What is the best exhaust size for an all motor VTEC car. I was under the impression that bigger is <u>not</u> always better. I thought 3in. would be too large, and wouldnl't have enough back preesure.

TIA

-Will

 

No, back pressure never good. Scavenging is. Pipe size is usually determined by engine displacement for NA and HP range for FI. Proper size creates proper vacuum affect in exhaust.

 

No like i said bigger is not better correct? and that is because it doesn't have the proper vaccum? For example a 200 whp civic doesnt need a 3in. exhaust.

Just making sure I am gettin all the info straight. Thanks for the help.

 

I did a lot of research before building my exhaust. I found the following rules of thumb (which I hope I remember correctly):

1.6L SOHC - 2"

1.6L DOHC - 2.25"

1.8L - 2.5"

2.0L - 2.75"

extensive head work- add .25" to above

NOX - add 1/2" to above (I don't know why this is)

TURBO - can never be big enough!

Among the people this advice came from was Oscar Jackson, of Jackson Racing fame. So if it sounds too small, talk to him.

The reason is as another poster said - smaller diameters result in higher exhaust gas velocity, which give you more "vacuum" during the exhaust stroke, better cylinder scavenging, and better power, especially low end torque.

A larger pipe may give you a bit more top end, if the smaller pipe is actually restricting flow, which is unlikely if it is a well designed system, etc. But it will be flat on the low end.

 

rock fish, your 2.0-2.75" is a bit off. You usually won't go 2.75" until about 3.0-up

 

Thanks for your help guys. i appreciate it.