In addition to the math in the FI book, a lot of text and editorial are provided about fuel injection. However, the correspondence that follows is from a ‘Fuel Injection Book’ customer. It is a demonstration of the extent that a tuner can get into from the FI book math section.

Hi Bob,
Ok I think I’m starting to get it if I take 32% less air, I must take 32% less fuel to keep the a/f ratio the same as baseline so if I had a total jet area of (.018 ) at baseline that is what I should be looking to maintain as I adj. % of fuel.

math:
.014480 = total eng. jet area = 79% of fuel to motor
.003848 = total bypass jet area = 21% back to cell
so 32% of 79% = 25% – 79% = 54% fuel to motor

.018328 – 54% = .009897 eng jet area
or .014480 x .68 = .009846 eng jet area
so .018328 – .009846 = .008482 is 46% fuel in return line
or .003848 + .004634 = .008482 total bypass area.
( .009897 + .008482 ) = .018379 same ratio

.009897 / 8 = .001237 area of jet dia.
look up jet area size on chart in back of fuel injection book
and closest is .001257 which is a # .040 dia. jet
same for .008482 is .008495? a # .104 dia. jet

then to double check:
.001257 x 8 = .010056 total eng jet area
plus bypass area .008495 (.010056 + .008495 ) = .018551
which is right where i want to be, but maybe a little less psi.

well at least i think that is right DP

Hi D, After a brief look, I agree with your logic. I did not check the math however. Bob

Hey Bob,
Thanks for all your responses to my questions about fuel inj. It all looks good on paper and I’m wondering how it actually will work out on the track but I guess that’s what all this math was all about to get the right ratio of air and fuel. I’ll let you know how it turns out in the spring when New England Dragway opens in April. Again thanks for all your help and for your books which have been great reading this winter. DP