Injector size?

[Boydster]

I'm in the process of rebuilding my 5.0. When all together (some of these items were already installed), it'll have 70mm MA & TB, Edelbrock 5.0 intake, Edelbrock 1.90 heads, Trick Flow Stage 1 cam, 1.6 or 1.7 roller rockers (depends on how the P-to-V clearance comes out), Keith Black 9.4:1 Hyper pistons, .030 bored, Pypes stainless headers & Pypes off road x-pipe. There's a lot more, but thats what applies to the question...

I have new 19lb injectors to go in it. Should I bump up a size? Or should the 19's be OK? I dont plan on having to do any dyno tuning because it's a pretty basic 5.0 setup. Already have an adjustable FP regulator and a 190 pump.

I know the safe call is to use the 24's. Just wanting to know if the 19's will be OK. I've heard the 19's are good to about 300 hp.

If I go to 24's, what's a good fuel pressure to start off with?

No plans for gas or blower.

Thanks.

 

[giddyup306]

How far does your AFPR go up to? If you can get 75 PSI there is no problem supporting 350 hp.

Injector Size Calculator

I had a similar combo to what you do, and 19# injectors worked fine.

 

[jrichker]

Fuel injector sizing & injector photos

Revised 07-May-2011 to add flywheel statement & improve injector table

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 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

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

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 Automotive Performance Software / Interactive Calculators to get the calculators used in these examples.

Here's the duty cycle explanation. Duty cycle is how much of the time the intake is open 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.

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



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See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring Mustang FAQ - Wiring & Engine Info Everyone should bookmark this site.

Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg

HVAC vacuum diagram
http://www.veryuseful.com/mustang/tech/engine/images/Mustang_AC_heat_vacuum_controls.gif

TFI module differences & pinout
http://www.veryuseful.com/mustang/tech/engine/images/TFI_5.0_comparison.gif

Fuse box layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif

 

[NIKwoaC]

Two things:

1. You can't change injector size without a tune or at least without a meter calibrated to the injectors. This is a common mistake that raises questions in here all the time. If you want to go from 19s to 24s, you either need a tune, or you need a new MAF meter that is "calibrated" to the new injector size.

2. Excessively raising fuel pressure is a band-aid fix for otherwise insufficient injector sizing. It is certainly a feasible option, but it tends to wreak havoc on part-throttle or idle A/F ratio. Again, bigger injectors with a tune or a calibrated meter is the better option.

 

[Rick 91GT]

I personally like a 24# inj in that combo.

 

[Boydster]

How far does your AFPR go up to? If you can get 75 PSI there is no problem supporting 350 hp.

I have no idea if it'll go that hi. It's a Paxton unit thats been on there for 15 years...

I had a similar combo to what you do, and 19# injectors worked fine.

Thanks...

 

[Boydster]

Two things:

1. You can't change injector size without a tune or at least without a meter calibrated to the injectors. This is a common mistake that raises questions in here all the time. If you want to go from 19s to 24s, you either need a tune, or you need a new MAF meter that is "calibrated" to the new injector size.

I plan on a new MAF meter to match the injectors if I step up to the 24's. Thats partly the reason I'm asking, cuz it's a big cash outlay if I dont really need to.

2. Excessively raising fuel pressure is a band-aid fix for otherwise insufficient injector sizing. It is certainly a feasible option, but it tends to wreak havoc on part-throttle or idle A/F ratio. Again, bigger injectors with a tune or a calibrated meter is the better option.

I've heard that before about raising the fuel pressure.
Thanks!

 

[Boydster]

Fuel injector sizing & injector photos
~snip~

Thanks, lots of good info there, and if I knew my HP, it'd be a cinch!

 

[Boydster]

I personally like a 24# inj in that combo.

Guess it's time to step up.

 

[giddyup306]

I have no idea if it'll go that hi. It's a Paxton unit thats been on there for 15 years...

Yeah a lot of them max out at 60 PSI, but I've seen some that go as high as 100 PSI. I'm sure you can "get by" with the 19's, but you also have to realize that some calibrated MAFs (like C&L) do a not so good job of "tricking" the computer. On the flip side, you could probably run 30's if you turned the fuel pressure way down.

When I lived in Lincoln, I spent most weekends at a dyno shop. A lot of Camaros and Mustangs with H/C/I combos only made around 275 RWHP. You might not be at the horsepower level where it makes a difference.

There is several ways you can make it work, but the final call is up to you.

 

[Boydster]

Thanks Giddyup. Still trying to make a decision... 19's at 40? 24's at 35? Hmmm...