2.5" downpipe vs. 3" downpipe

v8killer · Oct 20th, 2002, 10:58 AM

Hey guys,

I recently switched from the older style 5 bolt turbine housing to the newer Garrett 4 bolt turbine housing. With the 5 bolt housing, I was able to run a 3" downpipe right off the turbine outlet flange, using allen bolts for extra clearance.

It appears that with the 4 bolt flange, I will not be able to run 3" pipe right off the flange....as the bolt holes would be in the way of the 3" pipe. So, I am thinking of running 2.5" right off the turbo and then flare it out to 3". So, the downipe would be 2.5" for only about 1.5 feet.

Do you think this will hinder power any or is the short section of smaller piping negligible here because it is so short?

My other option is to use 3" piping off the turbo and heat up and indent the pipe where the bolts need room.

Thanks for your help,

psshhgoesmysr20 · Oct 20th, 2002, 04:59 PM

I believe it will cause a little excess backpressure, the reason being, the pipe is obviously smaller then a 3 inch, no matter how short it is, You can only force so much air into a 2.5 downpipe, it will never be able to flow as much as a 3inch pipe. Basically what I am trying to say is, when you have a 2.5 inch pipe before the 3 inch, you will just be choking the flow. Just IMHO

v8killer · Oct 20th, 2002, 08:16 PM

Okay, here is my thinking on that. The outlet of the turbine (the exit of the turbine wheel) is right around 2.5" in diameter. The distance from the exducer of the turbine wheel to the flange surface is about 1 inch or so. Technically, here we have a 2.5" "pipe" that is about an inch long. Let's install a 3" downpipe now. If your reasoning is correct, then we still have a "restriction" in the exhaust. The restriction is in the turbo itself and not in the downpipe.

I was thinking that the 12" section of 2.5" pipe I want to use is simply an extension of the turbine outlet. If this is not correct, then ideally you would want the turbine outlet to be 3" as well. I have never seen an outlet that big on a street turbo.

How about this: Dyno a car with a 2.5" downpipe and 3" exhaust. Dyno the same car with a 3" downpipe and 3" exhaust. The setup with the 3" downpipe would produce more power, wouldn't it? That's because the 2.5" "restriction" is only as long as the downpipe.

I think that the shorter the restriction is, the less power you will lose.

P.S. I am not looking forward to bashing in the dents for the bolts

But, in the name of having a 3" downpipe, it is worth it!

Thanks for your advice,

javierb14 · Oct 20th, 2002, 10:04 PM

check out this stainless steel 3" DP i made for a turbo'd b-series engine. the pic is kinda blurry but it start outs as a 2.5" flared out to a 3". the transition is only a couple of inches long. it worked really well...

more blurry pics at
http://www.public.asu.edu/~ramirez0/B_series