The AFR and Injector size Thread

[hijack3d]

Originally Posted by bardabe

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Injector Size Calculator

Flow Rate = (HP x BSFCX10.5)/ (#injectors x Max Cylce)

Flow rate = CC
HP = at the flywheel
BSFC = Brake specific fuel consumption (0.55 for turbo 0.45 for N/A)

Example:
2.0 Litre SR20DET
Flow= (500HP x 0.55X10.5) / (4 cylinder x 0.8)
Flow = 902cc injectors required
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Furthering on this, and let me know if it confuses some of you guys since I did just cut/paste from my old article on this.. but I thought it'd be good to add. I'd also like to add that, obviously, BSFC will be different for all engines, and some have actually found that the BSFC for their set up is theoretically lower than .55 estimated for turbocharged engines...

Let's insert a scenario. Hypothetically, I want 350whp from my manual transmission RB20DET-powered 240sx. What size injectors will I need? Since our calculation doesn't account for drive train loss, we're going to need to see how much of this horsepower will be at the crank. Statistics say there is a 15-17% loss for manual trannys vs a 20-25% loss for automatics. This is not set in stone, of course. You can manipulate these numbers accordingly.

We can look at this like our crank horsepower is really 15-17% LESS than what our actual horsepower is at the crank (BHP - WHP x (0.15 or 0.17)). Thinking of it like that makes my head hurt though, so we'll think of it another way: Our desired whp is actually 83-85% of our bhp. Following me so far? This is what our equation looks like:

Equation #2:
Calculating BHP
Manuals
BHP = WHP / (0.83 to 0.85)
Automatics
BHP = WHP / (0.75 to 0.80)

Continuing our Scenario:
Let's plug in our numbers to calculate what our BHP would be if we had 350 at the wheel.
BHP = 350whp / (0.83)
BHP = 422bhp

See? We need to be seeing 422hp at the crank in able to have 350hp at the wheel.

Now we need to plug our BHP into our first equation to see how big of injectors we'll really need to get.
[ 422bhp x 0.55 BSFC ] / [ 6 Injectors x 0.85 Duty Cycle ] = 45.51 lbs/min
(45.51 lbs/min) x 10.50 = 477.85cc/min