The 557 thread and discussion about airflow had me thinking about something we discussed on this thread many years ago.

To me Airflow cfm on a flowbench alone is a great tool when you think of CFM...but really in that it's simply a rate of flow in one direction at a standard pressure Of course its an effective tool for measuring total flow comparing one head relative to another....but it's really so much more than that.

Bench flow doesn't take into account (for example) the relative pressure (positive or negative) in the manifold at any given moment in a running engine or the effects of reversion or overlap and (also) the effect of the volume of the incoming charge REDUCING (pushing back) the instant the piston starts to move up from BDC on the intake stroke. This is why the mass and velocity of the intake charge "ramming" into the cylinder can effectively cause a supercharging effect.

More on that some other time..... but I acquired a lot of insight from a conversation with David Vizard about this.

But I'm sure a lot of Engine gurus have software programs and databases that simulate some if not all of these conditions, probably more from trial and error or for tuning and cam and manifold selection for a specific rpm range or racing condition.


For Example If the engine has 240 degrees of actual "off the seat" (lets call it @.050) duration, each valve is open almost exactly (720/240) = 20 seconds for each minute the engine is running...and that is (if you think about it) regardless of RPM. It also brings into perspective where in the rpm range you want to maximize efficiency (torque or 'packing the holes').

If it's say 270 degrees, the valve is open 22.5 seconds per minute. So in terms of port flow, the valve spends 1/2 of it's total duration only open between 1/3 open and 2/3 of peak. The rest is the ramp up, the ramp down and the 2/3 to peak and back to 2/3.

This is to me a good illustration of how little time there is to fill the hole....and that each intake valve is closed much more than 1/2 the time.

But if you examine the flow and you extrapolate the info here as a means of comparing heads.....it's an interesting way to look at it when you think about where the valve/airflow potential is relative to where the piston is in it's cycle; so a head that flows more at peak might not be as good as the head that flows a bit better between the 1/3 and 2/3 when you consider the amount of actual time that lift is filling (or emptying) the hole. And filling is relative to piston position/compression, intake charge, etc. I think guys who dominate stock and super stock classes understand this (or at least a particular head) better than most of us.

I recently did a lot of reading about the 1930's Auto Union and Mercedes Grand prix engine development which I found fascinating, I guess all this was in my head a while and the 557 thread got me thinking about it more.

I wrote this just in hope of inspiring some deeper thought about power production, I've always been more of a street strip type builder which in a way is more like a road race engine than a pure drag motor that is never below WOT and hopefully never much below 90% ve/torque peak.

Let the games begin....