LT1PWRD

iTrader: (2)

How much does a 3" vs 4" down pipe gain or loose?
I'm fabbing my down pipe and wondering if 4" will be a big benefit vs 3" @ my hp goals.

How much does a 2.5" vs 3" charge pipe gain or loose?
I ordered a 2.5" and was worried if it'll be a restriction vs a 3" at 7-800 fwhp

JoeNova

Either size on either pipe will be fine at 700hp. Go with what's cheapest and most available for you. Honestly with the downpipe I would try to fit a 4" but if fitment is a problem you aren't going to miss your goals with a 3".

Forcefed86

iTrader: (4)

120% of your turbine wheel diameter is the "rule".

LT1PWRD

iTrader: (2)

Ok good. That rule will put me right around 4" with the 3" outlet I have. I should be able to fit a 4 incher down there. Would like to run it to the bumper, may need oval to do that, we'll see when I get to that point.

rkreigh

4 inch downpipe is great for future goals on a single

hard to flow too much after the turbo

LT1PWRD

iTrader: (2)

Good points

Bimmerdude

iTrader: (1)

I saw a nice test once, I wish I remember what company put it together, I'll look for it later (was 4 or 5 years ago on bimmerforums).

Anyway, they tested different downpipe sizes on the same car, same turbo, but tuning for each setup, and found that downpipe and subsequent backpressure followed a number of expected trends:
-Ratio of "drive pressure" to "back pressure" was always fairly fixed (ie 10 psi turbine drive with 2 psi back pressure, with 4 psi back pressure they saw 20 psi, etc.)
-Larger downpipes significantly improved spool - even when the exhaust got smaller after mid-length (exhaust cools and shrinks in volume so no real friction increase)
-There were no downsides other than packaging and sound to going bigger - performance ALWAYS got better.

Charge pipe is a completely different story - you get into a spool vs top end debate on too many levels.