Custom EFI Project

Ric Rainbolt ricrain at
Mon Jun 12 19:13:24 GMT 1995

>Ric Rainbolt wrote:
>>I will be using eight Bosch fuel injectors (19lb/hr @ 3 bar).  Fuel 
>>pressure will be 3-6 bar, controlled by the EFI.
> <BIG snip>
>Calculation time!
> <BIG snip>
>Obviously we cannot expect 100% volumetric efficiency so mixtures will tend to
>be slightly richer than predicted so shorter times would be required.

Your calculations are correct but you failed to consider one important
factor... at idle the throttle is closed and the motor is only pumping air
at about 25-30% efficiency (0.3 bar absolute MAP * 90% efficient motor =
27%).  However, it did prod me into remembering something important about my
fuel calculations and now I am planning on adding a second set of injectors
(also 19 lb/hr) to compensate for fuel flow under boost.  After all at 1 bar
of boost, the motor will be operating at nearly 180% pumping efficiency.  At
8500 RPM, there is no reasonable pressure the single set of injectors can be
run at to obtain correct mixture.

By the way, I am using 11.5:1 as maximum enrichment (at operating temp) and
of course 14.7:1 as stoch.  Any comments?

Ric Rainbolt
ricrain at

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