toddk911

If it is better to run a velocity stack for air intake, would a reverse, or increasing diameter exhaust work better on the way out?

I.e.

2" header to 2.5 exhaust/down pipe, to 3" to 4" to muffler, etc. ?

Or maybe this would be better identified as a venturi effect?

nick_968

Is it not more important (for exhaust) to be bigger the closer you are to the turbo and then smaller on the way out. As the gasses cool they need less space and so the further along the exhaust the less diameter you need to maintain velocity?

toddk911

Ok, but wouldn't that create a bottle neck effect and increase backpressure?

racerxrick

I'll be interested in this thread as i am considering this exhaust:

http://www.lindseyracing.com/LR/Part...AUST4INCH.html

Mike Lindsey

It's pretty simple really. You want less back pressure. The ideal exhaust on a turbo is a short
velocity stack dump pipe. You see that on Indy Cars, Drag Cars, Tractor Pulling set-ups etc... No reason for a long exhaust system. It costs you potential power. However, we need it to exit out the rear of the car. Since we cannot end the exhaust part way there which would be better, we do the next best thing and make the pipe bigger which is working in the same direction or goal of not having a pipe at all. Bigger is better. Even 4" exhaust systems have back pressure, so we have 5" now.

We know... we know... somebody is going to say "but you can make 400 WHP on a 3" exhaust. That may be true, but if you put a 4 or 5 on that same car, it will make more with no other changes (other then it will need more fuel which you need to do one way or another) .

elargentino

This second paragraph is funny to me.
If I had a nickel for every time I heard that.
You can make 400 RWHP through a stock AAN Audi throttle body too...

toddk911

Glad you posted Mike as I was thinking about you guys when I was thinking of this.

Any chance you could try my theory out on your dyno and see???

So you are saying it would better to run full 3" or 4", for example, as opposed to running a gradually increasing diameter of say 2 -3 -4 -5 ?

That the trumpet style would not act as a collector so to speak and start to "vacuum" exhaust out?

toddk911

The reason I specifically said "velocity stack" is exhaust is all about velocity of the exiting gases, and not just volume of gases exiting.

Smaller diameter = higher velocity but higher backpressure. Larger diameter is the opposite.

So how do we get the best of both worlds? My theory is either a gradual increase in diameter or some staged exhaust that openes wider at high rpm much like a variable vane technology; but that type of exhaust would be really trick and expensive. lol

I guess an eaiser or cheaper way would be a standard exhaust with a Y piped in around the cat area and at the end of the pipe a spring loaded cap that would open to a large diameter pipe as rpm/volume increases and thus you would have smaller diameter for lower revs and higher velocity of exhaust and then as rpms rise and backpressure increases, the flap would start to open allowing flow into the 4 r 5" pipe and out and maybe tie back into the rear muffler or run an inline muffler in the large diameter pipe.

LS1Porch

The closest thing to what you're describing would be what the new(er) Chevy trucks come with. Have you seen that funny exhaust tip that's almost hourglass shaped? It creates a venturi effect at the end of the exhaust that speeds the gasses out.

It's fun to play with this stuff, but honestly i wouldn't expect a horsepower miracle in the real world. You could spend hours on it and end up with no noticeable difference in power.

Reimu

Has anyone here ever done a side exit? Probably not worth the couple of feet of exhaust it saves you but blowing flames into the oncoming lane would be interesting...

nick_968

For out and out hp the way I understood bigger is better but for mid-range response you need smaller. Hence the reducing size as the gasses cool to increase or keep velocity constant to pull the gas out. But hey its just a theory I have nothing to back it up!

333pg333

I would have to say on a 'seat of the pants' feeling that having switched from a full 3" system to a 3" Vband out, 3.5" downpipe and along for a few feet, into 4" oval to trans, then 5" out, that my car feels a little laggier...but have also gone up in size within GT3076 turbo range. Seems to be reflected in the dyno as well. It was a shame not to have been able to do a direct comparison with the same turbo when changing exhausts, but not possible.

Duke

On a turbo car the best exhaust is no exhaust. The turbo will always spool up faster with less restriction at the turbine outlet. The theory about better midrange with small exhaust does not work on a turbo engine.

blade7

Yes but when a potential restriction exists ie the downpipe is there much to be gained by significantly increasing the dia of the tube downstream of that downpipe section, particularly on a road setup ?.

Duke

All bends reduces flow and even straight piping reduces flow the longer it is.
So larger is better even after a bottleneck. Exactly how much that can be gained is hard to say.