Thread: Intake choices, new ideas...

Originally Posted by GeeXPee

All force induction system do is put more air in the same volume of space...CFM will go up (max out based on size) but the engine still can't exceed it's max CFM determined by the cu in size of the engine...that doesn't change. Volumetric efficiency doesn't go up just because the CFM is higher it's also because of the larger mass of air compacted into that same volume.

I'll take pics when I do my intake and let ppl know how it goes.

Thanks for the tips.

What you are saying with regard to the CFM determined by the engine displacement is this: CFM = RPM * Displacement (cu. ft) / 2 ... divided by 2 because 2 rotations to fire all 6 cylinders. However, the density of the charge in a forced-induction system is much higher than the density of the air coming through the air filter. Yes, your volumetric flow rate through the intake ports is going to be the same in an N/A or forced induction car, however it will be much denser air (greater mass) in an S/C car. To raise the density of the air we MUST pull more volume of atmosphere into the engine. Thus, a S/C car does require more volumetric flow pre-supercharger, and there is no limit to that based on engine displacement. The volumetric flow will be determined by the size of your supercharger and its efficiency. It's called conservation of mass. There's no such thing as conservation of volume.

The info from K&N seems bogus and makes me never want to buy a filter from them. Saying a filter "can flow" a certain volume of air makes no sense. The volumetric flow in any fluid system depends on the pressure differential across the restriction. Upgrading to a less-restrictive filter (which, according to them, won't help you) will serve to decrease the pressure difference across your filter, thus raising your vacuum pressure and increasing the efficiency of the supercharger. When they say "our filter flows 6 CFM per square inch" what is that? With what vacuum pressure? With what intake tubing? Bogus.

EDIT: okay, after I went on my rant about K&N I looked into their data and by "it can flow X CFM", he means it can flow X CFM on a flowbench before creating a 1.5"H2O pressure drop. However, I still hope he knows forced induction can flow twice, if not more, the engine displacement through the filter due to the compression. So his "451 CFM max" number is ridiculously bogus as soon as you get into boost, as Bio said. Times 10 if you're running a small pulley.

No to raise the density of the air you must increase the mass of the air while the volume remains constant...you are shoving a higher lb/min mass of air into the same volume in the engine. Pressurizing the air is what a FI system does...D=m/v...I really don't care what happens pre-supercharger. I'm talking about what takes place in the engine itself. But we can agree to disagree. It's all a moot point because I'm not really looking out for the FI guys anyways

I'm not trying to validate K&N either, I was looking to provide people info about the restrictiveness of the air boxes. I appreciated the fact that K&N was so forthcoming with their data whether or not anyone decides to use their systems I could care less. I trust that the CFM restriction on the stock intake system is accurate. I like to make decisions based on real data, I figured others might as well.

EDIT: Yeah I understand that in order for a S/C to provide a certain level of boost it must draw X amount of air in terms of CFM. Not really arguing that point. This is all skewed we are talking about the same thing just from a different angle.

I agree with your methodology, it is always better to think about the decisions and not just go with what others tell you. I'm just trying to help you get to the truth by sharing my own thought process.

I think everyone will attest that with a forced induction system there is much more flow through the intake when in boost. You are correct, the engine displacement doesn't change. But, how do we increase the density of the air in the manifold, except by drawing more air from somewhere else and cramming it in? That air comes from the intake. The s/c pulls a fixed volume of air from atmospheric pressure, and rejects it to the manifold at manifold pressure. If the manifold pressure is any higher than the vacuum pressure, the car is pulling more air through the intake than a N/A car would. To relate it back to what you were saying, to conserve mass, the volume flow rate of the less-dense air in the intake must be greater than the volume flow rate of the denser air in the engine.

But you are right, the point is moot if you have a N/A car. The volume flow through the intake will always be less than RPM*Displacement/2.

EDIT: To clarify, I am just trying to explain why the upper limit of 451CFM the guy from K&N quoted does not apply to a FI car. His method of computing that number only applies to a N/A car. And thus so does his reasoning behind filter selection.