Simple question, probably without a simple answer: How do you determine what # injectors you need?

Also, if injectors are sold as "high impedance", does that mean that they won't work with an otherwise-stock fuel system?

 

Ah, I see, nobody knows! It's a stumper!

TTT.

 

i suppose you would hook it up to some scanning software and see what % duty cycle they are running at. and if youre close to 100%, perhaps its time for a lil more injector?

 

You mean to tell me that there's not some general knowledge out there? Like, if you have 400rwhp or more, you need 36lb injectors, but at 550rwhp, you should go to 42lb? (I'm totally making up these numbers.)

 

all i know is what i was told. with the power im lookin to put out 28.8s should be alright for me

 

The formula is pretty good but remember to take into account fuel pressure (further down the page). Or, you can always just "eyeball" it using a stock motor as a reference and scaling up or down. IOW, if GM thought 24lb/h injectors were good for 275hp (stock LT1) and you are looking to make 550, than 48's ought to do it. And so on.

Rich

 

Good link. If I did the math right, 42lb injectors are good for about 537 horsepower on a naturally aspirated engine at 80% duty cycle.

Thanks Rob!

 

The sizing criteria tend to be a bit conservative. A careful selection of components and a good tune should get a normally aspirated engine down well below a BSFC of 0.50 #/HR/HP. Some types of injectors will continue to operate satisfactorily above 80% duty cycle. My rule of thumb is to multiply flywheel HP times 0.07 to get you where you belong. That corresponds to a BSFC of 0.48 and a DC of 85%. And it assumes you are running the injectors at the pressure they were flow rated at. And there is no real "down side" to using an injector that is a bit larger than you need, unless you get ridiculous.

With regard to injector impedance, the stock PCM will only drive "high impedance" injectors. To use low impedance injectors with the stock PCM you need to add an external driver box. Many aftermarket ECU's will work with either.

 

Is there an advantage to low impedance injectors? Do they respond faster or something?

 

Very simple answer #1: Put it on a dyno with a wide 02 sensor and if it's lean and you can't raise fuel pressure anymore (keeping a sane level) or increase pulse width by tuning, you need bigger ones.

Simple answer #2: (assuming about a 45psi fuel pressure)
Stock 24 lb max out about 390-395 rwhp and 6000 rpm in a stock block.
30lb injectors max out about 460-470 rwhp and 6500 rpm in a stroker motor.
42lb injectors max out.......... I'd be thrilled to know, ain't been there yet.

 

But "rwHP" means nothing with regard to sizing an injector..... how does the injector know whether you arrived at that 390rwHP by running 445 flywheelHP through an efficient M6 setup, or 500 flywheelHP through a loose TH400 with non-locking convertor. Obviously you can't use a stock injector for the 500 flywheelHP setup.

 

It doesn't.........This information comes from the school of hard knocks. I worked with Haltec and used my car as a mule to develope a fuel management system for them. My 97 SS 24lb injectors went static about 6000 rpm making somewhere in the 390 to 395rwhp range so we knew it was time to bump up to 30's.

In a 398ci combination my car went lean on the dyno at about 460-465 rwhp and just under 6500 rpm and we could not fatten it up by tuning. We went to 42lb injectors and made 482.

No math, no hocus pocus.......just trial and error. Dyno's, wideband 02's and scanners do work. Conjecture that most similiar ci combo's making similiar hp and at a certain rpm would experience about the same results.

 

Denny,

Fred's point was that the crank horsepower is the real determining factor. Driveline losses vary from one car to the next, so your 395rwhp and someone else's 395rwhp might not be coming from the same flywheel horsepower.

Since you did all your tests on one car, most likely without changing the driveline, your driveline losses didn't vary throughout your tests, so those values worked for you. Someone with a vastly different driveline might not be able to use your numbers, and since you didn't tell us anything about your driveline (auto? high stall? manual?), we can't make any assumptions.

 

That makes sense.

Guess except for auto or manual when it's all said and done there isn't that much difference between cars on this list regarding drive train loss. Realize someone will say they have a different rear end, different ratio or different tires, but for what we are talking about, a percent or two isn't going to make or break a deal but I certainly see the point.

I now have a LT4 CE 398ci C4 with 4.11 Viper gears and a six speed. The heads and cam (stock block) Camaro SS was factory 10 bolt also with 4.11 gears. Because of the independent rear suspension and gears, I would have tiny a bit more parasitic loss with the Vette.

 

So.... my 381 makes about 395rwHP through a 20% loss with the TH400. You're telling me my engine will run perfectly fine on 24# injectors?

But of course when I was running it through the M6 with only a 12% loss, and making 429rwHP, I should have been running something larger?

See my point?

You seem to have a total disdain for anything that involves a simple calculation. But all my calculations - at least for my car - are based on actual dyno testing that established the BSFC's and power numbers I use in my own calculations.