An aftermarket exhaust system simply makes a machine breathe better by allowing more room for the escaping exhaust, right?

When you install an aftermarket exhaust on a fourstroke, any fourstroke, does it require any re-jetting or other adjustments elsewhere?

Are there any special considerations that must be made if you were to ...gulp... build your own custom exhaust system?

The reason I ask is I have an old Honda streetbike motor that I would like to mount in a quad frame. However, it's going to require a total redesign of the exhaust system. I have the tools and know-how for the metal work, I am just concerned about things such as the proper size, length, expansion, silencing, etc...

 

it's the better flow that makes the power, more air in + more air out = more HP.

Whenever you change the air flow in an engine you must rejet, more air & the same amount of fuel as before modifications = BOOM.

On customs if it's a multi cylinder engine you will want to keep all tubes the same length & I would check with an expert on a design for the collector. these are both important for flow & scavenging.

 

Exhaust systems are far more critical on 2 strokes than 4 strokes. Try to keep the header pipe lengths the same, but don't loose too much sleep over it (there are millions of 5.0 Mustangs running around with unequal length headers). Keeping the pipe diameters the same as the OEM or an aftermarket exhaust would be a good idea. Slap a free-flowing muffler on there and then get to work fixing the jetting.