As you can see in my sig. with the setup that I have, I believe I'm running about 550-600 hp at the crank. This year I hope to add another 200 hp. I'm not sure if it will be with more boost or n20.

My question is when you use those injector sizing calculators and I'm sizing them for 800 hp, it tells me that I should use 72# injectors. I've been to different sites and used different calculators and they all say the same thing. But when I go to a site like this: http://www.cprracing.com/prod02.htm it says I can use 55# injectors.

Can somebody set me straight?

Scott

 

Very different situations between boost and N2O. Assuming a wet nitrous kit, there will be fuel nozzles and if your current injectors are adequate there is no reason to change them. If you go with a bigger blower I would recommend a set of large, low impedance injectors (and LJ's converter).

Rich Krause

 

Note it says Max. If you use the normal calculators and plug in 1.00 for the duty cycle as in always open it coincides with what they advertise...

 

The "ESTIMATION" is fairly simple... the two examples you are quoting appear to represents two different extremes in estimating criteria.

The calculation is very simple..... its what numbers you plug in for the variables that makes it or breaks it.

(flywheel HP X BSFC) / (no. of cyl X DC) = inj flow rating

BSFC = brake specific fuel consumption in #/HR/HP (pounds per hour per flywheel horsepower)

DC = duty cycle... essentially the portion of the available time that the injector is required to operate.

For example, the linked source quotes "max 427HP" with a 24# injector. If you back into the calculation, they assumed:

BSFC = 0.45 #/HR/HP.... not a bad assumption for a well tuned NA application

DC = 1.0 (or 100%)..... not a good assumption. 0.80 (80%) would have been better

(427 X 0.45) / (8 X 1.0) = 24.02 #/HR/HP

The correct way to use the formula is to guestimate the correct BSFC for your specific application, and to use a reasonably conservative basis for duty cycle. The standard online calculators assume a BSFC of 0.50 for NA applications, and a DC of 80%. And they recommend a BSFC of 0.55-0.60 for S/C setups.

Then you need to adopt a DC philosophy..... I see many people claim to be making over 400rwHP on stock 24# injectors. If you look at the numbers, this is impossible. How can they do it..... one possibility - they are simply looking at chassis dyno numbers. First, a chassis dyno pull only pushes the injectors to the max fuel demand for a fraction of a second - not enough for the evidence of injector breakdown to be seen. Second, a chassis dyno generally produces a nice curve of "SAE corrected" HP. That means that it is possible, because the pull was done on a very warm, humid day, with poor barometer, or at an elevation above sea level, the engine never actually produced the HP shown in the curve..... it only made that HP "on paper" through the magic of the SAE correction. In effect, it did not push the injectors to their true limit.

I personally would not use any more than 80-85% DC.

Based on some engine dyno testing, I found my relatively mild 381ci engine, running NA, had a BSFC of 0.44 at peak fuel demand. Running the same engine, with an additional 275HP of "dry" nitrous, we found the BSFC to be 0.53.

So.... first you need to determine how you will add the extra 200HP. As Rich points out.... running N2O "wet" will produce different results from running N2O "dry". And I suspect that adding it with an S/C will yield yet another typical BSFC, I would guess even higher than my dry N2O number.

I'm currently running just about 800 flywheelHP on N2O, and use 64# injectors running at 4bar (58psi) fuel pressure, which in effect allows them to flow almost 78#/HR. With that setup, I see a duty cycle of about 68%. I could push the setup over 900HP and still stay below 80% DC.

You need to pick the way you will add the 200HP, and use the BSFC that corresponds to that setup.

You need to determine how tight you want to push DC. You can make a case for keeping DC even lower than 80% on a sequential injection system, in cases where your aftermarket ECU allows you the capability to alter injector timing.

You need to determine what fuel pressure you want to run your system at.

Then you can calculate the best injector size for your engine.

 

Thanks for the replies. Your replies really helped out.
I'm leaning towards N20 wet system, that seems to be the way to go.

My other question is, is low imped inj. with LJ's unit be a better choice than high imped inj's? Inregards to better idle and responce time of the inj?

I understand that the PCM will only support high imped inj's and you need LJ's unit to use low imped inj's. I guess what I'm trying to say is there any difference in inj. performance with hi vs. low imped?

I plan on running 50 psi fp or more if needed.

Scott

 

This has some good info-
http://www.acceleronics.com/faq.htm#1

 

On a side note, how exactly do you determine what fuel pressure you want to run in the car? My car is boosted as well, and I'm going through the whole injector thing right now. Is there any idle/WOT pressure that I should be shooting for?

 

Thank you!!!!

That's exactly what I was looking for.