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Intake CFM: what does it really mean?

Many people in magazizes and advertisements, etc tend to talk about Head flow in terms of intake CFM at given valve lifts, say an intake port flows nice round 300 cubic feet per minute at .500 lift. But what does it really mean to the engine? Let's take a cam that is 240 degrees @.050 and .550 lift. Now what amount of that peak flow rate does the engine really "see". When you consider that in a minute of engine operating time on a 4 cycle engine that intake valve is open from .050 to.050 only about 1/3rd the time (720 degrees = 4 cycles/240 degrees =33.3%) the intake valve is closed 2/3rds of the time! So even if the port could flow 100% from .050 open to .050 closed (and it couldn't, unless it was pressurized) that 300CFM would net only 1/3rd of that flow (100cfm) at absolute best. Flow through an open/close cycle of a cam lobe really needs to be seen more in a Bell curve, not in a straight line chart as it is so often depicted.

I point this out only to bring to light that this is why subtle changes in cam duration (increase/dcrease the valve timing changes the whole % open equation) and rate of consumption (size of the cylinder being fed per valve cycle) all come into play.

Just an observation (and a GREATLY OVERSIMPLIFIED ONE AT THAT! ), nothing more.

Just curious if other people think this way, or is it just my crazy way of seeing things?


and some cams can also have more open area over the "curve", so that would also change things greatly.

I think intake manifold CFM is a different description in the overall "CFM" game. Someone back me up here, but isn't the overall flowrate of an intake manifold from carb plate to cylinder head surface? In other words, there is no X variable (such as a valve or its tendancy to stay open). They only make references to the capacity the intake flows. Modifications, of course, change these characteristics, but nothing in the realm of valves/timing/cam.

Yes, technically an intake runner should be considered as part of the intake tract, an intake can impede the total flow by flowing less but can't really enhance the flow if it flows more. Additionally, velocity and resistence to reversion can be maintained by the approach angle (ie, hi-rise), remember both air and fuel have mass and mass is not immune to gravity. But the intake and how air flows and reflects back to the plenum, definately are limited by that same valve timing parameter.

Introducing intake design has wayyy too many variables to go into here, but it's a valid point.

Panic (aka Ejit) has some really good info on this as well.

The only point I was trying to get across (in this again "over-simplified" example) is you technically 'need' 300 gross to achieve 100 net.

Take a 270cfm example with the same cam, you 'net' 90.


Imagine looking into the cylinder, like a half open pipe. see the piston start from tdc and rapidly decsend to bdc, and watch the valve open while this happens. You will see that at the pistons greatest speed right before bdc the valve is at or near its highest open point. So the piston is actually sucking on the valve and the port hardest when it is near its full lift. This is why porting changes make such a difference. If if was true that the port only worked a little bit, porting would probably have very little effect, as well as valve lift. And just for kicks, imagine when the spark plug fires and the piston starts descending down, the exhaust valve opens into a cylinder with a tremendous amount of pressure. So much so that the only time the exhaust valve really needs any spring pressure is at its highest lift and on closing. Pressure waves in the intake can make also make an engine seem to be down on power, even though you have good high dollar stuff. If it doesnt work together it wont make HP. Look at the magnum rt heads, they flow tons of air, even at higher lifts with a small slightly closed off entry. However, that entry being where it is speeds the air up and helps generate velocity. Velocity goes hand in hand with flow. Like HP and torque, improve flow, you should improve or at least not interupt velocity. Anyone agree/disagree?!

This makes sense to me, but getting back to the original post on this does the hypothetical 300 CFM amount per cylinder, per head, or for the whole engine? It seems to me that the analysis only accounted for 1 cylinder. Wouldn't the "other" CFM go to the other cylinders? Am I missing something? I'm no engineer, just a barely passable shadetree mechanic, so maybe I missed the "big picture" here.

streetwize is right, 300 cfm at max lift is kind of misleading. But whem people port heads they usually give the results in .05 or .1 increments I.E. .1 25 cfm .2 50 cfm... .5 300 cfm I think what streetwise is trying to say is that head porting and cam profiles should be more focused on flow throughout the whole open valve profile. I think this is probably too complicated for alot of people (Myself included). From what I've read cams open and close the way they do because of the velocities and momentum of the air. I think it is easiest to give people max and assume that it will probably flow better over the whole spectrum. There are also factors in the cam lobes, they cant be square, and the higher the lift the profile must be ramped differently. I assume that years of racing have proved that going for max lift has won out in design and longevity over max area under the curve? if not mayby you are on to something hehe, the next big thing, square camshaft lobes, perhaps


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