injector selection - Fuel pressure

[Anonymous]

Ok, I need some hel as i am having a hard time finding what im looking for or how to calculate it. I know flow rate i need for injectors, but how can I select a smaller injector and run higher pressure to equal my required flow rate.

Injectors are advertised for flow rate at 43.5 psi. If i need to run 60 psi, then my selection of injector tested at 43.5 psi will have to be one that is of a lesser flow rate to compensate for the increase in pressure.

The reason why I want to run ~60 psi instead of ~43.5 psi is because there is a reason why Ford used 60 psi on the 300 and only 45 on the V8's.
The increase in fuel pressure helps to keep the fuel from vaporizing. The 300 fuel rail is much closer to the exhaust than a V8, so the heat absorption is much greater.

This is also why they used an injector cooler on some of the 300's to keep the injetors cooler. There is a larger concentration of heat on the pass side of the engine.

Do I need to calculate out an Injector flow rate dependant on fuel pressure? If so, how?

Adam

 

[StrangeRanger]

Flow through an injector is proportional to the square root of the pressure drop. Most injectors are rated at 39 PSI and the 300 runs at 55-58 PSI, if yours are rated at 43.5 then:

Actual thruput = rated thruput * (58/43.5)^0.5

if you know your required thruput then

Rated thruput = Required thruput / (58/43.5)^0.5

Limit your duty cycle to no more than 90% and try to keep it above 80%

 

[Anonymous]

Ok..still having trouble here...something isnt coming out right or im just not doing something right.

If i try and find actual thruput of a 42 lb/hr injector the product is 39.5 at 58 psi. But if the injector is tested at 43.5, wouldnt the thruput be more than 42 lb/hr at 58 psi?

Im trying to debate between a 39 lb/hr or 42 lb/hr injector. But I want to run higher fuel pressure. Maybe a little more explanation is all I need. My results arent coming out nand making sense to me.

Adam

 

[StrangeRanger]

42 * (58/43.5)^0.5 = 48.5

At a BSFC of .50 and an 80% duty cycle that's big enough for at least 466 HP. If yuo push the duty cycle to 90% and the BSFC to .48, you could get as much as 545 HP out of the engine.

Whatinthehell kind of a monster are you building??

 

[Anonymous]

well..im building a turboed EFI 300...but my injector selection is my questionable parts selection.

But now I understand how you are doing this....Except If its a turbo I would want about 0.6 BSFC

Except Im not quite looking for 450 HP...oknly about 350...so I may have to figure out positively what size of Injector i need. Using most calculators I always end up with 39 to 42 lb/hr....so I am either doing something wrong or the calculator is...

 

[Asa]

well, the 14# injectors are good up to just shy of 200HP, so i would believe the 19# injectors would be close to correct

SR, got any input on that?

and is that 350HP figure at the flywheel, or at the pavement?

 

[StrangeRanger]

350 HP at a BSFC of .6 requires 210 lbs of fuel per hour or 35 lbs per hour per injector. At an 80% duty cycle that would require injector with a thruput of about 44#/hr.

But because of the 300's higher metering pressure, you only need a rating of:

44 / (58/43.5)^0.5 = 38 lbs/hr.

If you push the BSFC to .55 (a much more likely number than 0.60) and the duty cycle to 90% you only need about 36#/hr. or a rating of 32 #/hr.

Basically a set of FRPP 36# injectors should do the job with room to spare

 

[Anonymous]

Ok..I understand now...Why would the 300 use a lower BSFC turboed than other engines?

Just from researching and reading LOTS of stuff on fuel injection, most people say up to .55 for N/A engines and some people actually say up to .7 for turboed engines. I picked a medium that seemed reasonable compared to other engines but a little on the low side due to efficiency and the fact that the engine isnt OEM designed for a turbo application.

Why would .55 be a more likely number than .6?

I was originally going to use a set of injectors at 39lb/hr out of the new Mustangs, but I wanted to make sure I was selecting the correct injector before I bought them.

It was a close calculation at 39lb/hr but I realized that the fuel pressure obstacle had to make a difference.

 

[StrangeRanger]

a BSFC of 0.50 is a "safe" number for normally aspirated egines, frequently numbers run a bit lower, sometimes around .47-.48 at peak HP. For turbos, I've always seen 0.55 published as a safe number, 0.60 is more than conservative. 0.70?? really? I have trouble with that one on an EFI/turbo setup. You want the mixture to be just a tad rich to prevent detonation, you don't need to drown the engine.

 

[Anonymous]

You can get about 375-400rwhp out of a set of 42# injectors on a 6 cylinder engine. That about where the 3.8L V6 guys have been topping out @ 100% duty cycle with blowers, turbo guys are getting up to 450rwhp out of 42's. I'm only hitting 70% duty cycle on my 3.8L with 273rwhp.