3 inch versus 3.5 inch downpipe?

[supr91tt]

I've upgraded to champion ported iron heads and intake with 6466 turbo which appears to be flowing much more than the internal wastegate can manage. Since I'm now in need of an external wastegate setup I'm curious about the downpipe and wastegate sizes that I should consider. I see 3 inch DP is most common, but 3.5 inch is also available both with 46mm valve size.
What are the pro's and con's of 3 inch versus 3.5 inch?

TIA for your opinions

 

[1986 Buick GX1]

A quick search found this thread.

J

Thread '3" Downpipe VS 3.5" Downpipe ??'

Dec 11, 2017

any gain moving from a THDP (3") to a TA Performance 3.5" External Gate down pipe??


250 CU Stroker Motor
GN-1 Heads
E85
TA SS/Race Headers & 46MM Gate
6870 3 Bolt DBB Turbo
Pypes 2-1/2 inch exhaust --soon to be replaced with ATR 3" SS Crossflow

• jmendlik
• Replies: 26
• Forum: General Turbo Buick Tech

 

[dynoman]

A 3" dp only becomes a restriction on higher HP cars and if your signature is accurate you won't need a 3.5" until you start making north of 700mhp . We measured exhaust backpressure on my car and downpipe size wasn't an issue until we were well north of 850+hp . Mr Spool can elaborate on this subject and what we saw on my car .
There is NO good Backpressure .

 

[steve hughes]

When my car was running 9.30's with a 3" open down pipe , no exhaust i changed it out for a 3.5" at the track. Two runs with 3" and two with the 3.5" It picked up .05 et and .5 mph in the quarter.

 

[Chuck Leeper]

My tests were dyno based.
274", Champ heads, PTE GTQ70.
3" vs 3.5" = 5whp.
Previous test with 3" and the dump open, back-to-back = 5 whp.

 

[TurboTGuy]

A 3" dp only becomes a restriction on higher HP cars and if your signature is accurate you won't need a 3.5" until you start making north of 700mhp . We measured exhaust backpressure on my car and downpipe size wasn't an issue until we were well north of 850+hp . Mr Spool can elaborate on this subject and what we saw on my car .
There is NO good Backpressure .

That's a lot of MPH..... LOL