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      07-28-2012, 06:47 PM   #12
Brigadier General

Drives: 130i
Join Date: Nov 2011
Location: QLD, Australia

iTrader: (0)

Originally Posted by NYC6
Originally Posted by flinchy View Post
the difference between being .25" over is still better for power than being .25" under.

the biggest issue is back pressure, you ideally want none, no leftover gases in the cylinder chamber.

regardless of the turbo being able to push in tons of air, it doesn't PUSH out burnt exhaust gases (being the exhaust valve is open when the intake is closed, mostly)
"2.75 or even 3" full catback would show absolutely immense gains"
Your talking up to 1/2" increase and you're talking about immense gains. It aint gonna happen.
BMW is certainly adept at fine balancing its N/A engines. The are damn near close to optimum balance of intake and exhaust to get the most power out of its engines. Going larger in tubing size isnt going to help, it will hinder low end tq certainly and likely hp. IMO.

OK. Let us know how it works out for ya. Dont forget the before after dynos.
Go check out e90post, then come back to me about it, they all show plenty of gains with bigger piping, especially since stock is a bit under 2.5"

I mean, hondas (like my old prelude) are a 2.2L 200-220hp and they get large gains going up to a 2.5" or 2.75" lol

Most exhaust mods are only a 0.5" increase and that's enough for many performance cars to gain 10 or 20 hp

And i've got a 'before' dyno in about a week or two, and i'd say an after about two after that