I believe I said something earlier in the thread regarding making the quickest and smoothest transition to atmosphere. Sounds like you agree with that, then, right?

 

well, to be fair, the V8 had two four inch pipes. if it were a 5.0, then it's 4" for 2.5L I guess.

but as far as corky's comments about exhaust diameter, i simply disagree. I just don't have the means to test myself and prove it one way or the other.

matt

 

Brian, yes I do, if possible/legal. We have to have cats and mufflers, so you have to make do. My original comment was "bigger is not necessarily better". I stand by that. I have Corky to back me up too.

y8s - You are openly disagreeing with Corky's findings? :gay: More fool you!

That V8 would be bigger than 5L I think. But if it was, that is 4" for 2.5L. 1.6/2.5 = .64. 4" x .64 = 2.56". So your own example says you are oversized... hehehe.

By the way guys, I'm not having a go or trying to **** anyone off, I'm trying to help the guy who asked the question, and maybe learn some stuff myself.

 

You might be in error with that statement when you say "findings." I'm not sure, so I say "might." Please post his findings. Not his opinion. His findings. I'm very interested in seeing this proof. I won't be upset if I have to fab a 2.5" exhaust instead of a 3.0"..

Respect to that attitude. Personally though I wouldn't start calling people fools just yet.

Have a good one.

 

T25, again, you're going against the accepted knowledge here, so I'd ask you to prove to us why you wouldn't want the largest pressure differential across the turbine that you could possibly achieve. This is how a turbo spools, so explain to us why you wouldn't want that.

The larger the diameter post-turbine, the more easily a turbo will spool. If that isn't so, please tell us why.

 

In all the turbo documentation i've read, a cone 6" in length that tapers from the turbine outlet diameter out at an even transition is the best possible setup for a turbo. People see gains going from 2.5-3.0 exhaust even if the math doesn't seem right, gains are gains.

 

That's the thing, though, the math seems right...again, you want the largest pressure diff. across the turbine you can have. The way to do this is with a big exhaust.

Is there a point at which you won't see any more gains? Sure. But you will see no downside or lost performance from going that large, either.

 

people on the supra forums are going with 4-5 inch downpipes... if there was no point for them people wouldnt be doing it. I understand the supra is a 3 liter so ya 4-5 seems normal. All of those people are running huge single turbos BTW. Im a big supra guy so I visit there forums often. I have a feeling 2.5 inch will be too small for some reason.

 

According to Corky - a 3" downpipe on a Greddy turbo will be worthless. A tiny little turbo and a huge exhaust??? The 2.5" will work better. Now if you had a larger turbo (i.e. GT2560, GT2860) a 3" downpipe would be worth the time and money.
Stephanie

 

 

I don't think a 3.0" downpipe would be worthless, but you may have reached the point of diminishing returns at that size.

Of course the gains will be less going from a 2.5" to 3.0" than they will be from going 1 7/8" to 2.5".

 

The only person here who has first-hand experience with a 3" DP on a Greddy is WOT. To me, that
means his opinion > *

 

Let's say you didn't care about having a legal exhaust or just planned to use the car at race tracks or something. Theoretically, if you had a 'downpipe' that was 3" diameter that came off the turbo, went straight down and back for a foot (needs room for a o2 sensor) and then dumped straight out right by the transmission, you would get great gains over a full exhaust. Yes?
-Ryan

 

Yes, there would certainly be gains, although perhaps not "great," depending on how good the exhaust being used for comparison is.

 

Not to diminish any one's opinion - but Corky has a point. You have a turbine outlet that is 2.25". You want to mate it up to a 3" flange? Bad idea, bad joint, bad engineering.

If someone has there heart set on a 3" downpipe - make the turbine outlet flange 2.25", and every 10" swage the tube size up 1/2".
Stephanie

 

Not a bad idea, Stephanie!

 

better idea:

 

But wait, whats this?

http://forum.miata.net/vb/showpost.p...18&postcount=3

Oh look, the need for 3" downpipe? Horse's mouth. Enjoy

 

must be some weird effect at 299 rwhp that makes the gas flow differently

 

Baloney (or bologna, if you prefer).