Air induction systems primer.

[techno550]

There seems to be LOTS of confusion/misunderstanding of how the intake systems on cars work. So instead of continuing to post things scattered in various threads, it'll all go here (hopefully).

Like most things, bigger isn't always better. Yet we see people try to put the biggest exhaust, largest intake, and widest tires on the car they can manage to fit. I blame a combination of marketing and stupidity for this, but perhaps it can be fixed.

This thread will focus on the basics of intake design BEFORE the throttle body. I think I've covered intake manifold design here on BF.C before. I'll make some reference to the intake manifold design here as well, but I'll leave it as something that BMW designed and nobody here will bother to change. If there are any of questions on its design, I'd be happy to start another thread for that as well.

So, onward. The intake manifold is designed to take advantage of resonances that occur naturally in the intake. These resonances are caused by the drawing of air into the cylinders. As the intake valves open/close, they create pressure waves. The mass of the air in the intake is accelerated torward the intake valves on the intake stroke, and when the intake valve closes, that mass + velocity has momentum. This creates a pressure wave that reflects off the closed valve and back out of the intake runner. The harnessing of this pressure wave, and tuning of its frequency, allows the engineers behind the intake manifold to take advantage of it. The reflection of that pressure wave off the back of the intake manifold, combined with other pressure waves from the other cylinders in harmony, will create a pressure wave torwards the intake valve that will arrive just as it opens at certain RPM's. This lets the pressure wave assist with the filling of the cylinder, effectively cramming the air in, instead of just the piston trying to suck in air.

These intake resonance pulses don't just stay confined to the intake though. With the throttle body open, they travel outward through the intake pipes until they find something else to reflect off of. Usually they work their way through the MAF and find themselves bouncing off of the air filter. From this, you can see that changing the length, diameter, or shape of the pipe between the TB and the air fliter will change this frequency. That in itself shouldn't be a problem, but usually these intake resonances can cause slight issues. Most common of which is giving bad readings from the MAF sensor.The MAF is counting on air flowing in only one direction over its sensor, and reversions of the air will greatly confuse its reading. It will be seeing more air "going into the engine" even though that air is actually going away from the engine. This means that small reversions, which should be subtracted from the "air into the engine" number are actually added. The ECU uses this value for fueling, and without knowing why, gets it wrong.

This is where resonance control comes into play. Enter the Helmholtz resonator. The helmholtz resonator is there to attenuate the pulses that are outside of the intake plenum. A helmholtz resonator is a cavity and a duct connected to the intake pipe. Its the triangular thing hanging off the air box on the E36's. Its the gonzo nose looking thing on subarus and the likes. etc... Its the "random empty box" hanging off the intake pipe that most people look at and go "wtf?"
Yes, it has a purpose.

The diameter required for the intake duct is also an easily calculated bit. Its generally accepted that you want the intake velocities to not exceed 180 ft/sec. I'd be willing to bet that the factory was pretty damn spot on with the sizing of that stuff. So I'd be willing to "randomly guess" that somewhere around 3" is the right size for the intake pipes.

Next bit, Notice how straight and short the tract is from the stock airbox to the MAF? thats intentional. As is the air horn (velocity stack) inside the airbox. A bent piece of tube, even a mandrel bent one, will create turbulence. There is an exception to that though, and thats if you can get the air to swirl through the pipe, completing at least one full revolution through the bend. If one full revolution is completed, then all the air in the pipe has traveled the same distance and there is nothing to create a funk with the flow.

Hopefully this will prompt someone to actually "design" a proper intake pipe, as well as cut down on the random un-engineering of the intake system.

[delgadoduvidoso]

If one full revolution is completed, then all the air in the pipe has traveled the same distance and there is nothing to create a funk with the flow.

You're saying the funky flow is a bad thing?

[G. P. Burdell]

A helmholtz resonator is a cavity and a duct connected to the intake pipe. Its the triangular thing hanging off the air box on the E36's.

I always wondered what that was for.

Fascinating. Thanks for taking the time to write it!

[ettsn]

Excellent post, Mike.

I'm interested to see how quickly a well-written informative post like this falls off the first page to make room for more posts about 19" wheels and crank pulley threads.

-Paul

[M3Alpine99]

so no one makes a good intake right now your saying?

[techno550]

so no one makes a good intake right now your saying?

- Most seem to stop at "big pipe = more flow".
- Not one has any process for controlling intake resonance.

The general trend for all seems to be a combination of the above. "its a big pipe, so it flows more. Oh, and it SOUNDS cool. Its WAY LOUDER."

[ROBBOTHEHOTTIE]

so in your opinion... whats the best intake system??! from reading your post i think ill be putting the stock air box back on..

[beatniks325]

informative post, thanks good read.

it really seems like the stock airbox isn't "perfect" yet either because they designed it to dampen the intake noise right?

[ettsn]

Techno, perhaps you should start designing parts and sell the designs to these manufacturers. Or just start making them yourself. The dyno doesn't lie. If you make a better part, people will buy them.

-Paul

[badmonkey]

so no one makes a good intake right now your saying?

The parts are out there, if you know where to shop...



....

[seven217]

The parts are out there, if you know where to shop...



....

...and another clue is what I need. So that extra noise is most likely turbulance? Or is it noise from the filter? Can you recommend an intake system that is a better design than others? Great write-up!

[Mr.M]

- Most seem to stop at "big pipe = more flow".
- Not one has any process for controlling intake resonance.

The general trend for all seems to be a combination of the above. "its a big pipe, so it flows more. Oh, and it SOUNDS cool. Its WAY LOUDER."

+1

[Balthazarr]

+1

LOL

I've been thinking about going back to the stock airbox even with the 24lb injectors.

[ben1996328i]

always wondered what that was for.

so the velocity stack in the stock air box is a good thing? (i took it out but have the stock box, with like the half that faces the front cut off. got bored.)

umm i would love to engineer stuff like this

[Balthazarr]

always wondered what that was for.

so the velocity stack in the stock air box is a good thing? (i took it out but have the stock box, with like the half that faces the front cut off. got bored.)

umm i would love to engineer stuff like this

The Hemholtz chamber is that odd triangular projection that looks like a handle.

[CRYPTiC]

Next bit, Notice how straight and short the tract is from the stock airbox to the MAF? thats intentional. As is the air horn (velocity stack) inside the airbox. A bent piece of tube, even a mandrel bent one, will create turbulence. There is an exception to that though, and thats if you can get the air to swirl through the pipe, completing at least one full revolution through the bend. If one full revolution is completed, then all the air in the pipe has traveled the same distance and there is nothing to create a funk with the flow.

So you're saying that a Green Gizmo or Tornado might actually help a a CAI?

[elvirb87]

nothing is good for the car ha?

[xjeeper]

The dyno doesn't lie. If you make a better part, people will buy them.

.

[s50///mpower]

techno550, so would short ram or long tube be better? I heard some ware that the longer tube will allow more turbulence to build up, don't know if this the good turbulence or the bad though

[Concentric190]

Thanks Mike, good read

[techno550]

techno550, so would short ram or long tube be better? I heard some ware that the longer tube will allow more turbulence to build up, don't know if this the good turbulence or the bad though

the right length tube is best. The usual *compromise* between the long and short tube intakes is the source of the air. long tubes try to get air from behind the front bumper/foglight. This is usually dirty air though. The short tubes sit in the engine bay and get whatever washes off of the radiator, which is usually fairly warm.

i find it hard to believe that leftover air that didn't make its way to the combustion camber will come all the way back out to the maf. wouldn't the new air rushing into manifold collide with this air trying to escape and inefficiently(a lot of turbulence) but push the old air back in the manifold none the less. because some of the mass of the old air has already entered into combustion camber then the new air should have more mass and should push the remaining air heading toward maf back into manifold.

thanks mike for your time

The air from in front of the combustion chamber won't make it all the way out, but that pressure pulse can.

watch the fuel cloud in videos like http://www.youtube.com/watch?v=o_QyUD6V5_I
You can get a sense of the scale of reversal of flow in the intake manifold.

[M3Alpine99]

McCoy said my Dinan Long Tube Intake was supreme Ruler...

[328iJunkie]

interesting read. You never said which CAI/or intake for that matter would you use?

[techno550]

interesting read. You never said which CAI/or intake for that matter would you use?

On my NA cars, they've had the stock airboxes in place. I've drawn up some basic tube based intake systems, and done some basic analysis, but the gains are still quite small. Much of it being very reliant on the filter, and nothing beats a good large paper panel filter.
On the M3, a bigger panel filter would be my choice if I were to design an intake from scratch.

I've had a number of people ask about your typical tube type though, so I might throw some time that way. Need to pick a proper filter element though.

[M3Alpine99]

On my NA cars, they've had the stock airboxes in place. I've drawn up some basic tube based intake systems, and done some basic analysis, but the gains are still quite small. Much of it being very reliant on the filter, and nothing beats a good large paper panel filter.
On the M3, a bigger panel filter would be my choice if I were to design an intake from scratch.

I've had a number of people ask about your typical tube type though, so I might throw some time that way. Need to pick a proper filter element though.

Mike,

I thought we decided I had the Ultimate Creation of god for Intakes?