right now I have a mp t70 at 20 pounds of boost. I have a 3 in down pipe that goes through the k member and dumps behind the front tire no muffler nothing. would i gain anything by upgrading to a 4 in down pipe. car is a stock engine 04 cobra

 

you will almost always gain some by increasing exhaust size on a turbo engine

 

Actually you will be surprised how little you will gain from that. I have a friend with a Nova with a 406cu SBC with a single 88 on it. He had a 5" downpipe and exhaust to the rear axle. He got tired of the noise and ground clearance so went down to a 4" down pipe and exhaust to the rear axle. He lost nothing but noise. The car got quieter and ran exactly the same. It did not drop a tenth. Just some food for thought.

 

You should definetly gain some power by increasing your downpipe to larger than 3", as long as you tune accordingly. A single 4" pipe will flow about the same as a pair of 3" pipes. Cheers.

 

04red, I saw in one of your previous posts that you ran a 9.9 at 143 mph. At that power level a single 3" downpipe is certainly a restriction, but at least your pipe length is short. You're probably close to maxing out that turbo as well. Cheers.

 

First question would be ----what is the exhaust housing outlet size? Can't see any gain going larger than the housing diameter as that will be the bottle neck.

 

the exhaust housing outlet is 3 in also

 

A 5ft section of 3" pipe has more restriction than a 6" section of 3" pipe. The exhaust gas is cooling as it goes down the pipe which will add into the equation, but personally if I was trying to get to the back of the car I would have as big of pipe as I could.

 

True, but he said it dumps behind the front tire. Will there really be a gain in stepping the pipe from 3"-4"-5" ??
I totally agree if it is running the length of the car.

 

I wouldn't think so. My reply was more relating to a car that is going to the back. He might end up with 2ft of pipe, and that short I probably wouldn't worry about going any bigger than 3".

 

bigger pipe will help u spool faster too........

 

i have a gt-42(off a 60 series detroit)on a sbc 355 and i ran 3.5 from the turbo to the rearend and dumped it. it worked way better than the 3 inch from the turbo to the rearend. but the turbo had a 4 inch housing.