Exhuast manifold for turbo 1

[devittjl]

Most turbo applications seem to use a "stock" exhaust manifold. I believe the reasoning is to keep the turbo as close to the head as possible to aid in the spooling of the turbo.

I have an application where a "shorty" header is available.

http://www.focus-central.com/catalog/details.asp?cid=2&sid=&pid=89

Is there any advantage of using a header on a turbo application? My think is that there would be better and more even flow, thus quicker spooling with a header.

Thanks for the help.

 

[crysta1c1ear]

Turbo lag is from the time it takes to speed up the turbo rather than the time it takes the gases to get to the turbo so don't expect miracle results.

There should be less friction before the turbo with shorter pipes and less heat loss which the 'turbo' threads have stated is a contributing factor to the turbo efficiency, so you have two immediate direct advantages, though I doubt either of them is very significant. If things are hotter, you might just want to watch that they are not too much hotter.
We don't want things looking like this do we
Just kidding.

=

When a river gets wide and deep the current slows. When the river gets narrow and shallow it speeds up. The energy to speed up the water molecules at the narrow shallow parts has to come from somewhere. So generally, you are going to mess up the flow of something if the cross sectional area is continually changing.

The cross sectional area at the exhaust valves is probably as big as they could make it. With a turbo there will be more gas coming out of the engine through the same sized holes (valves). So unless we want an energy consuming shock and change of the speed of the 'current' I'd expect similar sized piping up to the turbo as the non-turbo version of the same engine. After the turbo it should be lower pressure and slower and that is where I would expect a significant change in pipe diameters. (A slower river needs to be wider or deeper to have the same flowrate.)

I am sure you'll get more informed answers, but my gut feeling is
1) don't expect too much so don't bother unless a tiny benefit is worthwhile;
2) yes it will have (small) advantages.
3) depending what is standard (ie same as on non-turbo version) after the turbo, there might be bigger gains to be made there - if that is not considered off-topic.

 

[patprimmer]

Like crystalclear says plus, an aftermarket tubular steel fabricated exhaust might not be robust enough to take the heat generated in a turbo exhaust manifold upstream of the turbo.

The original cast iron manifold will be substantially better from that point of view.

If you do fabricate a manifold, use tubing with a wall thickness at least twice as thick as a normal header for an NA motor, keep the pipes as short as possible, and merge them smoothly.



Regards
pat pprimmer@acay.com.au
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.

 

[Warpspeed]

There is one other factor to think about when designing an exhaust manifold, and that is exhaust reversion.

Exhaust manifold pressure will be higher than boost pressure in almost every instance, especially with anything you can drive on the street.

But exhaust reversion can be made far worse where there is more than one exhaust valve open, and where one cylinder can exhaust directly into an adjacent exhaust port that is just coming up to TDC on the exhaust stroke. This situation can be made far worse where a large amount of valve overlap is being used as well.

It can sometimes help to run individual pipes right to the turbine where adjacent cylinders both have the exhaust valves open together. The exhaust pulse has to travel right to the turbo, then back along the other runner. The time delay will be short, but it might be enough to make a difference with some particular cylinder layouts at particular Rpm.

An example of this, might be with three cylinders of a straight six or V6 engine feeding one turbo. If the exhaust cam duration is 240 degrees, and each cylinder fires every 240 degrees, one valve will shut, just as the next comes off the seat. There will be no interaction between the cylinders, and a compact "stock type" minimum volume cast iron manifold would probably be excellent.

Now consider the same engine with a healthy 270 degree duration exhaust cam. Two exhaust valves will now be simultaneously open for 30 degrees. One cylinder will be blowing down at very high cylinder pressure, just as the next is approaching TDC at the end of the exhaust stroke. An individual runner tube manifold might offer some measurable advantages.

The critical time seems to be when the engine is struggling onto boost with the turbo spooling up and wastegate shut, and there is sufficient time for considerable exhaust back-flow to occur. It might be thought that the big cam is the only thing causing the lag problem, but an independent runner exhaust manifold might help as well.

Putting the tube header on the stock engine with small cam probably will make no noticeable difference in this example. So it pays to think the whole problem right through with the cylinder layout and valve timing you plan to run.

 

[patprimmer]

Some good points by Warpspeed.

It just reinforces every decision you have to make when designing or building an engine is a complex set of compromises. It always ends up coming down to some calculations, tempered with with experience and a final judgement based decision.

Regards
pat pprimmer@acay.com.au
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.

 

[crysta1c1ear]

Sorry, when I wrote ...
We don't want things looking like this do we
Just kidding.
... I forgot to post the image, because initially I couldn't find it.

What Warpspeed wrote sounds right and he clearly knows more about this than I do, but I question whether it applies or distracts.

A link was given (

http://www.focus-central.com/catalog/details.asp?cid=2&sid=&pid=89)

to a product which reportedly increases horsepower.

Consideration will have been made in the design of this product of whether gas can come out of one cylinder and end up trying to get back into another. A net horsepower gain says that things aren't too bad, athough we don't know how it operates over the whole range, but presumably no worse if the product avoids customer complaints.

So as I see it, the real question is not the fundamentals of its design, but whether adding a turbo after it would cause significant changes to the advantages we assume it already has.

The pipes seem to merge quite close to the end.

For gas flow from one tube to undesirably interfere more (than in the non turbo version) with flow in another in the way Warpspeed described, I think flow speed (m/s) would have to be higher with the turbo.

With the turbo, there will be more mass of gas. When in the cylinders, its at a higher density and higher pressure. But does it come out faster? Or just at a similar speed maintaining high pressure?

(1) The higher pressure relative to the outside world (atmospheric pressure) is a reason the gas could speed up more compared to a normally aspirated vehicle.

(2) The blockage of the turbo is a reason the gas could speed up less compared to a normally aspirated vehicle.

Again, simply a gut feeling ...
the restriction of the turbo would cause high pressures to be maintained until after the turbo, before the turbo there wouldn't be an increase in the speed (m/s) of gas flow compared to the normally aspirated version with this same "shorty header" (so we wouldn't have the problems warpspeed descibes). What I'm saying, is that effect 2 - as a guess - overrides effect 1 in this case.

Taking some merely illustrative figures ...
If the "shorty header" alone gives you 2% more horsepower,
and the turbo alone gives you 50% more, then
1.5*1.02=1.53
then I suspect you might get a touch more than 53% more power with the two combined.

==

I've changed my tune a little since the first posting since I have cottoned-on that the "shorty header" provides a power increase in its intended application, probably with a mixture of warpspeed's understanding and pat's experience tempered calculations.

 

[willeng]

crysta1c1ear:

Nearly all engines, including std engines make the primaries glow, especially in a darkened room. Cast iron manifolds look great when hot also.

It's hard to compare any n/a exhaust system to that of a Turbo, there just so completely different!

The site says, 2 to 3 hp consistantly, one would have to argue whether it's worth the bother for such a small gain in performance & where is this gain.

I think the biggest stumbling block is that engines are as individual as people to the most part & require slightly different things to make them perk up. The trouble with this is that unless you have the exact same engine as the test engine was, the gains may not be there to have. On the other hand your engine might like the pipes & you'll gain more! Testing is the only real way to find out, you need to test several designs for your specific engine to see what it prefers!!

Any reversion, no matter how much kills power & the camshaft & exhaust choice is critical like warpspeed says!