Ancient History

[Todd Knighton]

> 
> I haven't dug up my stuff about VE and CFM and displacement, but seems to
> me that an xxx CID engine is a pump that always pumps xxx CI of air every
> two revolutions (assuming a 4-stroke).  All that changes is the density
> (i.e. throttle closed = lower density A/F charge).  And I thought that
> running R-E-A-L lean meant burning things up (I KNOW it does!).  So, it
> takes a certain amount of fuel to just coast, much less make any power.
> 
> Like I said, I haven't dug up the stuff, but I remember trying to mathematically
> anticipate 'good' mileage for my 351-W with a 3.50:1 ratio diff and was able
> to think 14 to 16 would be a reasonable goal with higher values theoretically
> feasible.
> 
> So, how does one take a 400+ CID engine and get that kind of mileage
> unless he's running a 1.7:1 rear end (i.e. L-O-W r's) -- even if he had
> 100% VE ?
> 
> Tom

Tom,
	who cares about CFM, it varies too much too worry about.  As well as
your air fuel ratio is determined by mass not CFM.
	You only run 100%VE at close to or wide open throttle, unless you're
turbo'd.  At very light throttle conditions, your VE is determined by
pressure AND rpm's.  Sure, your VE still follows your torque curve, but
your mass flow into the engine is very minimal at high vacuum
situations.
	For example, running down the road at 100%VE, or WOT, you might be
running 8 to 10 ms injector timing, where as at light throttle
conditions, you might see 1.5 to 2 ms.  Same RPM, different pressure,
thus less fuel.
	Sure it's the same CFM inside the manifold, but prior to the throttle
body (at atmospheric pressure) the CFM is much lower.

Todd Knighton
Protomotive Engineering