Sep 9, 2009
Hi all

Ive read a thread that states the best exhaust exhaust is straight through and less bends to max the air flow. My question is I'm going for a map soon and wonder should I keep my existing border 304, 2.25 straight through pipe (no cat and no mid box and goes under the axle and all slip fit) or go for a 2.5 system? Other question is Does the 2.25 compensate 2.5 without having the mid box and bends.

I have a b18cr with a 5zigen 4-2-1, 2.5 collector manifold.
silencers if they are straight through design wont claim power only weight.
less bends will help tho...
but the most important is the exhaust size/diameter !
less backpressure = more flow = more power !
in your case i would choose at least a 2.75'' or even bigger exhaust system this will let you room for future tuning.
So get rid of the border 304? Been wanting one for so long. Not much 2.75 about unless custom job.

Would it work if I chop the 2.25 pipe from manifold onward all the way to the back box ?
If your exhaust manifold has a 2.5 collector, you want to stay at 2.5, or bigger- the idea is never to "choke"down the exhaust piping size as the exhaust gasses flow though.
IMO, 3 inch on a naturally aspirated 1.8 is a bit big and tends to loose some exhaust velocity. There is a certain vacuum in the tail pipe that is properly tuned that can pull fresh air into the combustion chambers, aka "scavenging effect". Turbo cars need 3 inch or bigger however.
My favorite exhaust I had for a 1.8 B18C, was a 2.5 from the headers back through the straight portion ("b" pipe), that went into 2.75 custom mandrel bends ("c" pipe) into the muffler. Using the theory that air slows down when taking turns, the larger diameter bends near the rear axle compensated for lack of straightness.
Good luck on your map!