Help with injectors size suggestions

i'm going through the same delema. i have 24# injectors on my car, and even with the mods in my sig they're still too big. my tuner leaned my car out as much as he could and my a/f is still 10.0:1. i JUST got done rebuilding some 19# injectors that i'm about to throw in with a 70mm mass air meter. hopefully that'll fix my a/f and give me back a bit of power. from what people have told me on here, 19# injectors are good up to about 350hp. from what you've listed, you're car is probably no where near that. so if you're keeping it n/a, some 19# injectors should be plenty. if there's more to your car than you listed, 24#er's might be what you need. whats your a/f at right now?

Injector HP ratings: divide flow rating by.5 and multiply the result by the number of injectors. This uses a 100% duty cycle. These ratings are for naturally aspirated engines.

Example:
19/.5 = 38, 38 x 8 = 304 HP
24/.5 = 48, 48 x 8 = 384 HP
30/.5 = 60, 60 x 8 = 480 HP
36/.5 = 72, 72 x 8 = 576 HP
42/.5 = 84, 84 x 8 = 672 HP

The preferred duty cycle is about 85% maximum, so for a safety factor multiply the final figure times .85.

304 HP x .85 = 258 HP
385 HP x .85 = 326 HP
480 HP x .85 = 408 HP
576 HP x .85 = 490 HP
672 HP x .85 = 571 HP

Remember that the above ratings are at 39 PSI. Increasing the pressure will effectively increase the flow rating. Example: a 19 lb injector will flow 24 lbs at 63 PSI, and a 24 lb injector will flow 30 lbs at 63 PSI.

See http://users.erols.com/srweiss/ to get the calculators used in these examples.

I agree that the 30's are way to big and the 24's will suffice also ditch the C&L Meter for a PMAS 75/24 unit

Your 30's are fine. Any issues you can thank your "calibrated" mass air meter for.

The larger your injectors, the further you get away from you stock computer's tune for 19 lb injectors.

Follow Jrichker's chart!

24 lb injectors would be plenty.

Killercanary, on this site, only ran 82% duty cycle with 24 lb injectors on his stroker with nearly 360rwhp. He uses a very accurate computer tuning device that is even more accurate than those little hand held devices.

Good Luck.

i had a c&l and a pmas..the pmas wires came undone from the harness but before it got screwed up it was good,,, the c&l was garbage man,,,, straight garbage

according to the injector calculator you could in theory swap out your 19# injectors at 38 psi for some 24#'s at 24 psi and it would run the same???

n2o-89fox said:

according to the injector calculator you could in theory swap out your 19# injectors at 38 psi for some 24#'s at 24 psi and it would run the same???

Click to expand...

Nope, it would give you the same fuel delivery potential I imagine, but it needs some degree of pressure. Maybe Jrichker will be able to explain it for us...

The computer (so I have heard) likes to see 38-40 psi of fuel pressure at the regulator.

n2o-89fox said:

according to the injector calculator you could in theory swap out your 19# injectors at 38 psi for some 24#'s at 24 psi and it would run the same???

Click to expand...

In theory, yes.
The problem is that as pressure drops off, so does the spray pattern. Lower pressures tend to make the injectors "dribble" and have poor atomization of fuel. This can have a negative effect on performance at low intake port velocities. Typical case would be flooring the throttle at 30 MPH in 4th gear. You may encounter a stumble in acceleration until the airflow through the intake ports picks up enough speed to help atomize the fuel.

jrichker said:

In theory, yes.
The problem is that as pressure drops off, so does the spray pattern. Lower pressures tend to make the injectors "dribble" and have poor atomization of fuel. This can have a negative effect on performance at low intake port velocities. Typical case would be flooring the throttle at 30 MPH in 4th gear. You may encounter a stumble in acceleration until the airflow through the intake ports picks up enough speed to help atomize the fuel.

Click to expand...

Thank you for clearing that up...

jrichker said:

In theory, yes.
The problem is that as pressure drops off, so does the spray pattern. Lower pressures tend to make the injectors "dribble" and have poor atomization of fuel. This can have a negative effect on performance at low intake port velocities. Typical case would be flooring the throttle at 30 MPH in 4th gear. You may encounter a stumble in acceleration until the airflow through the intake ports picks up enough speed to help atomize the fuel.

Click to expand...

Sorry for hi jacking this thread but I have one more question. So if you were to do it ( lower pressure, bigger injector) the only problem that might happen is when you go wot from really low rpms in a high gear? Would drivablilty be comprimised? The reason I ask is because I bought a maf sensor out of a 94 cobra that said it was calibrated for the 24 injectors which come to find out it is the same one that comes in the gt and is calibrated for the 19's and the cobras computer makes the adjustments. I ordered some 24's to go along with the maf because when I removed my 19's they were all cracked to *****. I just want to be able to make the 24's work without buying a calibrated maf or some new 19's. I just need this to work until i get the money for the tweecer so I can program the car for the 24's.

n2o-89fox said:

Sorry for hi jacking this thread but I have one more question. So if you were to do it ( lower pressure, bigger injector) the only problem that might happen is when you go wot from really low rpms in a high gear? Would drivablilty be comprimised? The reason I ask is because I bought a maf sensor out of a 94 cobra that said it was calibrated for the 24 injectors which come to find out it is the same one that comes in the gt and is calibrated for the 19's and the cobras computer makes the adjustments. I ordered some 24's to go along with the maf because when I removed my 19's they were all cracked to *****. I just want to be able to make the 24's work without buying a calibrated maf or some new 19's. I just need this to work until i get the money for the tweecer so I can program the car for the 24's.

Click to expand...

What I described is a drivability problem. Your problems may be similar or not.

with the combo descibed by the original poster i would run 19s, they will feed it just fine.

IMO i wouldnt even mess with injectors without a tune. me and friends of mine have had nothing but problems when trying to add bigger injectors with a "calibrated" meter.

the EEC from the factory thinks it's running 19s and it WILL continue to think it does until you tune/ get a tune telling it otherwise.

the calibrated meters do nothing more than fool the engine into thinking its getting less air that it really is. so it will try to dump in less fuel. but with a higher flowing injector it would "in theory" get the right amount of fuel. but this isnt the case. its a bandaid fix and jacks with load% in the EEC and it will use the incorrect spark advance/retard tables. for example it would think it was at 70%load when it could infact be at 90%

if someone spends enough money and etc to get out of the capabilities of the stock 19s then its HIGHLY advised to get a tune or tune it yourself to make sure the fuel it needs. there is no other way to FOR SURE know the fuel is correct. the stock EEC is tuned for a BONE STOCK engine.

even a mild bolt ons/exhaust 5.0 could benefit slightly from computer tuning, it would be a waste of time because those mods dont really affect anything but the fuel delivery could be optimized slightly. nothing you could feel but you get the point. as mods get into bigger heads and cam and intake, the car will benefit alot more from a tune. a blown car will really benefit from some tuning.

a mild HCI combo could benefit from a tune but it might not be worth it really. if the person plans to keep on modding i would wait until the car is considered complete or close to it. a more elaborate HCI like 200cc heads and big cam could probably benefit greatly seeing how they can ingest more fuel.