Intake pipe selection: what do you think?

[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!

[Daox]

I mostly agree with your assessment. I would however keep the intake pipe long. I'm not sure how long yours is, but I don't think I'd shorten it.

[California98Civic]

Originally Posted by Daox

I mostly agree with your assessment. I would however keep the intake pipe long. I'm not sure how long yours is, but I don't think I'd shorten it.

The WAI is about 17-18" of smaller-bore pipe, and the stock was more like 29-30". If the WAI is smaller bore, might it make up for the shorter length? Or are these dimensions not related in that way?

[PaleMelanesian]

Resonance is entirely dependent on length and not related to diameter. I'm thinking of how a brass wind instrument works. There are student models with a narrow bore (easier to play) and professional models with a large bore (requires stronger lungs but makes better and louder tone). The both make the same pitch, but the volume and quality of that note are different. I think you want the lower frequency here, which would require a longer pipe. If it gets too long you get restriction so you'll want to go fatter as well. I doubt you're going to run into that limitation here, though.

[Daox]

I was going to say if I was going to do it, I'd want to be around 32".

[DonR]

I'm a little confused, are you talking about making your own intake manifold?

The intake runners they are referring to start at the back side of the valve & are measured to a point where a pressure wave would bounce off, or when the pipe opens up into the plenum, not sure which. Both of which occur downstream of the throttle plate. All the stuff from the air filter to the throttle plate really does is quiet down the intake of air.

It is possible to use first or second harmonic waves in an intake. Sometimes you can hit two usable harmonics with the same runners, just at different RPM's. If you time an exhaust harmonic in between them you can get a pretty flat torque curve as well.

An extreme example of long runners is the chrysler cross ram intake manifold

[California98Civic]

Originally Posted by DonR

I'm a little confused, are you talking about making your own intake manifold?

The intake runners they are referring to start at the back side of the valve & are measured to a point where a pressure wave would bounce off, or when the pipe opens up into the plenum, not sure which. Both of which occur downstream of the throttle plate. All the stuff from the air filter to the throttle plate really does is quiet down the intake of air.

It is possible to use first or second harmonic waves in an intake. Sometimes you can hit two usable harmonics with the same runners, just at different RPM's. If you time an exhaust harmonic in between them you can get a pretty flat torque curve as well.

An extreme example of long runners is the chrysler cross ram intake manifold

I love this forum for exactly this kind of schooling. Can't express the gratitude. No, not my own intake manifold, just the pre filter-housing intake tube is all I mean. Would this mean in your opinion that the length and diameter of the pre-housing tube is not very significant, or just that this article is not relevant to my issue and that I have misunderstood it? (BTW, also seems like there is a mod possibility in your observation about the post throttle intake pipe.)