I want to discuss in this thread people's experiences and/or knowledge on tailpipe and downpipe sizing with turbos. From what I've read, you want as little back pressure as possible aft the turbo. I've read that an exhaust gas velocity of 250ft/sec should be the goal, but most of us can't readily test or calculate that. So I wan't real world dyno/track/butt dyno experiences. I think we can divide it into 3 categories: Front mount single, front mount twins, and rear mount. Each has their own circumstances so please state what kind you're using.

I read over on tech that the peeps at forcedinductions.com suggest going with a tail/down pipe that's 1.25 x exducer bore dia. Does that rule of thumb apply to rear mounts?

So lets say you have a turbo with 3" exducer bore, then you should run a 3.75" downpipe and tailpipe. Well, 3.75" pipes aren't readily available, so you'd have to go with a 4". If a 4" is good then is a 5" even better?

I think there's considerable power to be had by paying attention to your downpipe and tailpipe situation. The more deltaP you can get across the turbine the quiker the spool...right?

 

Well, I used to have the following set up:

3inch Down pipe – no cat – 3inch Y-Pipe with an Electric cut out - 3 inch pipe strait into a Magnaflow Muffler with no baffles in it – split to two 2.5 inch pipes out the back of the truck

Having the Electric Valve gave me two different power curves. According to the ever so accurate Butt Dynometer with the valve closed I got lots of top end power, with the valve open I lots of low end power. I also did not have any Turbo whine through the exhaust unless the valve was wide open…

Now, I have the following set up:

3inch down pipe – no cat - 3 inch pipe strait into a Magnaflow Muffler with no baffles in it – split to two 2.5 inch pipes out the Side in front of the rear wheel

I reused the same muffle from the old set up but, this set eliminated Six feet of pipe and the Exhaust valve, compared to the old set up I got a power increase across the entire RPM range… Oh and the Turbo whine is so loud that it makes Honda’s cry as soon as they hear it!

 

Well since everyone has so much info to give, I think I'm going to go with a 4" downpipe so I can have some room to grow.

 

interesting discussion.....I wish i knew more I did call STS and discuss the idea of a muffler after the turbo and they loved it.....after the muffler is 3 inch mandrel bent tail pipe.

 

I used a 3" dynomax bullet and a " turn down and lost .5psi over the straight 10" long pipe that came after my turbo. My spool time was hurt a little too. Shoulda went 4" It is quiet tho

 

well considering how much power parish gain through a cut out with his setup i would say the bigest you could fit. i'm going 4" when i do mine. and useing my 4" magnaflow desiel muffler.

 

i really dont think you can go too big after the turbo. the shorter and bigger the better. in the real world a 4" dp is about all your going to get in there and it fits prety tight. a 3.5" fits prety well.

i ran a 3" dp with open cut out and then a 3.5" with open cut out on both a 6.0 and 6.7L. all the set up's maxed out in the 600-650rwhp with a t76. the turbo seemed to be the limiting factor more than anything. the larger motor spooled the turbo much faster and had a ton more tq down low.

on my pt88 set up i had a 4"dp to a 4" cut out and 3.5" exhaust thru 2 bullets inline and a dump. all about 6' long. opening the cut out gave me over 50hp.

 

What about a straight through dynomax bullet before the turbo? It lets you hear the turbo sound beter.

 

I'm not really interested in how it sounds. I care more about if it's robbling 50hp from me at the top end. I could probably fit a 5" in there, maybe, due to the turbo location, but it'd be tight.

My thought is that if you go too big, you might slow the exhaust gas too much, to where it'll create a little pressure in the pipe. If that happens, you might lose flow since you'd have less deltaP across the turbine. 5" is awefully big, and 4.5" just seems like an odd and hard to find size.

 

I'm installng a 3.5" electric cut-out to the turbo...nopiping inbetween and a 3" exhaust with muffler out the back. When it's all set-up I plan on doing dyno pulls with the cut out open and with it closed...I'll post up the number difference when it's finished, hopefully a month or so.