Calculating injector duty cycle....

fastsc92

Registered User
I'm beginning to feel that I'm running out of injector on the top end and I'm wondering what duty cycle my injectors are actually running at. What's the best way to determine this? Where is the best source for 60lb injectors?

The car hasn't been on the dyno, but I'm assuming it puts down around 350+ at the wheels. Running 42lb injectors with the following mods:

.493 cam
ported heads
75mmTB
73mm MAF
Custom Inlet Plenum
94/95 blower (MPI Porting, 16psi)
5% Pulley
Rasied top
Front mount
Long Tube headers, no cats
2.5-3.5-2.5 exhaust
75 shot n20 (haven't used in a few years)
 
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I didnt see you list a 255 lph pump but I am assuming you already have this. You need 60's and I would expect horsepower to be around 315 rwhp with the mods you listed.
 
yes there is a 255lph pump in there along with many other stuff, but this list is to just get an approximation. The 350+ was refering to the use of nitrous.
 
right on with the juice it would def be higher than 350 hp. I would still expect your suspicion of it running out of fuel with the 42s to be correct. I have never seen them support much over 300rwhp. Is it tuned? If not you know who you need to talk to. ;)
 
I do tuning myself... I may have to look into an 80mm system or even a 85mm system with some 60 pounders behind that. I'd like to keep my 75mm TB. I don't think the 80mm TB will flow much more. I guess the quesiton still remains....what's the best way to calculate duty cycle?
 
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Dave,

Very informative thread. In the past the rule of thumb for injector size has been as follows.

30# 300 flywheel HP
36# 360
42# 420
50# 500

Calculate 15% drivetrain loss for manual and 20% for auto.

According to your findings...what changes should be made to the old rule of thumb ?


David

Here is what was posted in that forum.
 
To figure out how much power your injectors can support, you take your number of injectors times the poundage for each. Then you multiply that by a constant called "Brake specific fuel consumption" (BSFC). That varies, and for our cars it's usually approximated at 0.60. That number is rather high, as our cars have to spend some power internally to drive the supercharger. Finely tuned race cars can get into the 0.40s or 0.30s. Anyway, BSFC is expressed in pounds of fuel per horsepower. So 100 pounds of fuel x 0.60 pounds/horsepower = 60 HP.
 
If the 350rwhp is without the nitrous you are way out of injectors.

I'm assuming the 350+ is WITH the use of nitrous. I haven't run the nitrous this year, but I'm having issues getting my commanded ratio. I'm thinking that I may be running out of injector at the top end. The ratio is dead on if I do a short WOT in 2nd gear. If I run an extended pull such as 3rd or 4th gear. The ratio will lean out as the RPM's increase. Adjusting the MAF clip or the fuel table does not change the ratio in the upper range. It's not that lean, but I'm commanding a 12.1 ratio and the richest I see is only 12.7ish.
 
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