Super short runners for high boost?

[obanion]

I have been running lots of simulations on a (I think) very nice engine simulator called "Engine Analyzer Pro" from Performance Trends.

Specifically, I did a comparison between longer (~13&quot runners and super short (~2&quot runners. What I found is most interesting.

At atompheric pressure, the longer runners certainly win through pulse tuning. This can be observed by looking at the intake port pressure waves, which you can break down at any given RPM and observed over the whole 4 stroke cycle.

Now it gets interesting. As the boost gets higher, starting at 15psi, and testing all the way to 45psi, the shorter runners start to perform better and better. To the point that at 45psi, my tests showed almost 30% more peak HP on the shorter runners, with no other change.

The reason for this is in the outputted intake port pressures. As more and more pressure exists, there is a greater and greater drop in the pressure at the intake valves, with the longer runner. The shorter runners don't suffer from this drop.

On thinking about this, it makes sense. When the valves open, the column of air in the runner has to accelerate to speed to start supplying the air to the cylinder. With a longer runner, compounded with greater mass (from the higher pressure), there is a much greater mass in the runner, while slows the acceleration. With the short runners, the pressure drop is sudden, and the pressure at the valves stabilizes quickly. With the longer runners, the pressure drops as much as 15psi lower, and takes longer to come back up.

The result, is VASTLY improved volumetric efficiency with the shorter runners. The more boost you run, the greater the advantage.

Now, is this software correct, and my theory as well, or is this a bunch of fantasy? The engine I'm building will run at very high boost levels (45-55psi), so I most certainly want the short runners if this is true.

 

[patprimmer]

Pulse tuneing does not work in the real world with forced induction, as it depends on pressure waves travelling at the speed of sound. The speed of sound through air varies wit either temperature or pressure (I forget which, but by the combined gas laws, they are directly related, so it does not really matter which).

As boost and induction temperature is quite variable, you can never maintain the required tuned length when running the engine.

Also the main advantage in tuned length pulse tuneing is to improve exhaust scavaging at about, TDC overlap.

The positive pressure from forced induction dramatically improves exhaust scavanging, to the extent that wide lobe centre cam designs are often used to reduce this so as to avoid to much unburned fuel air mixture from escaping the combustion chamber before the exhaust valve closes.

Having said all that, I would expect the shortest, most direct manifold to give the best result

Regards
pat

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[Andy330hp]

Some questions:
Turbo or Super?
RPM range?
Application? (race, street, both?)

 

[obanion]

Compounded turbos (one blowing into another)
45-55psi boost
Rev limit at 7-8k
Application is both. Compounded turbos gives very good spool, as the smaller turbo acts effectively alone at first. Willing to sacrifice some low end performance from loss of pulse tuning in order to reap gains on top, if above theory holds. If the gain is more like 5% on the top, then I'd rather run a stock manifold (cheap and readily available) with 13" runners.

 

[Andy330hp]

30% certainly sounds "optimistic" to me.

I can't help but wonder if there is some problem with the simulation software near special cases. What happens if you enter a runner length of zero, or near zero (0.001 if zero is not allowable)?

Have you wandered around a racetrack and looked at what other people are doing?

 

[obanion]

It won't let me set a 0" runner. I'll see if I can fuddle the lengths around a little, see if I'm just catching a calculation error.

 

[patprimmer]

A real test of it's program intelligence would be to try a negative number.

Regards
pat

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

 

[obanion]

OK, I ran many more simulations. Kept increasing the runnner length in 10mm increments, from 25mm all the way to 300mm. No sudden changes indicative of a calculation error. Everytime it got longer, I lost a little top end power, and gained a little on the bottom. The advantages of short on the top end were much better than the losses on the bottom end.

 

[EfiOz]

I know that shorter runners and manifold do wonders for turbo lag.

 

[obanion]

As in shorter runners reduced lag?

 

[patprimmer]

Of course it reduces lag. The less the volume in the manifolds, the less the gas needed to be pumped to fill them.

Regards
pat

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.

 

[mlangeani]

Earlier crude intake models used to view air inside the runners as mass in a mass-spring system. Therefore, the higher the air density the lower should be the system frequency and your deduction might explain the facts. However, the intake process is quite complex. There are pressure waves traveling along the runners, one cycle being affected by previous one, one cylinder being affected by the others, intake process being affected by the exhaust process, and so on. Meaningful conclusions need knowledge, experience, and carefully analysis.

Long runners produce lower pressures, but if properly tuned they do provide higher pressures at intake port at valve closure time, tending to improve charge efficiency. Try to change the runners length, little by little, and find the optimum lenght for each boost pressure. However, be aware that each length has its own optimum cross section and valve timing. For better understanding, try to isolate effects. For instance, eliminate the connections between cylinders through the intake system, eliminate the connections between cylinders through the exhaust system, run only one cycle, increase valve size, and so on.

 

[Azmio]

Obanion,
Here is another way that you can do with your performance simulation software. First you can open up your intake valve earlier to take advantage of the scavenging effect from the exiting exhaust gas. Next you should also close your intake valve later than the NA engine. This way, the positive pressure in the intake system can further push the air into the cylinder even while the piston is moving up. If you have a VVT system it's even better because you can get just the right balance before the piston starts to push the air back into the intake port.
I understand that you run up to 45psi and therefore idle and part load are not your concern.

AO