Injector HP ratings: divide flow rating by.5 and multiply the result by the number of injectors. This uses a 100% duty cycle. These ratings are for naturally aspirated engines.
Example:
19/.5 = 38, 38 x 8 = 304 HP
24/.5 = 48, 48 x 8 = 384 HP
30/.5 = 60, 60 x 8 = 480 HP
36/.5 = 72, 72 x 8 = 576 HP
42/.5 = 84, 84 x 8 = 672 HP
The preferred duty cycle is about 85% maximum, so for a safety factor multiply the final figure times .85.
304 HP x .85 = 258 HP
385 HP x .85 = 326 HP
480 HP x .85 = 408 HP
576 HP x .85 = 490 HP
672 HP x .85 = 571 HP
Remember that the above ratings are at 39 PSI. Increasing the pressure will effectively increase the flow rating. Example: a 19 lb injector will flow 24 lbs at 63 PSI, and a 24 lb injector will flow 30 lbs at 63 PSI.
Just as the wrong sized jets in a carb can cause decreased performance and driveability problems, so can incorrectly sized injectors. The following calculation can be used for approximating fuel flow per injector based on horsepower (HP) and Brake Specific Fuel Consumption (BSFC).
Note:
1) Engine HP must be a realistic estimate.
2) BSFC is determined from engine dyno measurements. It typically ranges from 0.4-0.6 for gasoline engines. A BSFC of 0.5 is a safe, initial estimate.
BSFC =
Pounds of fuel per hour
--------------------------
Brake Horse Power
3) The 0.8 multiplier for the "Number of Injectors" helps derive a practical "Max Injector Flow Rate" for each injector based on an effective real world injector operating pulse time and fuel flow. It is unrealistic to establish the fuel flow to an engine based on an injector operating pulse time of 100% (wide open all the time). This calculation uses an injector operating cycle of 80%. Some full race engine management systems may operate at 85-95% duty cycle, but extended operation may eventually overheat the injectors and cause irregular flow rates and poor low rpm operation.
Injector Flow Rate (lbs/hr) =
HP * BSFC
---------------------------
number of injectors * 0.8
With a known injector fuel flow rate you can get a rough estimate of the systems capacity by using:
HP =
IFR * number of injectors * 0.8
---------------------------------
BSFC
where
IFR = Injector Flow Rate (lbs/hr)
Increasing the fuel pressure can often provide increased fuel flow and better atomization. If you know an injector's static (non-pulsed) fuel flow at one system pressure you can find its static flow at another pressure with this:
Good question. I'm going to upgrade my '88 with an Edelbrock Performer intake and heads, stock cam with 1.72 roller rockers, 65mm TB, 70mm SN95 MAS. If I don't have to upgrade injectors I would be happy with leaving them alone.
From the Ford Racing catalog, also in synch with jrichter, 19lb injectors will support up to 275 flywheel HP at a 90% duty cycle. 1998whtgt's sig shows 275rwhp for his combo which is a cam ahead of what I plan on doing. Will my motor's horsepower bump into the 275 NA flywheel horsepower limit?