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The number of mufflers, the size and length of the exhaust and the flow are very significant issues when it come to serious tuning.

What exhaust tubes are you using? I first had a 50mm tube with 2 mufflers which i found very good for torque! When i swapped the intake and exhaust manifold after a short time i increased the piping to 55mm. The engine responded great. Power throughout the powerband.

I got 55 mm since 2002. We all know that I got enough power with that. Never changed for smaller. Have seen 50mm systems having a hard time getting to 100 bhp/litre, performance over 7000 rpm seemed bad.

55 means 55mm in the Exhaust Header also right?

Hmmmm.... think header actually had a smallest 48mm part. Not sure if its only local at the cat flange or entire tube... at least original gasket can be used, which is 50mm inner diameter if I am correct. So tubing is < 50 there. Next time I place cat I try to see.

ok, but my Cat is far from the engine , not at the header as some car models just to clarify. Yours also right?

Correct. It's not on the exhaust manifold like d15z6 (ek3).

It all depends on what header you're using .Look at Bisimoto's STock D15 engine with 2.5" exhaust and cold air.He made 16WHp gain with his custom v2 header and a lot of torque gains too .And that's with a 2.5 exhaust!
I'm using 2.5" all the way ...

Just 2.5 inch all the way hehe... to me that sounds big man .
That is what's under my B16B.

Dodo Bizar wrote:Just 2.5 inch all the way hehe... to me that sounds big man .
That is what's under my B16B.

It is Big for a little engine but like i allready said if you are using a Great header it extracts a great amount of gas out of your cylinder and you def can use that 2.5" exhaust.If your using a general 4-2-1 exhaust you'll be better of with 55mm or so.