When that Injector Pulse Width number gets past 100-percent on the data logger, it’s time to prepare yourself for a piston or two to spray its glittering guts all over the track photographer. If you are in the middle of a boost curve, or worse yet, a big blow of nitrous oxide, prepare for block parts to join the spectacle.
The culprit is a lean condition that occurs when the injectors hit 100-percent duty cycle, or wide-open 100 percent of the time. Mathematically, this number can be much higher, but that doesn’t matter. They aren’t going to deliver any more fuel. When you engine goes lean, it begins to misfire and create a lot of heat, usually enough to melt your pistons. You can prevent these and other atrocities by designing a fuel system that doesn’t fail at redline. If you can’t produce enough fuel pressure or the injectors aren’t sized to run at a maximum of 85 percent duty cycle, you need to fix the problem. Yes, there is math involved, but you paid attention in school, right?
The science starts with our favorite question: how much power do you make, or wish you could make? We are going to use a boosted 1,000 horsepower engine as an example because it’s a nice round number. The first calculation shows the injector size required to feed that kind of supercharged power running on gasoline:
(HP x .6)/.8 = x, x/8.
Calculating for our theoretical 1,000 hp engine:
(1000 x .6)/.8=750, 750/8=94 lb. /hr.
Injectors are typically rated at 43.5 psi of fuel pressure, but you can adjust the pressures up or down to change injector flow. For example, a common Chevy LS 88 lb/hr injector will flow 94 lb/hr at 50 psi fof fuel pressure (√50/43.5 x 88=94). For boosted applications, you will also have to factor in a 1:1 boost reference. At 1 bar, add 14.7 psi to the fuel pressure requirement for a total of 64.7 psi.
Fuel pumps are rated in either gallons per hour (gph) or liters per hour (lph) at a specific fuel pressure. To feed 1,000 horsepower, a supercharged engine requires roughly 0.600 to 0.650 pounds of fuel per horsepower, per hour at peak torque, or 600 lb/hr plus 50-percent because this calculation is for static flow:
600 x .5=300,
That number can be converted to gallons by dividing by 6.2 pounds per one gallon or 145 gph. Finally, we can compare our flow requirements of 145 gph at 65 psi to the flow chart supplied with the fuel pump. In this case, Holley’s P/N 12-155 fuel pump flows more than 120 gph at 65 psi. It’s at the low end for an EFI supercharged street/strip engine with a 12-volt charging system but great for your carbureted 8-71 big-block.
10 micron filter 162-567
10-bolt flange fuel pump 12-155
100-psi regulator 12-851
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