How exactly do you calculate the injector duty cycle?

Is it based on Pulsewidth?

 

Injector sizes do not carry a horsepower-specific rating, as brake specific fuel consumption plays a factor, and that can vary from motor to motor based on the motor's efficiency. Read this link:

http://www.westechperformance.com/pa...ding/bsfc.html

 

wouldn't most of the shot be from nitrous anyways (caught myself from saying nawwws )

i know it mixes with gas...are you doing wet or dry? don't the wet shots have their own injectors or something?

 

Wet kits have a separate solenoid and orifice to run the fuel in before being mixed with the nitrous at the nozzle. Horsepower added by a wet nitrous setup would have no bearing on injector sizing. It would, however, have an effect on fuel line/fuel pump sizing.

 

Wouldn't it depend also on what kind of tranny you have? All other things being equal I'd say you would be ok with 15-20 rwhp more in a manual over an auto because of driveline losses, that is if you are giving a rwhp cutoff point for stock injectors.

 

Wo, weaving in and out of the topic here

I have one more question to add to the jumble. Wouldn't increasing fuel pressure lower load on the injectors? Just a thought. I know there is a limit to this approach, but it would seem to help your 100% duty cycle problem for a temp fix...

 

Yep. Bumping up the fuel pressure increases the amount of fuel that can be pushed through at the same injector pulsewidth time.

 

True, but only to a point. Beyond a certain fuel pressure, the injector won't be able to close fully after opening. It could stick partially open and leave you with the same situation as a maxed out injector (i.e. going dangerously lean).

I've said it before here, but WHY are people willing to spend thousands on parts only to skimp on $200 worth of injectors? It just doesn't make sense in the least.

BRAD

 

Good point you've brought up I totally overlooked the problem with a Fuel Injector not being able to close. I am a firm believer in doing things the right way as well....

 

I was maxing out my stockers. I don't have a dyno slip but I'm only trapping 113 mph. This is at 3000ft DA, hot, and some other small problems so I can't imagine how it would've turned out in good weather

FPR's cost money too, I'd rather just spend that cash on a real solution. With everyone here willing to stretch the fuel system past its limits it's no wonder the blower guys go through engines like candy

I went straight up to 42 lb Lucas injectors when Racetronix was having a sale. After just a small amount of tuning the car behaves just as well as with the stock 24lb Multecs and I have LOTS of headroom I wouldn't go much higher than this size for high impedence injectors though. If I'm going through the trouble and money of injectors I would want to make sure, at the very least, it's under 80% DC. I don't like seeing over 85% if I can help it.

 

That would only be in cases where the pressure was bumped WAY over stock. In other words, 60+ psi, or 20+ over stock, which shouldn't be done. I totally agree that you should size the correct injectors for the engine, however, a few psi increase would do no harm, and could provide a margin of safety in a pinch.

All this goes back to my original point that there is no concrete horsepower-to-injector flow rule. There are guidelines, but the actual injector size will differ from engine to engine. Pretty much all comes down to A/F ratio and duty cycle desired.

 

So can someone explain how to calculate injector duty cycle?

 

Good info! I've got stock injectors and running most of the time 380 something RWP with a max of 399 rwhp. Car runs 115 mph in the 1/4 full weight. I'm sure mine are maxed out as well but I keep a close eye on the Scanmaster and it doesn't go lean on me.

I wonder why such the large discrepancy between engines? Hmm???

 

Brent,

I posted a link earlier in this thread that solves some of that question. Brake specific fuel consumption will vary from motor-to-motor based on fuel quality, combustion chamber work, and tuning to the engines powerband. All this alters a motors efficiency, changing the amount of fuel needed to get to the same A/F ratio as a different motor.

 

Yeah, I saw that and that's why I mentioned "large" in my reply. I wouldn't think one engine could be that much more efficient than another? I will check out that link though. Thanks!!!