Twin76mmTA

iTrader: (1)

I know the formula to figure out the CFm that can be flowed through a 4" pipe but since i don't know the velocity of the exhaust exiting the turbo i can't figure it out.

So as a rule of thumb how much HP can a 4" DP support?

smokinHawk

iTrader: (48)

allot

probably around 1400-1500rwhp, maybe more but backpressure will increase

EspaceF1Fan

Will you have one or two 4-inch downpipes?
Whats your power goal?
Also, keep in mind alot of 4 banger guys find 4 inch downpipes to add some power even at a lowly 450-500hp(thats the point they see 3inch as a restriction), so maybe a 5 inch is in your future? I'm sure a 4 inch has to support up 700-800-ish atleast.

RealQuick

iTrader: (14)

3" has supported 1000rwhp depending on cubic inch.

jason99frc

iTrader: (12)

In a regular exhaust application a single 4" exhaust is good for 675 FWHP before a restriction in noticed. I would expect the same or a little higher for a Downpipe on a turbo setup so 1300 - 1400 ish should be on target.

Slowhawk

iTrader: (12)

It also matter's how long the pipe is too.I've seen people run low 7's with a single 4" pipe but they ussually go 5-6" around then.

EspaceF1Fan

On a SINGLE 3 inch pipe? If so they were really hurting themselves...

RealQuick

iTrader: (14)

Not saying its optimal, but it has been done. The mustang guys 306ci-347ci have pushed the single 3" pipe.

EspaceF1Fan

Gotcha...