Ideal downpipe size?

[sly91sc]

Sorry if this is a newby question. I did a search and didn't see any threads that talked about anything but a 3" downpipe.

I've got a Garrett, Stealth 70 (68mm compressor) with a Turbonetics .96 exhaust housing and I'm fabricating a complete setup on a currently non-turbocharged car. Because I'm fabricating the whole setup, there are really no limitations on the size of downpipe I can run. What's the ideal size for a downpipe? 3" seems a bit small to me but 4" would be a bit harder to work with if it was overkill. Is 3" enough for 600whp? or would 3.5" be better?

Dan

 

[RUQWKNF]

Depending on drivetrain losses, 600 rwhp is going to put you somewhere in the 680-700 flywheel hp range. At that power level, I highly recommend you go with at least a 3.5" downpipe for reduced back pressure and an easier breathing motor. I used to always tell people that a 3" downpipe will work fine down into the 10.00s. I know this for a fact because my old '86 was a 10 flat car and only had a 3" downpipe with a Tial 38mm wg and dual 3" cat back exhaust. I'm sure I would have picked up a good bit if I'd have switched over to a 3.5" downpipe as my flywheel hp level was around 760-775 based on car weight with driver and mph it was running.

I would say once you get into the 650 rear wheel hp level, (730-740 flywheel hp level) it's time to think about a 3.5" downpipe. Until you get there, you are wasting your money. Keep it simple. You will see more gains with a more freer flowing exhaust system than you would going to a 3.5" downpipe at the 550-600 flywheel hp level. Think straight through mufflers and 3" piping.
Just my O2's worth.

Patrick

 

[Boost231]

3.5 will work good for what you are trying to do.

 

[sly91sc]

Thanks for the input. I've currently got a dual 2.5" mandrel bent full exhaust and I was planning to connect the downpipe to a reverse y-pipe and go into that. So I'll need to find a source for 3.5" mandrel bent steel tubing.
I see 3" and 4" on ebay but no 3.5". I'll need two 45s and a 90 to get me to my exhaust setup.

Dan

 

[Boost231]

go with 4" if you can get it to fit.

 

[bison]

go with 4" if you can get it to fit.

A 3.5" is plenty for 800 hp

 

[JZ 97 SS 1500]

Sorry if this is a newby question. I did a search and didn't see any threads that talked about anything but a 3" downpipe.

I've got a Garrett, Stealth 70 (68mm compressor) with a Turbonetics .96 exhaust housing and I'm fabricating a complete setup on a currently non-turbocharged car. Because I'm fabricating the whole setup, there are really no limitations on the size of downpipe I can run. What's the ideal size for a downpipe? 3" seems a bit small to me but 4" would be a bit harder to work with if it was overkill. Is 3" enough for 600whp? or would 3.5" be better?

Dan

What turbine wheel is in the unit?

If its a p-trim, based on the turbine map, you would need ~ 3.175" exhaust to keep the back pressure at its lowest point to at around 650rwhp.