What HP are they good too...i read the 39# will max out at 480ish....so i am wanting to know the max on 42#s

 

If they are the Ford Injectors they are the same just rated at different pressures.

 

so not all 42# injectors are not created equally?!?!

 

H.P.= lbs. per hr. x 2.04 That's per cylinder. So, 39lbs. x 2.04=79.56 x 8 =636.48 h.p. This is at 100% duty cycle. At 85% this is 541 h.p. These figures are flywheel h.p.

 

Bosch or Lucas 440cc Injectors (Ford has used both)

Ratings

39 lb per hour @ 39.5 psi (Ford Std)
41.5 lb per hour @ 43.5 psi(Industry Std)

The Ford 39/42 injectors are the same injectors just rated differently.
They are 42 lb Injectors by the industry standard and 39 lb Injectors by the way Ford rates them.

 

that is some great info...the question i am wondering is what is the drop off to the wheels. I know like wheels and tires can make a dif but lets just say a stock gt ... so if you have 541 at the flywheel what % loss to the wheel...just looking for an average

 

Lots of people say 15%. However I tend to disagree based on my experience.
For a 5spd car I would say around 10% Max in that power range.
For a stock Mustang its around 13%.
For a Stock 450 bhp Viper its around 11% (Despite heavier drivetrain, wheels, and IRS which uses more power)
Drivetrain loss is neither a flat figure nor a set percentage.

BTW calculation he used above would only be accurate for 39.5 psi (Stock Ford Rail Pressure) it would rise from 541 bhp to 576 bhp if you raised fuel pressure 4 psi (43.5 psi). It would also vary with the BSFC of the engine. Some engines are more efficient than others.