twincharging estimated results 12

[bear1a]

ok, so, if you have a twincharge set up on a chevy small block 400 cu. in motor, turbo feeding roots blower... roots by itself at 4000 rpm produces 6lbs boost...turbo by itself at 4000 rpm produces 15 lbs boost..what happens when we put them in series...turbo feeding roots at 4000 rpm ..what approximate boost will be get?...i'll know soon enough i'm building an engine..but would like some theoretical input

 

[Warpspeed]

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.

 

[patprimmer]

Opps

I forgot to divide 454 by 2.

I wondered why it needed such a big blower or OD

Regards

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[Warpspeed]

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.

 

[patprimmer]

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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[Warpspeed]

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.

 

[rpmag]

would you be able to use a lysholm SC?

 

[patprimmer]

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.



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[bear1a]

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.

 

[bear1a]

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...

 

[Warpspeed]

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.

 

[bear1a]

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..

 

[patprimmer]

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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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[bear1a]

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....

 

[bear1a]

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...

 

[Warpspeed]

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.

 

[Warpspeed]

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.

 

[bear1a]

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?..

 

[Warpspeed]

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*.

 

[patprimmer]

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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[globi5]

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?)