I've just finished reading an article about injector size compared to flywheel horsepower. They use the following equation:

HP/ no. of cylinders x BSFC / Duty Cycle % = Lbs/hour of fuel req'd.

Works great for a stock LT1 rated at 305 HP with a BSFC (brake specific fuel consumption)of 0.50 (0.50 lbs. of fuel per hour per HP) and a duty cycle of 80%. The stock LT1 has 24 lb/hr (3.799 grams/sec)injectors.

Ex: 300 HP / 8 x 0.50 / 0.80 = 23.8 lb / hr.

Now for the question:

How well do you guys think this works for higher horsepower motors?

I have an LT4 conversion with the GM hotcam and various other bolt-on mods. The head package is "advertised" to give 425 HP at the flywheel. Add the cold-air kit, 52 mm throttle body, 160 deg. thermostat, throttle body coolant bypass, etc. May be closer to 450 HP. Datamaster files show around 472 HP but I think that's stretching it compared to my Dyno-proven 509 hp 461 Olds '77 Trans Am.

Using the above equation:
425 HP = 33.20 lb/hr (4.199 grams/sec)
450 HP = 35.16 1b/hr (4.448 grams/sec)
470 HP = 36.72 lb/hr (4.644 grams/sec)

I've read on another site where someone suggested the the BPW at 6000 rpm would be 20 ms at 100% duty cycle. At 6000 rpm my BP ms reaches 17.73 ms which is 88% duty cycle with 30 lb/hr injectors and pulling an AFGS of 277.

If you back out the HP by using the equation with 30.123 lb/hr injectors with a 0.88 duty cycle (17.73 ms) it shows a flywheel HP of 424.

30.123 x 8 cyl. / 0.50 x 0.88 = 424 HP

Is it time to move up to 36 lb/hr injectors or is this equation BULL? I'm sure some of you guys can share some "practical" experience compared to the mathematical answers.

Does a 75 or 100 Nitrous wet shot require larger injectors?

Thanks

 

Pretty cool info

And a wet shot wouldn't require larger injectors b/c you are adding fuel per a fuel solinoid. Just make sure your fuel pump is up to par!

 

The equation you quoted is extremely conservative and "safe" for normally aspirated applications. It is not safe for blowers, turbos, or dry nitrous, where BSFC's are normally well above 0.50 #/HR/HP, approaching 0.60 in turbo applications. I've got actual BSFC numbers for my engine, and they were 0.46 running all motor at 500flywheel HP, and 0.53 running the engine with a 300-shot of dry nitrous.

The 80% duty cycle recommendation is also very conservative. Depends on the type of injector.... ball/seat, pintle or disc. Some injectors can function satisfactorily at 90% DC.

Obviously, if you looked at the size based on 0.46 and 90% DC its going to give you a smaller injector size. As a point of reference, I run 78# injectors, and see a shade under 70% DC with the full 300-shot of dry nitrous. As noted in the post above, a wet nitrous system isn't going to affect injector sizing.

 

I would use those equations for rear wheel HP, not flywheel.
A set of 30# Ford SVO injectors will be plenty for your setup.

 

That's not the way they were intended to be used. Say you had a 500HP engine.... with an M6 I got 425rwHP. With a TH400 and a loose convertor, I got 390rwHP. Same engine, same fuel demand, different rwHP.

By using rwHP, you are essentially driving your duty cycle higher in proportion to your drivetrain losses..... with 80% drivetrain losses, you would be guaranteeing yourself a 100% DC.

I agree you don't have to use 0.50 for BSFC, and not all injectors die at 80% DC, so make your adjustments to the equation by putting in accurate numbers, not relying on a number like rwHP. Running an injector at too high a DC shortens its life, affects the spray pattern and can reduce the actual flow capacity of the injector.

 

I know the heads are the limiting factor and I'm only pulling +/- 280 grams of air. Based on this, is it time to move up to bigger injectors?

 

The BSFC value depends on the effiency of the engine and the WOT AFR used, like 12.8 for an NA motor. A good acceptable BSFC for an L98 engine is about 0.5, where as for the LS1 engine, a value of 0.45 is more appropriate.

As mentioned above, a BSFC of 0.5 is conservative for a high Hp motor, a well tuned modified L98 could use a value of 0.48. However, because forced induction motors usually are setup to run richer AFR than 12.8, a good BSFC here is 0.55.

For example, my LS1 car dynoed at 400RWHp,,, divide by .88 (tranny lost, etc.) and the Hp at the flywheel would be about 454Hp. I am running at 85% DC, so if you run the above equation (BSFC=0.45), I calculate that I should have 30# injectors,,, that is exactly what I installed in 2003.

BTW: Its better to buy a slightly bigger injector than you actually need, as opposed to one which is a bit too small.

Anyway, this is my take on this stuff.