Help Me with Injector Size

Hey guys. whats the best way to figure and calculate what injector size I'll need?? I can give motor specs if needed....

Thanks
---Ryan

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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

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

Don't forget that the BSFC will vary depending on how efficient the engine is. I'd use the following figures as a guide:

Bone stock 5.0: BSFC 0.51
Full basic bolt-on 5.0, stock H/C/I: BSFC 0.48
Mild H/C/I 5.0: BSFC 0.45
Higher HP H/C/I 5.0: BSFC 0.42

Using the 85% maximum recommended duty cycle and stock 39psi fuel pressure:

19lb injectors are good for up to 270hp (full basic bolt-on 5.0).
24lb injectors are good for up to 390hp (higher HP H/C/I 5.0).

You could increase the fuel pressure with an adjustable regulator to raise the HP threshold but there's a limit to how far you can go without risking the injectors sticking open.
e.g. Ford Racing 340hp crate combo (GT40X heads, E303 cam, Cobra intake). Assuming a BSFC of 0.45 (Mild H/C/I), stock 19lb injectors would be at 100% duty cycle at stock fuel pressure. Even if you raise the fuel pressure to 50psi, the injector duty cycle would still be too high at 88%. If you use 24lb injectors at stock fuel pressure, the duty cycle will be a comfortable 80%.

If you're likely to err with the injector size, it's better to err on them being a little too big rather than too small. As long as the MAF is calibrated for the correct injector size and the O2 sensors are functioning properly, the engine computer's adaptive learning memory will ensure that the A/F mixture is close to optimum under all conditions.

Just tell us what your engine setup is and we will tell you what to use.
No reason to over complicate something so simple

Sounds good, I'm slow when it comes to calculations::
Stock bore 302, Factory Crank and rods with Forged Aluminum Pistons (factory dish), Ported E6 heads, X303, 1.6 alum roller rockers, Gt40 intake, 70mm TB, bigger MAF (haven't bought yet obvisouly), and I'll be running a wet kit.... I'm assuming somewhere close to 100shot. I also have an Aeromotive Adj Fuel Press Reg so I can play with my pressure. I had it set to 42psi before motor went down for rebuild.... I think I've got everything in there.
Thanks for the help guys.
--Ryan

You'll probably want 24 or 30lb/hr injectors for that build.

Keep in mind that a "wet" nitrous kit supplies its own fuel, so the engine's injector sizing doesn't need to be influenced by how much nitrous is added.

I don't know what your plans are for a fuel pump, but you're going to need something in the 190LPH range (but going bigger doesn't hurt). Fuel pump sizing IS influenced by the added nitrous system.

Assuming you have a headers-back exhaust, your 302 will be putting out roughly 320-325 flywheel HP (all motor) so you'll need 24lb injectors.

I will be running a Walbro 255lph fuel pump.

I am running long tubes with header mufflers for now for exhaust

the E6s weren't my first choice, but I bought them with all the machince work done to that to support my cam selection, and from the reading and research I did on them, I was lead to understand that they are not that great of heads in stock form, but if the exhaust side was opened up that they flowed REALLY well, which these have been..... I will eventually run a set of TF heads or AFRs, but I couldn't justify spending half my motor and suspension budget just on heads..... the car has been down for about 8 months now and I'm gettin itchy. haha.

Thanks for all your help guys..... I will probably run the 19s for now and in a few months buy a set of 24s.... I'm not worried about making crazy power right now (although I'd like to). this is my first motor build and I understand the saying that "there will always be somebody faster"..... Thanks for your help again.

--Ryan

Ya, the 19's sound right.

I would go with the 24 lb injectors. They are easy enough to be tuned. Plus you can pick them up pretty cheap. It will also help you out if you decide to do some more mods. The 24s will last you for a while even if you start upgrading engine components.

You size injectors for use now, not later.

so would I be better off just running factory e7's machined to run dual springs..... you guys got me worried about these e6s now. the X-cam doesn't ever build power til like 2200, and if I'm gonna hit a wall at 4, then whats the point?

If money is tight, i'd look for some gt40 irons and slap them on.
IMO get the cobra style heads, not the P's, so you don't need to blow 200 bucks on headers.
Heads should be around $350 or so
The E6's are pretty useless, and the E7's aren't far behind.

Money is tight, but I can get it if I need it, and after readin y'alls replies, I started lookin into the AFR 165s and the TFTW heads... You think they would be a good head for my setup?? I just spent the money on the forged alum pistons and don't wanna have to send them back and spend more for bigger notched pistons if I don't have to, but I still wanna make this $3k build worth the time and effort on the fun-o-meter. I'm aiming for mid to low 11s on spray

--Ryan

if i was you i would go with a set of brodix heads.for the money your gonna spend on afr's or the tftw heads the brodix heads are much better and come with alot better valve train than the other 2 come with. it would be better to spend the extra couple hundred to go with the better heads. I mean after all the head and cam package are the eart and soul of any build.

Well, using tw's or afr's would certainly be worth alot of power, probably another 50rwhp than what you were previously setting up.