I'm getting ready to redo my current exhaust system, mainly to get the mufflers tucked up a little higher behind the rear bumper cover. Since it's going to be redone in stainless and I don't want to do it again, I thought it might be a good time to make sure it was sized correctly.

I've read and read and searched the topic for several months before doing it the first time back in 2004, and I've also been reading what people have been saying about it since then. The problem is...nobody has a formula or a chart to determine what size is needed for what power level, they just say bigger is better or I made this much power with whatever CID, turbo and exhaust size. That info is what I used to size the original exhaust.

I have a dart block 347 with AFR 205 heads, Spyder intake and a PT76GTS turbo that has a 3.5" downpipe (was originally 3") that extends to about the middle of the trans pan. Downpipe is connected to a single 3.5" pipe that runs to the rear of the car and just before the diff (IRS) it Ys off to dual 3" pipes with a pair of 3" Magnaflow non chambered mufflers. This is mainly a street car that has a hydraulic roller cam that limits rpms to around 6800. 99% of the time I'm driving the car on the street at around 14.5-15.5 pounds of boost (aprox 600 rwhp), but ocassionally I do take it to the track just to see what it can do. I'm fairly certian the exhaust size is fine for how I'm using the car on the street, but I'm not sure that it's big enough when I increase the boost to 20-24 psi (aprox 800 rwhp) on the track. Highest boost I've even tried so far is 20 psi (with 3" downpipe), but eventually I'd like to see what it does with the turbo maxed.

I'm pretty much stuck on the size of the downpipe, because there just isn't enough room to go any bigger than 3.5", but it may be possible to use a 4" pipe between the downpipe and the split to dual 3" mufflers. If it's not going to make at least another 40 rwhp then I'd rather stay with the 3.5" pipe for more ground clearance. This post from the FAQ section seems to indicate that a 3.5" single exhaust was large enough to keep pre-turbine back pressure down to an acceptable level with an even larger motor using the same turbo.

https://www.theturboforums.com/smf/index.php?topic=63459.0

In keeping with the "bigger is better" theory, I had thought going progressively larger with the exhaust size between the turbo and the tailpipes was the correct way to go, but now I'm reading where some people actually drop down to a smaller size after exiting the downpipe, so I just don't know what is correct.

I think the dual 3" mufflers are big enough, so the real question is if the 3.5" single pipe between the downpipe and mufflers is large enough for my combo or does it need to be increased to a 4" single coming off the downpipe ? I may also be able to fit dual 3" pipes all the way from the downpipe to the mufflers, but it would be a very tight fit getting past the fuel tank.

David

 

3.5" is plenty. splitting to two 3" is plenty.

 

Thanks for the input.

David

 

David, if it means anything to you to help your decision, i made over 900rwhp through true dual (twin turbo) 3" with borla pro xs mufflers.

Anthony

 

I think you will be fine with the 3.5'' into dual 3''.

 

Good deal...it will be much easier to stay with the 3.5" center pipe.

Thanks, David