I hope this article clears up a few things for people that think that CAI is a great mod and/or are wondering why cars don't come stock with such air intake systems. The guy that wrote this article is an engineer and knows what he's talking about. He posts on bimmerforums.


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. __________________

 

Nice find.

 

Exactly why the V2 see's bigger gains than other intakes...Also, i understand that it may seem like using a club instead of a knife (reverse engineering), but sometimes a club still gets the job done better. I mean, you can't argue with comparative dyno's that say that many brands of intakes give gains.

 

you may see gains, BUT a standard paper filter does a better job at filtering. Also, you will not see a gain in all RPM ranges. You may get a bump in HP in the top range, but sacrifice some in the lower range. The point is, when you install a mod - there's always some sort of a sacrifice.

 

V2 has increase over stock at ALL rpm's just take a look at the dyno. But yes, standard cai's will see a hump and decrease, the just generally see it earlier in the powerband where you can hardly notice anyways.

 

i'd take a comptech airbox over a V2 anyday. too bad comptech went out of business. lol.

 

I'm not saying it's the best, just that it's good, works on that principle, and sees gains AT ALL RPM.

 

may be that is because AEM is one of the few companies that have engineers on staff designing their products.