There are a couple of constants that we need to be aware of before we determine what size injectors we need.
These are really simple but I'll touch on them anyway.
The Duty Cycle that is mentioned in these calculations starts from the position of the crank where the injector first begins to fire until it needs to start to fire again. That is, two rotations of the crank or 720 degrees. So if the injector is continuously open it is operating at 100% duty cycle - that's not a good thing. It is common practice to size injectors for a maximum of 80% duty cycle.
All or I should say most injectors are rated by the manufacture at a fuel pressure of 43.5 PSI. So if your LS-1 FPR is running at 58 PSI then your injectors are capable of much more fuel.
You can calculate the change by applying the following formulas.
divide your fuel pressure by the rated pressure
in our example 58 divided by 43.5 = 1.333
find the square root of that number
in our case the sq root of 1.333 = 1.154
You next need to multiply the flow rate in pounds of fuel per hour at the manufactures rating at 43.5 PSI times the 1.154 derived above.
if we are looking at 28.8 lb/hr injectors then 28.8 X 1.154 = 33.235 lb/hr
Next we need a very brief discussion of BSFC
Brake Specific Fuel Consumption is a measure of how efficiently your engine converts fuel to Horse Power. Most engines do this most effectively at wide open throttle when the intake and exhaust are doing their job in the most effective manner possible - in other words filling the cylinder completely - at6 all other times the BSFC is worse because the combustion chamber pressure is lower. i.e., compression is directly proportional to efficiency - or something like that. So what we do is to assume that the BSFC of a street engine is .50 pounds of fuel per horse power per hour. Really good race engines and airplane piston engines get this number down to .47 or lower, but for us we should use .50
With an understanding of the above we can then do the math.
The question is - how big an injector do you need to make 500HP at the flywheel.
We will assume that you have the necessary parts, heads, cam, intake and exhaust goodies and most important of all displacement to make the 500 HP. Fuel injectors can make HP by targeting the back of the intake with a heavy stream in place of an atomized spray but drivability will suffer and that's a whole different story.
multiply the desired HP X the assumed BSFC
our example 500 X .50 = 250 pounds of fuel per hour
then multiply the number of injectors X the assumed duty cycle.
our example 8 injectors X .80 duty cycle = 6.40
then divide the pounds of fuel per hour by the injectors X duty cycle
our example 250 / 6.4 = 39.063
Keep in mind that you will need a set of 40 lb/hr injectors only if you are running your fuel rail at 43.5 PSI. if your running it at 58PSI then you can use 34.5 pound per hour injectors or be reasonable, use the next larger size like 36lb/hr injectors.
If you have any questions take a look at WWW.rceng.com/technical
Russ does injectors better that any one I know.