todahead

Got a 3'' exhaust on my b18c and tune it on the dyno, car made good power 202whp on a dynapac
but exhaust is very loud cousing problems with the police.(police ears are sensitive in my area)
i'm curently using a big oval rear muffler and a 22'' long middle resonator.
thinking to change the rear mufler with a smaller diameter one 2,5'' to tone things more and adding a 100cel cat.
Just wondering if there will be performance loss running a 3" straightpipe and the muffler is only 2.5" input
since most b series are running 2.5'' exhaust systems.

JRCivic1

There will be many people who will argue with what I am about to tell you... from this and many other forums... but I have DYNO PROVEN these results. Any bolt-on 60mm cat-back will work perfectly (and will not lose any HP or Torque) and be MUCH quieter than your current 3". The key to this is that your exhaust system is built with mandrel bent piping... each corner is the same internal diameter as the straight piping. If you go to a local exhaust shop and ask for a 2.5" exhaust system, what you will likely get is 2.5" straight pipe and crush bent turns which reduce the inside diameter of the pipe in this area to about 2 to 2 1/8" inner diameter. Your exhaust system will not flow any better than the smallest choke point in it... meaning that your 2.5" exhaust is actually 2" !!! This may be too small for the exhaust flow generated by a 200 wheel HP engine, so you may lose power if your exhaust is constructed this way.

Short story: I built a mild K24/20 engine to replace the 371K mile original K20A2 in my RSX-S... and even though I KNOW better, I read so many threads stating that you absolutely have to have 3' exhaust to make maximum power and there will be a significant power loss if you go smaller... I went with a 3" cat-back. With 3" exhaust, my K24/20 made 259 wheel HP and 195 ft/lbs. Torque. It was ridiculously loud... and after six weeks of driving this car daily, I couldn't stand it one moment longer. I was willing to lose 20 wheel HP if necessary just to quiet this car down. A week later, I went to the dyno with an Apexi WS2 60mm cat back in the trunk and did a "Back-to-Back" test. We strapped the car down to the dyno and did a baseline pull with the 3" exhaust still in place. The results were 259/195. We unstrapped the car, pushed it back on the lift, removed the 3" cat-back and installed the Apexi 60mm cat-back. Strapping the car back down 2 hours later, making NO changes to the tune at all, and merely making a pull resulted in a .8 wheel HP loss (yes, 8 tenths of one HP) and a gain of 1 ft/lb of torque !!! ESSENTIALLY, NO CHANGE. These results are within dyno error of 1%. There was a significant loss in power in the midrange around the VTEC crossover which we were able to tune out with altering the VTC settings... and ultimately, we were able to eliminate this power loss and matched the previous HP/TQ values at every RPM. End Result: 2.375" exhaust made the EXACT same power as 3" exhaust at nearly 260 wheel HP. You make way LESS HP, so your results will be no different than mine. Make it quiet... you will be happy that you did... and still have 200+ wheel HP.

Final note: You CANNOT make your exhaust system quieter by bolting a "silencer" into the exhaust tip with a 1 or 1.25" hole in the middle... this type of system WILL reduce engine output.

todahead

Thanks for the reply.
Even if the crush bends lose diemeter still a 3'' piping will be adequate for most builds.
The problem whith the 3'' exhaust is the noise level and only that , i'm at the same route as you since i dd the car now.