Confused with Injectors and Fuel Pressure

[93gtmustang]

Below in bold is from jrichker

The following examples are only for naturally aspirated engines.

Injector HP ratings: divide flow rating by.5 and multiply the result by the number of injectors. This uses a 100% duty cycle.
Example:
19/.5 = 38, 38 x 8 = 304
24/.5 = 48, 48 x 8 = 384
30/.5 = 60, 60 x 8 = 480

The preferred duty cycle is about 85% maximum, so for a safety factor multiply the final figure times .85 (cheat on the math and use .588 instead of .5 and you don’t have to multiply the end result by.85)

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.

I'm not sure if my fuel pressure is correct.
My engine is rated at 345 flywheel horse power.
I'm running 24 lb injectors at 39 psi.
Does that mean that at 85 percent duty cycle that my injectors are good for only 326.4 horse power?
Should I increase fuel presuure?
Is there an equation to figure the proper amount of fuel pressure to duty cycle for injector size?
My car is run mostly at WOT at the drag strip.
Am I running lean?
I thought that 39psi was correct, but now I'm not sure.
Thanks for the help

 

[5spd GT]

It is a good question

My first question is, are you running the Ford Racing 302 engine with the GT-40 heads?

If so, that horsepower rating is done with no accessories or drivetrain (for sure) losses factored in. So in reality, you could be seeing 20-25% bhp loss.

So if that is the case, you are just fine.

Now, on to fuel pressure calculations. Joe (jrichker) typically post:

A 19 lb injector will flow 24 lbs at 63 PSI, and a 24 lb injector will flow 30 lbs at 63 PSI.

Take your brake specific fuel consumption number (.4-.6)and multiply it times the injector rating in pounds per hour. Then multiply that by the number of cylinders.

19/.5 = 38, 38 x 8 = 304 HP
24/.5 = 48, 48 x 8 = 384 HP
30/.5 = 60, 60 x 8 = 480 HP

So we can work this backwards.

If 63 psi on a 19 lb injector can support the same horsepower of a normal pressured 24 lb injector, that is a 61% increase in fuel pressure for 24% more horsepower support.

So anyways, 63-39 (assuming 39 psi is the baseline pressure), is 24 psi.

There is an 80 horsepower difference between what 19 lb and 24 lb injectors can support.

80/24 = 3.3

So for every 3.3 added horsepower support, one needs an extra 1 psi.

Joe will help make corrections and this is not a set formula that I know of, but it is just working with what is already stated.

Good Luck.

 

[88mouse]

From what i came up with you need them to be 26.9lbs.

I got this off of another web site. Its the formula for finding the injector size.

345/8=43.125
43.125*.50=21.5625
21.5625/.80=26.953125

Here is the web site to. http://www.flowtechinduction.com/technical.htm

 

[5spd GT]

That formula is quite accurate as well

However, I am assuming that the 345 HP rating he is referring to is without drivetrain and accessory losses.

He would need to be in the 300-310 rwhp range for 345 bhp with accessories factored in.

 

[88mouse]

That formula is quite accurate as well

However, I am assuming that the 345 HP rating he is referring to is without drivetrain and accessory losses.

He would need to be in the 300-310 rwhp range for 345 bhp with accessories factored in.

That is true. I just done it based off the known hp.

 

[93gtmustang]

That's right. My engine is the Ford Racing GT-40 B-303 Cam Crate motor.
I believe the horse power rating is at the flywheel.
I'm running 39 psi fuel pressure. Is that a good setting, or should I bump it up some? I just want to make sure that I'm not running lean.
Thanks for the help!


1993 GT Mustang / Ford Racing GT-40 Crate Motor B-303 Cam / 24 lb. Injectors / Stock Ignition / MSD EFI Blaster Coil / Edelbrock 5.0 Upper and Lower Intake with BBK Phelonic Spacer / Professional Products 70mm Throttle Body / C&L C.A.I. w/76mm MAF / Stock Air Box w/K&N Panel Filter / Holley Adjustable Fuel Pressure Regulator / Electric Fan / BBK Underdrive Pulley's / A.C. Removed / BBK Short Headers / Flowmaster Exhaust / Tremec T-5Z / B&M Short Throw Shifter / Alum. Driveshaft / 3.73 Gears / Eaton H.D. Posi / Moser 31 Spline Axles/ Subframe Connectors / Line Lock / Driveshaft Loop / B&M Short Throw Shifter / BMR Upper and Lower Control Arms / Lakewood Adjustable Drag Shocks/ Holley 190 lph Fuel Pump /

 

[5spd GT]

Now that we know your engine set-up...

You are just fine

It is a very close set-up to the '93 Cobra engine. They had GT-40 heads, Cobra intake, larger maf, etc. They come with 24 lb injectors from the factory, and they have it set-up with some good slack.

 

[93gtmustang]

Now that we know your engine set-up...

You are just fine

It is a very close set-up to the '93 Cobra engine. They had GT-40 heads, Cobra intake, larger maf, etc. They come with 24 lb injectors from the factory, and they have it set-up with some good slack.

Thanks
I really appreciate all the help!!!