Chevy Small Block Part 11 - Pt. 11: Tuner's Delight

F.A.S.T. EFI And A Comp Cams Hydraulic Roller Turn 8.5:1 Compression Into 480+HP

Mike Petralia Aug 1, 2003 0 Comment(s)
Sucp_0308_15_z Chevy_small_block_part_11 Fuel_regulator 1/14

Fuel pressure was controlled by this return-style SX billet regulator (PN 15404). Also shown is the MSD billet crank-trigger distributor (PN 84697).

BSFC And Proper Injector Sizing
This can get tricky. Fuel injectors are rated by how many pounds of fuel they'd flow wide open in one hour at a given pressure, typically around 43.5 psi. Yet, horsepower output, fuel pressure, the engine's Brake Specific Fuel Consumption (BSFC = pounds of fuel consumed per horsepower per hour), and the injector's duty cycle, all determine proper injector size.

Since we know DM worked very efficiently with its Demon carb running in the low-to-mid .400 BSFC area, we wanted to test the EFI in the same range. We figured BSFC would be around .400 and we'd need a minimum injector duty cycle of .85, which means that at max power, the injector will be open 85% of the time. Keep in mind that fuel flow through the injector is affected by fuel pressure, as pressure goes up, volume goes up and as pressure goes down, volume goes down.

We ran most of our tests at 50 psi so the actual amount of fuel flowing through the injectors was higher than its rated figure. You can plug your own figures into this equation to estimate injector size needed for your application. It's safer to estimate a .500 BSFC, not the .400 we used, unless you know your engine can run that efficiently. We used the following formula to estimate injector size for Danger Mouse.

Sucp_0308_16_z Chevy_small_block_part_11 Crank_trigger 2/14

The F.A.S.T ECU can work with any electronic distributor, but you get the most accurate trigger signal and fuel/spark control from a crank-trigger like this MSD PN 8600.

Injector size = (HP x BSFC) / (Number of Injectors x Duty Cycle)
Ex: 480hp engine w/ 8 injectors
BSFC = .400
480 x .400 = 192
8 x 0.85 = 6.4
192 / 6.4 = 30 lb/hr

The math says that with 8 injectors operating at 85% and .400 BSFC, Danger Mouse would need a 30 lb/hr injector. We also tried larger 36 lb/hr injectors at both 43 and 50 psi (Test 23), which made the a little more peak hp, but averages fell off and low-end torque dropped enough to be noticed on the street.

Keep in mind that horsepower estimates in all these calculations are at the flywheel, not the rear wheels. A less efficient engine, (i.e. higher BSFC figure), will need more fuel pressure and bigger injectors.


Westech Performance Group
Mira Loma, CA
Comp Cams
Memphis, TN 38118
SX Performance
St Louis, MO 63143
Russell Performance Products
Daytona Beach, FL 32114
Harv's Performance
Whittier, CA 90606