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3 inch versus 3.5 inch downpipe?

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supr91tt

Member
Joined
Jan 5, 2012
Messages
37
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 :)
 
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 .
 
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.
 
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.
 
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
 
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