Engine What size fuel injectors?

You don't need a tuner, just the right calibrated meter and matching injectors.
While you very may need 30's, what makes you say you have injector problems?

Swhitney said:

I seem to be having some injector problems, and would like to know if I should jump up in injector size? I assume the engine is making somewhere in the low 400s to the crank, so instead of putting on another set of 24lb injectors, I could jump in injector size?

Quick and dirty about my setup:

1992 mustang GT
347 stroker
360 hp to the ground
victor jr heads
Performer RPM 2 intake
24 lb injectors

Click to expand...

Fuel injector sizing & injector photos



Revised 26-Dec-2014 to add statement about figures are for flywheel HP and not rear wheel HP



Injector HP ratings: this flywheel HP, not rear wheel HP.

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 at the flywheel.



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.



19/.5 = 38, 38 x 8 = 304 HP x .85 = 258 HP

24/.5 = 48, 48 x 8 = 384 HP x .85 = 326 HP

30/.5 = 60, 60 x 8 = 480 HP x .85 = 408 HP
36/.5 = 72, 72 x 8 = 576 HP x .85 = 490 HP

42/.5 = 84, 84 x 8 = 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/calcpchg.htm to get the calculators used in these examples.



Here's the duty cycle explanation. Duty cycle is how much of the time the intake valve is open that the injectors are turned on. The 85% figure means that for 85% of the time the intake valve is open, the injectors are spraying. The idea is that you want some percentage of the duty cycle left over so that you have some room to grow the process.



If you are at 100% and you need more fuel, all you can do is turn up the fuel pressure. That means the whole fuel curve from idle to WOT is affected. Maybe you are already too rich at idle, and turning up the fuel pressure makes it worse. If you had some injector duty cycle left to play with, a custom tune could use that where it is needed. That would not over richen the whole range from idle to WOT.



If you did turn up the fuel pressure, you might be able to change the injector duty cycle to get the air/fuel mixture ratio you want since the injectors will have extra fuel delivery capability.



With larger than stock injectors or higher that stock fuel pressure, you will need an aftermarket MAF that matches the injector size. The MAF “lies” to the computer to get a fuel delivery schedule that meets the engine’s needs and isn’t too rich or too lean. The best strategy is an aftermarket MAF and a custom tune to insure the best air/fuel ratio over all the RPM range.



Don't forget to increase the fuel pump size when you increase injector size or significantly increase the fuel pressure







Diagram courtesy of Tmoss & Stang&2birds





See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/ Everyone should bookmark this site.



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http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg



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Swhitney said:

After initial fire and when I shut off the car it has trouble firing back up, unless I hit full throttle. It will then fire up but rev up to around 4000 RPM. Also, the fuel pressure bleeds off over some given amount of time.

Seems to me that I need a set of 30 lb injectors.

I read an article about the mass air flow meter, and that matching them it really not the ideal way to go. They said you simply need a tuner to do it right.

Click to expand...

Couple things, at idle injector sizing isn't going to affect start up. You have issues elsewhere.
And calibrated meters have been used pretty much since the since the beginning of the EFI mass air foxbody.
I've run more aggressive parts than you currently have and it ran just fine with 30's an 80mm pro m.
If you want to use a tuner go ahead, but if it doesn't run right without it, don't expect it to run right with it.