General theory on EFI

[Tlsalt at aol.com]

Andy Wrote:

OK, I have been to the site, and have lurked here for a week.  I am very 
interested in the PC based EFI system, as is a friend of mine.  However, I 
am still unsure about how exactly it all works.  From what I can see, you 
can gain inputs from the throttle pedal, air mass sensor, manifold pressure, 
temperature, barometric pressure, crank/cam position, and oxygen content in 
the exhaust amongst others which I have not yet thought of.  The only 
variables in output I can see are the injector timing, and ignition timing.  
The injectors use a pulse width which would be of a set length depending on 
the outcome of an algorithm using some or all of the above inputs, and can 
either be set off at a time relative to the engine position (eg 10BTDC) or 
at a set time (ie. static time gap between them).  The ignition has to 
happen at a time relative to engine position (eg 10BTDC).

Am I missing something here, or is it really that simple?  Also, how do I 
get close to the sort of injector width I want at a certain time?  How 
should I go about developing an algorithm?  Are there any good sites I can 
visit which will explain to me (in intricate detail - I like that kind of 
thing) how to get in the ball park of fuel requirements as I have no wish to 
blow more than a couple of engines playing with this (Fords, and Austins are 
2 a penny here, but getting rid of them is more tricky).

Thanks in advance for any help you can give me, I think I got everything in 
there.

Hello Andy,

    Yes, it really is that simple.  Determining pulsewidth is best done by 
dyno testing, but there are formulas to determine pulsewidth at maximum 
torque using engine size VE and injector size.  If you have the proper size 
injectors (another simple formula based on horsepower), it's really just 
picking a number from one to ten ms, say starting with 2 ms sloping up to 8 
ms, and the engine will run.  In practice, it takes a bit of experimentation 
to arrive at the optimum settings and this is a function of the 
sophistication of your equipment and your experience.  If you are looking for 
a place to start, try Jeff Hartman's book and Bosch's Gasoline engine 
management.

Paul Saltwick 
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