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# Injector size calculation

### EFI Tuning Fundamentals

Discussion and questions related to the course EFI Tuning Fundamentals

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Hi, this is my first question, enjoying the modules thoroughly but I must be missing something in this formula, please advise.

If the value 0.85 in this formula is our max desired injector duty cycle (85%) in the example shown then we require an injector size of 735cc.

However: When I use the same formula to calculate the size of the injector needed for a max desired duty of only 75% the the results shows a smaller injector (649cc) should be used, which doesn't make sense.

Also: As explained in an earlier module Ethanol, having a different Stoich value, requires more fuel to be injected. What does this do to the value of BSFC?

You've actually found an error in the way that formula is drawn. The correct formula is:

Injector size (lb/hr) = (HP * BSFC) / (# Injectors * Max IDC)

This is why there is a correction in the actual calculation in this module. It has been corrected in our reshoot which should be online in a few weeks. On ethanol we are burning a lot more fuel to make the same power that the engine made on gasoline. In general for the same amount of power you will need around 35-40% more fuel on E85.

Thanks Andre, that solves it.