7 + 10 or 26.7 psi of boost...right?....

My figures for an 8:71 are 436CI per turn or about 500 CI per turn if it's high helix retro Teflon stripped.

At 1:1 it will act as a restriction. You are going to need about 70% OD to get a 1.67 boost multiplier factor.

You really need at least a 10, or even a 14:71 if you want to run small OD ratio.

The data for GM based Roots blowers are:-

6:71 small diameter
Rotor dia=5.505", length=14.975", displacement per full turn of rotor=339CI.

6:71 big diameter
Rotor dia=5.778", length=14.975", displacement per full turn of rotor=411CI.

8:71
Rotor dia=5.778", length=15.905", displacement per full turn of rotor=436CI.

10:71
Rotor dia=5.778", length=17.000", displacement per full turn of rotor=466CI.

14:71
Rotor dia=5.778", length=19.000", displacement per full turn of rotor=521CI

This is theoretical displacement.

Retro or high helix will change this considerably.

A worn blower will reduce this a little.

A very good tight new blower and a Teflon stripped blower should be about the same.

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In every twincharged installation that I have been involved with over the last ten years, the pressure reference for the turbocharger wastegate has always been taken from the combined final boost pressure going into the engine.

That pretty well fixes the maximum boost that the engine ever sees, and holds it constant over a very wide power band. That is one important tuning parameter.

Another tuning parameter is the supercharger drive belt ratio. That alters the amount of boost produced by the supercharger which can influence boost threshold, and turbo response. But much more importantly it can raise boost pressure with respect to total turbine exhaust back pressure, and that is extremely beneficial to engine volumetric efficiency, combustion stability, thermal stress, and the detonation threshold.

The third tuning parameter is exhaust turbine a/r, which also effects the turbo boost threshold and response, as well as final turbine back pressure.

Despite the obvious cost, complexity and packaging disadvantages, it has huge performance advantages, especially for a very tractable high horsepower road car. It is especially good with small capacity engines.

The biggest advantages are the extreme tractability and low end response that the supercharger provides, as well as the massive top end airflow that only a suitably large sized turbocharger can give.  you get the advantages of both, without the disadvantages of either.

Superchargers by themselves all suffer from a rapidly falling volumetric efficiency at high engine Rpm, that is inescapable.

Turbos, (within a fairly limited flow range) can offer increasing flow and pressure, with rising rpm. Wonderful for top end airflow, but excessive turbine back pressure, a high boost threshold, and lag, can be less than wonderful.

But combine the two in the correct proportions and it is pure magic. Getting the proportions right is just a case of  experimenting with the blower drive ratio and the exhaust turbine a/r. This is not difficult, but the results are always extremely rewarding.

Plumb the exhaust up to the turbine section of turbocharger. Don't hook up anything to the compressor. Install pressure guage on discharge side of compressor.( no restriction) Guage reads zero. Now introduce a restiction. More restriction higher guage reading. Now lets assemble it all together in the scenario described. Install guage in conduit between turbocharger dischage and roots blower inlet. If guage reading is above atmospheric then the roots blower has to be providing the restriction. Where is the flaw in this logic?-------Phil

My figures come out a little different ???

Let's assume you want to end up with 20psi boost on a 568 CID engine.

That is 14.7 psi absolute pressure, needs to be increased up to 37.7 psi absolute pressure (20 psi higher).  That is a total pressure ratio increase of 37.7/14.7 = 2.56

If the supercharger and turbo both have similar pressure ratios, each will need to be the square root of 2.56 = 1.6

So as a starting point a 568 CID engine will have a displacement per turn of 284 CID.

My figures for an 8v-71P are 436.15 CID per revolution.

Now a bit of guessing will be required here, because of blower rotor leakage, and engine valve timing, but to fill 284 CID to a pressure ratio of 1.6 requires 454 CID of air.

As the blower "theoretical" displacement is 436 CID, it will need to be driven at something like 454/436 = 1.04 or 4% overdrive.  I am thinking that 1:1 might not be such a bad first attempt.

 

Opps

I forgot to divide 454 by 2.

I wondered why it needed such a big blower or OD

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Phil, in your thought experiment, the pressure between the turbo and the roots blower could end up positive, negative or at exactly atmospheric pressure.

Take the smallest sized turbo you can find, and feed it into a 12-71 blower driven at 12,000 Rpm.  It will the turbo that will be restricting the supercharger inlet.

If a supercharger is correctly sized and driven for any given engine to produce some usable boost, it can never be restrictive when boosted further by a turbo. Trust me, I have done this many times.

My revised figures for an 8:71 are 436CI per turn or about 500 CI per turn if it's high helix retro Teflon stripped.

A 454 pumps 227 per turn which is 1.9 multiplier.

You could get away with a 6:71 at 1:1 drive ratio
or even under driven a little.

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It is notoriously difficult to estimate a final boost pressure for a given blower drive ratio.

The relative displacements will get you within sight of the ball park (on a very clear day).

But what it really comes down to is real volumetric efficiency of both blower and engine, and both can vary hugely with how they are set up.

Thermal effects come into this as well. Hot air expands, and drives up the boost pressure, while also decreasing air density. Intercoolers shrink the air causing an often unexpected fall in supercharger boost.

Another wild card is exhaust back pressure.

Everything after the supercharger is just dumb back pressure as far as the supercharger is concerned, and the exhaust back pressure is just an additional restriction in series after the engine.

If you can fix the exhaust so that the exhaust back pressure falls by 5psi, then you will probably mysteriously suddenly lose almost exactly 5psi of boost.

Estimating the boost pressure of any supercharger installation is fraught with uncertainty.

I have to laugh to myself when I see a formula in a book to  calculate pulley sizes to obtain a certain boost level. It NEVER works out like that in practice.

The best indication is t consult racers that have run a similar setup. If a big block V8 with an 8-71 is said to produce about 10psi with 1:1 drive, it probably does.

Block up the exhaust with some mufflers, or a an exhaust turbine, and the boost could  rise considerably.

That is why the whole twincharge thing is so difficult to design theoretically on a clean sheet of paper, and the blower drive ratio is the most elusive thing of all.

I can tell you one thing for sure. Whatever boost the supercharger produces by itself on an engine, will be greatly magnified by adding a turbo. So if you only have 4-5psi, that will swell enormously with denser air into the supercharger, and some added turbine back pressure.

would you be able to use a lysholm SC?

Yes a screw type blower will work and is also positive displacement.

Exact boost is hard to predict, but one can only take their best guess.

Also, as far as the blower is concerned, it is very easy to adjust by simply changing pulleys. Once you have a number for your combination, an accurate correction is predictable as the VEs change very little with a change of drive ratio.

Regards

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let me confirm this... so.. if i have a 454 cid motor..227 cid per rev..and i mount a 436 cid roots blower to it..that..i now have a 1.92 multiplier at 1 to 1. which means then i have a virtual 872 cid motor, 1.92*454..and at sea level ..on paper..i am producing approx.  14.7 * .92 or 13.5  psi.

Is 20 psi 20 psi?...

Here’s my question...my goal is to move the torque curve left, up, and keep it flat as possible throughout the rpm range, and I’m looking to produce about 20psi..so..

With the twincharge set up, I can generate 20psi many ways..there are several permutations..the waste gate, and pulley size allow me a lot of flexibility, and I’m sure I will be playing with many combinations..but I’m looking for some direction...

i can generate 20psi and have little waste gate action..or I can generate 30 psi and blow off 10psi, or 40psi or 50psi etc....i can generate 5 to  10 with the s/c..and then whatever with the turbos...and just use the waste gate to blow off the rest...so...is 20psi in one scenario..the same as 20 psi in another...

The engine sees three things, the 20 psi final boost pressure, some exhaust back pressure that it must work against to power the turbines, and some direct mechanical power loss to drive the supercharger.

If I was doing this myself, the first thing I would do is select a pair of turbos that will handle sufficient airflow at around half the expected final boost pressure. I would then fit either the largest a/r turbine covers available from the range, or the next largest. At least something a fair bit larger than mid size.

I would then eyeball the flow area inside the exhaust housings, and choose wastegates with about roughly the same flow area. This assumes that the turbos will hit full boost at about half redline rpm. At redline, roughly half the exhaust flow will be through the turbines, and half through the wastegates. This is a very simplistic assumption, but the wastegates will need to be fairly large, because the required flow will be high, and hopefully the exhaust back pressure driving that flow fairly low.

I would initially drive my supercharger to produce about one quarter to one third of the total final expected boost pressure, knowing that the  turbo will increase that up to something approaching half the total boost pressure when everything else is hooked up.

If my engine was 454 CID, I would probably aim for about 6 psi theoretical supercharger boost to start off with.  That is a pressure ratio of 1.4  In theory that is going to require 318 CID of air. If the blower displacement is 435 CID, then the blower needs to be underdriven at perhaps 0.73 crank speed. A 6-71 may be a more suitable size for twincharging if you have one, but I cannot say so for sure.

After that, it is just a case of testing what you have. See at what rpm the turbos spool up to full boost, and the relative boost contributions measured across both supercharger and turbo compressors. Adjust the supercharger drive ratio or the turbo exhaust housings to suit the application.

People doing this for the very first time almost always make the supercharger too large, and the turbos too small. Sizing both for a twincharge is nothing like sizing either to be used by itself on the same engine.

The supercharger will work much harder and generate more boost pressure across itself when assisted by the turbos. It can be made smaller, or significantly underdriven compared to what you would normally expect to run supercharged.

Likewise the turbo needs to be enormous on both the compressor and turbine side. Turbos compressors are rated at a pressure ratio of 2.0 A twincharge will run at high airflow and an unusually low pressure ratio, and the bottom of the flow map is not usually a friendly place way out in the choke region.

The same turbo on the same engine run without the supercharger might not see any boost at all below 7,000 rpm, it would be huge. But the supercharger will produce enough extra flow to lower the boost threshold, usually to about half what it would otherwise be.

Realise that the supercharger kicks the exhaust turbine with extra flow.  And the turbo stuffs some extra dense air into the supercharger as it spools up. They assist each other in the most miraculous way. This results in a very fast boost buildup at rpm a lot lower than you might expect for such large turbos.

This all has to be experienced to be believed, but be conservative with supercharger sizing, and be very bold indeed with sizing the turbos, and it will all turn out about right.
  

i think i got it...

454 cid motor, 436 cid roots...at .74 crank produces about 6.2 psi, thus virtually creating a 653 cid motor..so i need two turbos whose combined boost gets to 10 psi(goal 20 psi overall).  6.2 s/c + 10 *1.4 turb = 20.4 total boost...in terms of sizing turbos...each of them need to handle  653/2 cid - that is the post s/c boost virtual engine..

Warpspeed is spot on with this.

You are partly correct, but the multiplier acts on density, not pressure. The air density is the same no matter what the temperature, but as you compress the gas, it increases the temperature considerably, which also increases the pressure.

My original statements discounted effects such as temperature change and cam timing etc so as to keep things simple.

bear1a

I do not know your background, so apologies if this sounds condescending.

You need to know and understand the combined gas laws to get a good understanding of the relationships of volume, density and pressure.

As said earlier, a Roots blower is positive displacement, so putting aside leakage between the rotors and the housing, it will build whatever pressure necessary to flow the mass of air it ingests. This means that within reasonable limits, at a certain rpm it will pump the same mass of air no matter what the outlet restriction is. Theoretically this looks like an engine will produce the same power irrespective of port efficiency, but with low flow cylinder heads, the blower does produce more boost to flow the same mass, so it takes more power to drive the blower. This power is drawn from the engine as a parasitic loss.

As a turbo is not positive displacement, as the pressure builds, the flow will drop off, so at the same turbo rpm, the higher the pressure the lower the mass flow.

Regards

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patprimmer no apologies needed..i'm learning..i love this stuff..and i am very fortunate to have found some incredibly talented engineers on this forum....
 

so in our example if we have taken a theoretical shot at the roots..6-71...at about 1:1..i'm thinking  gt42's..as a start...for the turbos...   

I have never myself had any personal experience twincharging such a large engine. But a small rotor 6-71T (339 CID)or 6V-71 (327 CID) driven at 1:1 and a pair of GT4288's with the 1.34 a/r exhaust housings would be my best first guess.

Those GT42 turbos are rated at 800 Hp each (at 15 psi boost), so they should come fairly close to your 1,400 hp target.

bear1a,

The GT42 may not be up to the job at the 1,400 hp power level you are anticipating. It runs out of flow well into the choke region. But it would work fine at lower power.

I have just been looking a bit more closely into this. 700 Hp worth of air per turbo is about 1050 CFM, or in nice round numbers 80 Lb/minute per turbo. Ten psi boost works out to a required pressure ratio of 1.68

Take a look at the GT4508R, it has a nice juicy fat compressor map down in the area we are interested in. It also runs down as low as 20 Lbs/minute at that pressure ratio, so it also a wonderfully wide flow range at good efficiency.

thank you...looks like the 6-71 with gt45's...here's one for you...do you think i should inject above the roots at all for cooling and lubrication?...i could run 8 above and 8 below?..

I really don't know the answer to that. This power level is way out of my league. One EFI injector per port is the simplest way.

Extra injectors discharging into different air pressures presents some rather unique problems for correct fuel mapping.

Another consideration is dry air is pretty safe if the engine spits back. A blower and plenum full of fuel may go *bang*.

In drag racing with methanol mechanical fuel injection and a Roots blower, it is normal practise to put at least some if not all in above the blower. It is reasonably common to also put some into the ports.

The idea of putting some in above the blower is to cool, lubricate and seal the rotors.

How you map for EFI with injectors into 2 different pressure zones is a problem. Maybe 2 different ECUs. a basic primary unit to put some fuel into the blower and a more sophisticated unit to accurately trim fuel for correct cyl to cyl distribution. Sounds expensive and complicated to me

Regards

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Water injection above the blower would cool, seal and it wouldn't affect fuel mapping. (Obviously, lubrication is a different story but to what level does this have importance?)

Water may be of doubtful value here, because detonation is definitely not going to be a problem.

Water will certainly seal the blower rotors and raise boost pressure, but as explained earlier, getting sufficient boost from the supercharger is never a problem when twincharging.

Where water really comes into its own is when a roots blower is run up to obscenely high pressure ratios without intercooling, or when a straight turbo engine is forced to operate at unreasonably high exhaust back pressure. Twincharging solves both these problems in a very neat way.

Twincharging is very benign. Combustion stability is easy to maintain up to very high boost pressures, because of the excellent positive scavenging possible during valve overlap. Fresh cool clean charge every cycle, with all those nasty hot exhaust residuals blown right out through the turbines.

Most people only think of mass flow and volumetric efficiency when planning a high power engine.  But combustion stability and thermal management can go a very long way to gaining reliable horsepower that lasts.

like i said this is fun, when you talk about positive scavenging and valve overlap, i presume you are referring to the exhaust stroke of the motor, where some of the intake is actually blown out the exhaust side, right?..i can see how that is affected by cam duration, but how is it affected by a different type of forced induction, i.e twincharging, vs. straight roots or turbos...

It has to do with the relationship between boost pressure and the back pressure in the exhaust manifold that the engine has to fight against.

With a supercharger, boost is always going to be way higher than any exhaust back pressure.

With a turbo, the unavoidable back pressure created by the turbine can be greater than boost pressure, and very often is. That traps a lot of very hot exhaust in the combustion chamber that simply cannot escape. Brightly glowing red hot exhaust manifolds and turbine housings are not that uncommon. Realise that gas flowing through those glowing headers, will be hotter than the pipes! That "red hot" trapped gas dilutes the incoming charge, and undoes mostly all the useful cooling that the intercooler may have achieved. It also takes up space that could otherwise be filled with fuel and extra oxygen during cylinder filling.

This is all far worse in a low compression engine, because the unswept combustion chamber volume will be greater for a given engine capacity.

We all know how turbo engines make greater power (at a given boost level) whenever a larger turbine, or larger turbine housing is fitted. The really big disadvantage of doing this, is that the engine then becomes more peaky and laggy.

But adding a supercharger after the turbo compressor increases boost pressure, and lowers exhaust back pressure, because the turbo no longer has to create all of of the required boost pressure all by itself. So the exhaust turbine has to work a lot less hard.

Boost can be very easily kept considerably higher than exhaust back pressure over a very wide rpm range without the disadvantage of a high boost threshold or lag. The turbo is still there, providing the massive top end airflow that only a turbo can produce.

This is why earlier in this thread I suggested you monitor exhaust back pressure as well as boost pressure. If boost can be kept a few psi above exhaust manifold pressure, that is hugely beneficial to sweeping out all of the exhaust during valve overlap.

The engine will have a much higher specific power output, because every induction stroke gets a full charge of clean cool fresh air, not diluted by superheated trapped exhaust gas.

The temperature in the combustion chamber at the end of the compression stroke will be lower, and there is far less chance of combustion temperatures reaching the critical point of instability and detonation. The result is a safer and more powerful engine, as well as potentially a much wider power band in both directions.

Supercharging looks ideal for really high horsepower, but they all have the unfortunate disadvantage of falling volumetric efficiency as rpm rises. But a big turbo can really perk up the top end airflow.

To the turbo freaks that insist turbos are more powerful, that may be true, but at the expense of having an absolute minimal power band.

Where are all the four second turbo cars ???

Twincharging will give the flexibility, response and wide power band of a supercharger with the high specific top end power of a turbo. The advantages of each, without the disadvantages of either.

Where that boost pressure actually comes from, is extremely important to the final results. My own experience and success leaves no doubt that twincharging is superior to either supercharging or turbocharging.
 

Will scavenging lower the boost a little? With a lot of valve overlap, we will have some air going straight through. Pressure can't start to build until the exhaust valves close.
I understand the benefits of scavenging, but will replacing a cam with little or no overlap with one with a lot of scavenging overlap lower the boost a little? Is this one more of the complexities that make the difference between experts and beginners, or am I missing something?
respectfully

You are quite correct.

But losing a little induction air is much better than having very hot trapped exhaust residuals displacing and diluting the fresh incoming charge.

In fact, if the whole thing is set up to perfection, no significant air volume need be lost. The exhaust valve is kept open just long enough, to see the last of the hot exhaust exit at low pressure, before it closes.

The degree of valve overlap is tied to the pressure differential between induction and exhaust. Exactly the same thing occurs with tuned headers and a high overlap cam, except it is the timed negative exhaust reflection that is the driving force. This only works over a limited rpm range, and the effect can be relatively weak.

With tewincharging it is the higher boost than exhaust pressure that is the driving force. And this is far more consistent in action over a much wider rpm range, and delightfully easy to control with the blower drive ratio.

So very much depends on the pressure differential, and valve overlap. But it is certainly far easier to set up than critical header and collector tuning, and vastly less valve overlap is required for it to be effective.

A blast of relatively cool air over the exhaust valves is beneficial too. The fuel injector can be timed to open only after the exhaust valve has fully closed, so it is only air that is lost, but the benefits of doing this are huge.

When optimising an engine to accept a Roots blower there are 4 important issues, reduce compression ratio to avoid detonation, reduce valve overlap so as to limit scavenging to the point where almost all exhaust gas is expelled, but little fresh charge is lost, increasing exhaust flow capacity and cooling the charge to avoid detonation.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

as a follow up...i'm assuming a twin screw supercharger would work the  same as a roots in the twincharging set up..yes?

Yes, but it will have a lower parasitic loss to drive it to the same boost.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

that's a good thing i presume..:)

It certainly is.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

as i'm getting close to pulling the trigger on parts..i'm trying to figure out the smallest size blower i can use  for the system...whipple makes a 5 liter...a bit smaller than the 6-71 in physical size as well.and has a rear air intake which is nice...any thoughts as to whether the whipple will be enough

My best guess would be a Whipple 3300AX would be about right.

This assumes a 400 CID engine, and an initial "guess" at a  required (unaided by the turbo) supercharger pressure ratio of 1.5 That is a rather high, but let's be conservative here.

That is going to requires 300 inches of air to fill 200 inches of engine per revolution. Or going metric 4,916cc of air per rev.

The 3300AX has the rear entry option, and flows a theoretical 3,300cc of air per revolution. So the blower will need to be overdriven at 1.5 x crank speed.  As this blower has a maximum continuous rated speed of 11,000 rpm, that allows a conservative 7,300 rpm engine redline.

I would start off with a blower overdrive ratio closer to 1.3, knowing you can go up comfortably to at least 1.5 if required. but the actual boost you will end up with in the car is impossible to predict with any accuracy. There are just too many difficult to pin down unknowns.

warpspeed, you say it is unrealistic to calculate boost from a positive displacement blower.

i have done a few calcs and seem to have found a method which gives ballpark figures. i posted this on another forum so i will just C&P it here.

please prove/dis-prove this method.

note that a SC14 is a toyota positive displacement blower capable of displacing 1.4L of air/rev. the 3sgte engine is a 2litre 4 cylinder DOHC toyota engine.

"im trying to calculate the effective boost of running a SC14 on a 3SGTZE engine

im looking at running a belt ratio of 1.2:1, overdriven

so one rev of the crank forces the SC to displace 1.7litres of air (1.2*1.4)

since the engine consumes a fixed volume, i thought (suggested by someone else) it must be possible to use the formula P1V1 = P2V2,

the pressures are in absolute so;
P1 = 14.7 psi (atmo pressure)
P2 = unknown
V1 = 1.7 litres (amount consumed by SC)
V2 = 1.0 litre (volume consumed by 2.0L engine @100VE)

P2 = (P1*V1)/V2
= 14.7*1.7/1
= 25psi (abs)
= 10.3 psi boost

this figure sounds ballpark but i havent looked at any system losses. if the VE for the SC and engine is the same (probably not) , then they cancel as a fraction because the V1 would multiply by a number eg 0.9 and V2 would also multiply by a number ie 0.9..... therefore, 0.9 would be on numerator and denom so would cancel.


its seems so simple but ive read that calculating PD SC boost is complicated and not accurate at all?


i would like anyone with a SC engine (prefer SC14 or SC12) to provide me with engine size, SC type and SC pulley ratio so i can validate this formula


just something ive dug up about another forum member's setup

he has a 1g-gte (2 litre I6 dohc), ~1.7pulley ratio and claims 16psi boost

putting the formula to the test;

P2 = (14.7* (1.4*1.7))/1.0
=34.986psi (abs)
= 20psi boost

close!

from that formula, the only way that value can be lowered is if the denominator is bigger (meaning the engine consumes more than 1.0L air/rev - should be opposite) - so prob not that case.

or, the numerator is smaller. this makes sense because its highly likely the efficiency of the SC deteriorates with RPM. since the atmosphere pressure shouldnt budge from 14.7psi and the pulley ratio cant change..... the calculated displacement of the SC is;

~31psi (abs ) = (14.7* (actual SC disp. * 1.7))/1

actual SC disp = 31/(14.7*1.7)
= 1.24L/rev

this equated to 88% VE

seems ballpark also

so now to redo this with engine VE of 95% (realistic?)

~31psi (abs ) = (14.7* (actual SC disp. * 1.7))/(1*0.95)

actual SC disp = 29.45/(14.7*1.7)
= 1.178L/rev

equals 84% efficiency

now i shall use the same forum member's efficiency figure for my calculated boost

P2 = (P1*V1*SC VE)/V2
= 14.7*1.7*0.85/1
= 21psi (abs)
= 6.5 psi boost
 
maybe the SC VE is better when less boost is run?



sounds about right to me but seems too simple to be accurate. anyone like to check my math?

any ideas how thos SC VE's equate in real life?


ideas and thoughts welcome


cheers

brett
"

Brett

Your method P1V1=P2V2 is what I use, but there are several sources of error.

1)  The air is heated during compression so that increases the pressure at the compressed volume.

2)  The blower has some clearance between case and rotors, so there is some leakage.

3)  Restriction in the cylinder head will resist flow, but as it is pretty much positive displacement, the mass of air must flow through the engine, therefore the pressure will continue to build until the mass flow into the engine equals the mass of air at atmospheric and displaced by the blower.

4)  Valve overlap allows boost to escape across the chamber and out the exhaust, thereby reducing boost.

5)  Fuel added to the manifold will both displace air and cool the charge.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

Pat is spot on. Plus probably the biggest source of potential unknown error of all...  exhaust back pressure.

Think of it this way.

The supercharger pumps a "fixed" volume of air into two restrictions connected in series downstream of the supercharger. The first restriction to flow is the engine itself, the second and sometimes much greater flow restriction is the exhaust system. The pressure drops across both these restrictions are directly additive. The actual boost pressure at the supercharger outlet is not created directly by the supercharger, but is a direct result of the ultimate total restriction to flow downstream from the supercharger. And that will be directly dependent on exhaust back pressure.

Nobody talks about this, and I have never seen it in any of the reference texts on supercharging either. but there is almost a 1:1 correlation between a change in exhaust back pressure and a directly resulting change in boost pressure.

Any blower drive ratio formula that only considers supercharger and engine displacement can be way off, if the exhaust system is restrictive. Realise also that exhaust back pressure rises almost square law with any flow increase. Double the horsepower will raise total exhaust back pressure FOUR times. This simply cannot be ignored, and often makes a complete nonsense out of any preliminary boost pressure guestimate.

Pat mentioned thermal effects, and an intercooler can cause a dramatic drop of boost pressure. The air literally shrinks in size as it cools. The drop in boost pressure occurs right at the supercharger discharge, it is not measured as a pressure drop between intercooler inlet and outlet. Although obviously there will always be some finite pressure drop across the intercooler.

I suppose you could assume that the original engine was equipped with an exhaust system sufficient to handle the original amount of exhaust flow.  But some standard factory systems can be fairly restrictive at flat out maximum power.

My own experience is that I initially guess a suitable supercharger drive ratio in the way you suggest, based on relative supercharger and engine capacity. Then just road test it with a boost pressure gauge and an exhaust back pressure gauge. Armed with some figures, it will then become obvious what needs to be done to get it where you want.

It is a most curious thing to modify the exhaust system, and  then have a very significant reduction of boost pressure, yet have the car accelerate much faster as a result.

But getting back to twincharging. The exhaust turbine and wastegate will have a major impact on total exhaust back pressure. And that directly effects the supercharger. The whole thing is just far too complex to try to design on paper. Give it your best shot, but be prepared to change both the supercharger drive ratio and turbine cover a/r to get optimum results.

pat & warpspeed, your replys very much appreciated.


i am now seeking some advice on a suitable turbo


the engine i will be using is a toyota 3SGTE, 2 litre I4 DOHC. the supercharger (which i already have) is a SC14. it displaces 1.4Litres of air per rev.

it would be good if you could recommend something from the garret range (type, trim, a/r). im new to reading turbo maps so im unsure how to go adding in a SC to the map and its consequences :S.


im aiming for a peak output of about 400hp at the fly. the standard injectors are 540cc so 540*4/5 = 430hp.



im assuming with wastegates, bigger is better. somthing around the 40mm diameter would suffice?


regards,

brett

First, I doubt if the injectors will be up to the job. The engine will require around 6cc of fuel per minute per horsepower. Flat out at 100% duty cycle that is 550 x 4 = 2200cc/minute. Divided by 6 is 367 Hp. Run at something more sensible like 80% duty cycle that would be only 293 Hp worth of fuel, a long way short of 400 Hp.

I have no idea what combination of boost pressure and maximum rpm you plan to push the engine to achieve 400Hp.  The decision you make there, is going to have a very great influence on success, and also reliability. It is the very basis of everything else, including selecting a turbo.

The SC14 with its two lobe plastic rotors was designed to only run intermittently for very short periods on the original engine. Do not expect it to last very long if it is overdriven, or used continuously. A three lobe Eaton M90 is about the same size (1474cc/rev), and is a far better and more robust design in many ways.

Just wanted to chime in and say that this is a very interesting, and valuable thread!

I have been developing supercharger systems based on Lysholm type compressors for several years now.  The twincharging idea is intriguing, but I will probably not get into that for a while yet.

What are your thoughts on a twincharged setup, but with the throttles located AFTER the positive displacement blower/compressor?  I know that the sizing, and operation, of an appropriately controlled bypass valve (or valves) will be absolutely critical in this type of application.  

Are there any other considerations that might come into play on top of what would already be necessary to make a blow-thru the throttle positive displacement setup work by itself?

Thanks,
Steve

Steve,

Any supercharger setup will always benefit from improved throttle response by having the throttle(s) as close as possible to the intake valves. That is the simplest and most obvious way to do it with a centrifugal supercharger (or a turbo). But it requires a little more thought when a positive displacement supercharger is involved. The benefits of improved throttle response are quite significant, and well worth the trouble. Moving the throttle ahead of the supercharger is always a retrograde step. Sometimes it is unavoidable, but doing so is never an improvement.

The basic problem is fairly obvious. With a positive displacement supercharger, and a suddenly closed down stream throttle, there would simply be nowhere for the supercharger discharge air to go.

Some type of air bypass system around the supercharger is required to recirculate any air not consumed by the engine.   This system can also relieve the supercharger of any back pressure (boost) upstream of the throttle at small throttle openings, if it is set up to do that. This can completely unload the supercharger and reduce the pumping and supercharger drive power at light throttle when no boost is required. The benefit is less supercharger heat, noise, and vibration, and significantly improved small throttle fuel economy (expect ~10%).  All supercharged factory production cars have such an air bypass system fitted, but few if any of the supercharger kits do.

The idea is simple, but putting it into effective practice usually baffles and defeats the do it yourself hot rod fraternity. It is much easier for the Auto manufacturers because they can use the existing electronic engine management system to control the air bypass. They can build in some nifty extra features, such as not allowing boost in reverse gear, or reducing boost if engine water temperature is too hot or too cold.

An effective bypass needs to be very positive and progressive in opening and closing, and it needs to be sensitive to engine load.  Crude blowoff valves simply slam open and shut and are extremely unpleasant to drive. They usually also usually allow full boost upstream of the throttle when plenum vacuum is negative, for example during constant speed highway running. The extra wear and tear, engine harshness, and fuel consumption is never good.

I have experimented with all this for years, and have developed a very simple solution that anyone can copy quite easily. More on this in my next post.

The ideal thing to use for a positive displacement supercharger bypass is an external turbocharger wastegate. This should be mounted so it bypasses the flow directly around the supercharger when open. It does not need to be very large.  Try to imagine how much boost pressure you would lose if you drilled a one inch hole in your supercharger pipework!  

It absolutely must be mounted so it flows in the correct direction.  That is, increasing boost pressure would try to force the poppet valve off its seat.

Now most if not all external wastegates have two pressure inlet ports, one above and one below the control diaphragm. And the internal wastegate spring always holds the wastegate shut.

The trick here is to connect two control air lines, between the wastegate to directly across the throttle body. The idea here, is that the wastegate diaphragm senses the pressure drop across the throttle body, and increasing plenum vacuum opens the wastegate against the internal spring.

It should be obvious that at wide open throttle, the wastegate diaphragm sees zero differential pressure, no matter how high the boost pressure rises. That is the secret....  The air bypass is load sensitive and depends more on the combination of airflow, load, and throttle position, and  not so much on just raw manifold air pressure.

At engine idle there will be sufficient vacuum to snap the wastegate wide open. The supercharger bypass will completely unload the supercharger at engine idle.

At light throttle constant vehicle speed there should still be enough vacuum to hold the bypass open far enough so there is zero measurable boost upstream of the throttle.

A little bit more throttle, and the wastegate closes slightly further and boost may just rise slightly.  More throttle still and the wastegate fully closes, giving full available boost pressure.  it is all wonderfully smooth and progressive in action.

Same when you lift off, the wastegate opens. For a more violent type of throttle movement, the wasteagate slams shut instantly at full throttle, and springs instantly wide open on throttle opening, as in gear changing.

Any boost spikes up stream of the throttle force the wastegate off its seat, so it acts as an over pressure relief valve.

Getting it working properly depends entirely on fitting a suitable spring to the wastegate, and that requires a great deal of thought and attention to get right.

The first requirement is that the spring be stiff enough to hold the wastegate closed against full boost pressure without leakage. But it also has to be weak enough to open the wastegate against plenum vacuum at part throttle.

The problem is the ratio between poppet valve area and control diaphragm area. A ratio of at least 2 x diameter (4 x area) would be a minimum requirement, and many cheaper wastegates do not have sufficient difference in areas to work properly in this application.

But you can work all this out for yourselves. You may wish to be able to drive around with the supercharger unloaded with plenum vacuum higher than perhaps 5 inches Hg.  That is 2.5 psi (roughly) But it must also seal against say? 10 psi of boost. You can see that if the two area have only a four to one ratio, it might be very difficult trying to juggle a spring that can achieve both these requirements.

So try very hard to find yourself a wastegate with a huge diaphragm diameter, and a reasonaby small poppet valve flow diameter. The Chinese gates with the crap diecast bodies and  nasty low temperature rubber diaphragms work fine in this application, provided the area ratios are sufficient. And that can take some searching.  The more expensive "brand name" Japanese gates are mostly excellent, but far from cheap, not all meet the area ratio requirement so be careful.

It may be possible to build a "Frankenstein" hybrid wastegate by mixing parts, to get a higher than standard area ratio. The larger the ratio, the less critical spring selection will be.

While setting all this up, drive around with the wastegate on the passenger seat or floor, and watch what it does. It will respond instantly and progressively to throttle opening, just like the needle on a vacuum gauge does. Get the spring sorted, then install it onto the engine.  I think you will be extremely pleased with the results.

This system works equally well on supercharged or twincharged engines. I have been fitting this to street vehicles for well over eleven years now, with great success, and others that have copied it are also very pleased with the results.

Sorry for the barrage of words.  It is really a very simple idea, just difficult to express in very few words.

More on the spring thing.

A particular spring will have a specific opening pressure, and a rate of opening that are quite independent.

For instance, a very long soft spring can develop the same force at the same loaded height as a much shorter stiffer spring.

How all this feels in the car can make quite a difference.

For instance, boost could rise smoothly with gradually increasing throttle opening from just above idle. That may feel very smooth to drive but may not be ideal.

A lighter spring may allow you to use throttle openings up to and above 50% while seeing zero boost, then have the last 25% of throttle opening really throw you back into the seat.

That lets you easily keep up with the traffic flow without using the supercharger at all. Fuel economy will be as it was originally. But flat out it will really get up and go.

Spring selection really decides how quickly boost builds with increments of throttle opening, as well as the throttle opening where full available boost is reached.

The whole thing is really wide open to driver preference,   but with a bit of fiddling around, an ideal spring can absoluttely transform how the vehicle feels to drive.

It is really fun to play with some different springs.  But first watch  how the wastegate behaves with it sitting on the passenger seat. Get a feel for how it moves with the throttle under different driving conditions.  

One thing is for sure, whatever originally came in the wastegate will be far too stiff.

Unless it was designed to control the original turbo boost pressure to perhaps something like 2psi or 3psi. That is highly unlikely.  The spring you end up with will be much lighter, and you are going to have to find something yourself by experiment.