i have some 19Lb/hr Ford fuel injectors, and i want to know if they would run as 21lb injectors at gm's fuel pressure, if i remember correctly the ford fuel pressure is 39 and gm is 43.5 so with that i get the equation 43.5 X 19 = 826.5 / 39 = 21.2lbs.
is that correct? i would be running these injectors, and they would act like 21lb injectors?

 

http://www.rceng.com/technical.htm

 

Your calculations appear to be correct. Most injectors have flow rates which are relatively linear as a function of pressure, at least through their acceptable pressure range.

Also, most injectors are designed for at least 3 Bar (43.5 PSIG) inlet pressure, and can operate reliably up to at least 48 PSIG (or higher for some types). 3 Bar is also the internationally accepted "standard" test pressure for injectors, but not everyone adheres to that. Most aftermarket injectors are rated at that pressure, but OEMs may not be.

You also need to be very careful about your presumptions of stock fuel pressure ratings. Ford has used 36 PSIG in the past, but not on all engines, and not in all applications. The Probe, for example, being a Mazda product, used 3 BAR to rate the Denso injectors that came with the car. Turbo applications, like some of the T' birds, used 3 Bar as well. Yamaha-engines SHOs used 3 Bar and Denso. Even some "domestic" Fords used 3 Bar for flow rating, so a blanket statement like that can be misleading.

Even GM products had varying flow rating pressures.

It's best to decode the injector part number and determine the actual flow rating at 3 Bar for an accurate comparison.