At WOT, such as at the dragstrip, open headers beats all.I think a lot of people confuse back pressure with Scavenging.
2.5" is recommend to about 650rwhp by many companies. Of course everything varies and depending how far past that your power will determine your gains.
I use to have tons of info on this but over the years I have lost it. I know some of the bigger power guys gained a lot at the track when swapping from 2.5" to full 3". I am sure somewhere I have the info on my car before and after my switch.
If a full exhaust is needed, such as on a street car, or even a road course, then exhaust scavenging becomes extremely important. You want your exhaust to be tuned for optimum performance throughout throttle manipulation.
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