[California98Civic]

What do you think? Researching my replacement muffler because it is becoming time to replace the old one, I came across the below on intake pipes. I have been fine-tuning the WAI. The below article makes me think the optimal design would be a bit different than mine, which is a little smaller-bore than stock and about 2/3 the quite long stock length. The summation is this: "install long small bore pipes when you want power and torque at low to medium revs, long large bore pipes for mid-range torque and short pipes when you want performance at higher revs."

The Intake Pipes
Pipe Sizing Calculator

You've read it all before, the intake pipe diameter and length is very important because it affects volumetric efficiency, impinges on pumping losses, mean effective pressures, etc, etc. You have also heard of rarefaction waves, reversion pulses, syphoning, blah, blah, blah. Well guess what we actually incorporated a lot of this stuff in the calculator to make out we know what we are doing. This will also hold truw for the exhaust side of things where we claim a proper sized exhaust will suck harder than the piston itself.

Alrighty heres what we have thrown into the soup to come up with the intake pipe sizer:

Long inlet pipes increase volumetric efficieny at low speed, but decrease VE at high speed (VE is volumetric efficiency);

Mean effective pressure increases proportionally with volumetric efficiency;

The amplitude of pressure waves increase as pipe length increases;

Pressure peaks occuring around the inlet closing occur later as the pipe length increases;

Cylinder vacuum comes on later in the stroke as the pipe length increases, until the number (frequency) of pressure waves per engine cycle changes and it reverts back to the original angle and starts coming on later as the pipe continues to be extended.

Later inlet valve closing is better with long pipes and earlier with short pipes. This is because the arrival of the pressure wave is later with long pipes

Which when translated means you install long small bore pipes when you want power and torque at low to medium revs, long large bore pipes for midrange torque and short pipes when you want performance at higher revs (at the expense of losing some of your low end performance).

So what about all this pressure waves stuff you ask? Well that's simple too, because some else spent a load of some government's money when dinosaurs walked the earth and came up with some observations that you can get a whopping 15%-20% increase in mean effective pressure by just making the pipe a length that encourages a third harmonic based on the speed of sound and engine rpm. And better than that the fourth and fifth harmonic suck too!