Cold Air Intake

[P10DET]

Originally posted by Cue-Ball
No, George, i believe you are incorrect.

A certain volume of air is already in the tube. If the tube gets smaller, the air has nowhere to go but through it. In order to fit the same (large) volume of air through a smaller space, the air must speed up. This effect is used often in air intakes.

I'd dig up my book on this subject and quote it, but i'm leaving for California tonight and won't have time. If i remember i'll try to post some references when i get back from CA on the 20th.

I don't think I am. The nozzle of a jet engine is an expanding venturi to speed up the flow IIRC.

Also, you are assuming a fixed volume is flowing through different sized orifices (sp?) at the same rate. I don't believe that is the case with an automotive intake.

I'd be interested in what you find out though.

[P10DET]

On second thought, you may be right.

There *is* a fixed volume passing through the orifice at a fixed rate in the case of an engine.

I guess I'll step back and just say "Ah ain't rightly sure now."

Please post what you find when you get back and enjoy your weekend.

[G-Forces]

It's an interesting problem. On a push effect through a pipe if the diameter gets smaller then yes it will defenitly speed up. That how water cutters and jet engines work. Even turbos and superchargers to a degree.

However the engine alone is a pull effect it will attempt to injest as much air as it can. Making the intake pipe to big might not effect the efficiency as much as making the pipe too small and effectively chocking off the air supply.

Imagine sucking air through straws of different shapes and sizes. Some will be easier to suck through and some will not. But none of them will put more air into your system than you can handle. That's what turbo and supercharging systems do.

[T4 Primera]

The velocity will increase in the smaller pipe to maintain the mass flowrate (not volumetric flowrate) which is the same on both sides. This will be accompanied by a pressure drop and temperature drop in the smaller pipe but that's another topic.

Geo, the divergent nozzles you have seen are most likely on supersonic aircraft. They are actually convergent-divergent nozzles (although they look just divergent from the outside) which are needed to produce supersonic exit gas velocities and also to cater for afterburner peculiarities. You'll find that most subsonic jet aircraft have convergent nozzles.

[G-Forces]

Originally posted by T4 Primera
The velocity will increase in the smaller pipe to maintain the mass flowrate (not volumetric flowrate) which is the same on both sides.

Ahh, right if talking specifically about velocity. I guess I kind of got off track. I still maintain the plenum will cancel any velocity benefits of a reducing diameter pipe.

[T4 Primera]

A tuned intake system works just like a tuned exhaust sytem in reverse. And just like an exhaust, a pipe to large can affect low end torque because the velocity is not there to help produce pressure pulses at the intake valve. OTOH, a pipe too small can choke things at higher revs.

Ever noticed how the plenum tapers down away from the throttle body?
This is because as each cylinder removes part of the air stream, the mass flowrate changes at that part of the plenum. Therefore the plenum x-sectional area is reduced to compensate and ensure that the cylinders all still get the same velocity and pressure appearing at the intake runners.

On a sidenote. My Mums MX-6 has VRIS. It's a 3-stage variable resonance intake system which changes the volume of the intake plenum at certain rpm. As a result, you get 3 torque peaks instead of 1 like a VE engine ... these are mostly installed on V configured engines but there must be a way to ..........

I think I'm gonna do fluid dynamics and acoustics papers this year - I want the ability to calculate all this stuff

[g20infiniti91]

i know what you mean i need something for my infiniti g20 91 i want it to go fast but theres not much to do right now

[G-Forces]

Originally posted by g20infiniti91
i know what you mean i need something for my infiniti g20 91 i want it to go fast but theres not much to do right now

You mean there's not much you can do to it, because of money, time, etc. Or there isn't much you can get for your G20? There's a tonne of good stuff out there for your car.

[G20-KIWI]

Ok firstly I'm quite sure air increases in velocity when the diameter decreases. A mate of mine has an intake going from like 4" to about 2" something and it resulted in him overboosting but it worked really well once tuned (mitsi 4g63-t).

Question: The factory cast airfilter adapter with the MAS that like restricts the air temporalily then increases in diameter, has anyone experimented by smoothing the roughness of the metal or can you machine it out????? Just curious because it seems quite restrictive and I cant afford to get an aftermarket computer to eliminate the MAS.

[ViEt-G20]

george was right.. velocity decreases when you go from larger to smaller...
just think about it.. when you have a cone and pour water into it slow it goes out fast.. but if u fill teh cone up all the way and then let it run out.. it goes slow... we had a discussion about this in calculas the other day.

[jasestu]

Originally posted by ViEt-G20
george was right.. velocity decreases when you go from larger to smaller...
just think about it.. when you have a cone and pour water into it slow it goes out fast.. but if u fill teh cone up all the way and then let it run out.. it goes slow... we had a discussion about this in calculas the other day.

Hate to break it to you, but calculus is not engineering, and the conclusion of your discussion is wrong. If you have a set flowrate of an incompressible liquid (like water) going through a pipe, then as the diameter decreases the velocity will increase, I promise you. Of course pumping water through a pipe is a different situation to a motor sucking air (which can be compressed) through a pipe, so I'll refrain from speculating on that... Actually, no I wont - the velocity will increase as the diameter decreases, there will be limits, but I'm fairy certain that it will occur.