I kno the principle... Having a 2.5" full catback exhaust is going to have more back pressure than a 3" full catback exhaust due to the smaller diameter... But can you tell me how that effects your performance... Whats better?

 

there is something called a scavaging effect, on any exhaust you want the exhaust gas to travel the fastest velocity it can, if you are turbo the exhaust gas is travelling extremely fast so you need a large diameter exhaust to allow the velocity to keep up. Now if you kept that same size exhaust and put it on a NA motor the velocity would slow down because it will have more volume per section of exhaust effectively slowing the exhaust gas down, the same thing happens when you run to small of a pipe. The exhaust cant travel at its maximum velocity creating a bottle neck. I hope this helps

 

Exhaust back pressure is created when exhaust gases are trying to escape through too small of an exhaust pipe, and the exhaust flow actually slows down as a result.

There are two parts to air flow, flow, and velocity. Flow is how much air is being pumped out, and velocity is how fast it is being pumped out. The two work in conjunction with eachother to make power. You don't want a pipe that is too large because the velocity won't be that high, and you don't want a pipe that is too small because the flow will be too low.

Usually turbocharged engines or engines that rev high need larger exhausts. Turbochargers because air is actually being shoved out of the exhaust by the exhaust turbine, and hi-revving engines because the higher the RPM, the higher the exhaust output is.

A smaller exhaust will feel good at low RPM's because the flow and velocity will be optimal for the amount of air being pumped out of the motor at that RPM. With a large exhaust, you will feel a drop in power at low RPM because the exhaust isnt able to escape fast enough. At high RPM's, the effect is reversed. The exhaust gases will experience a bottlenecking effect at higher RPM's, whereas with the large exhaust the flow and velocity will be optimal at the upper revs and you will make more power.

You want to look for a pipe diameter that will optimize low RPM and high RPM power...which for a naturally aspirated honda motor is between 2.25" and 2.5"

Hopefully this has explained the concept to you...

 

well i tried to explain it lol

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Soccerking3000 &raquo;</TD></TR><TR><TD CLASS="quote">well i tried to explain it lol</TD></TR></TABLE>

Yours was good but I wanted to expand on it

 

Yep that was a good explaination Bauley. Basically backpressure is associated with restriction. A little restriction will help increase velocity sort of like placing your thumb over the end of a garden hose and watching the water shoot out instead of pour out. In a boosted engine, velocity is essentially artificially maintained by the mechanical air pump (turbo or SC) so flow is the main consideration when looking at exhausts and bigger tend to be better. In a non boosted engine, velocity must be maintained by many things to make good power but the exhaust system is very important and must be sized correctly and is usually a compromise between low and high end power.