[Diy_efi] New project

[Brian Dessent]

Dennis Noordsij wrote:

> One thing you mentioned, a VE table for SD. If you take the MAP, compare it to
> the ambient pressure, isn't there a nifty formula that takes the size of the
> throttle opening and angle of the butterfly to calculate exactly how much air
> would have moved in due to pressure differences? Which would give you the
> exact amount of air almost as accurately as a MAF could? Every time you

No, it's much more subtle than that.  Volumetric efficiency captures the
entire path of the air from the atmosphere through the air filter,
intake ducting, throttle body, plenum, runners, intake manifold, ports,
valves, cam timing/overlap, etc.  If air was incompressible and
inviscid, then you would be correct that simply knowing the displacement
of the cylinder and air's temperature and pressure would be sufficient
to determine exactly how many air molecules entered the cylinder.  In
fact this is the definition of 1.0 or 100% VE, the amount of air that
would enter the cylinder if the air was some ideal fluid that has no
compressibilty or resistance to flow or other dynamic effects.  But
that's not the case, and it's not something that you can calculate
analytically.  You could get a rough initial map by assuming a constant
VE (of around .7 or so) for all spots but it will certainly need some
refinement.

Brian

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