Fuel System, need advice

What are you HP goals?

The stock 19 LB injectors are good to 300-320 HP.

Past 300 HP I would choose 24 LB injectors for an engine without forced induction or NO2.

Only use 30 LB or greater injectors for forced induction or if a little blue bottle (NO2 dry kit) is in your plan.

Even if your stock injectors will work for your new combo, I'd recommend sending them out for a good cleaning. A standard 255lph pump will be more than enough for now and will leave you room for later upgrades.

Lynx331 said:

yea thats probably a good idea since they have been on there for 130,000 miles. Wouldn't i be able to clean them myself?

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For the price you will pay to get them cleaned you could just get some used ones off a newer GT. I got some off a 2000 GT with 1000 miles on them from a buddy for free. If you search around you should be able to find a set cheap.

Edit: 255 should be good as stated.

You will want to change the injectors out when you get the heads...24lbers with a recalibrated MAF...

155lph would suffice with those given mods...or a 190lph...

Oh and I run 30lb injectors on my h/c/i...24's might present a lean problem on my car in the upper rpms...

5spd GT said:

You will want to change the injectors out when you get the heads...24lbers with a recalibrated MAF...

155lph would suffice with those given mods...or a 190lph...

Oh and I run 30lb injectors on my h/c/i...24's might present a lean problem on my car in the upper rpms...

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He wouldn't need more than 21's with the mods he's looking at. What does your A/F look like on the dyno? Just how many RPM's are you turning that make you think 24's wouldn't be enough?

SlowGT said:

He wouldn't need more than 21's with the mods he's looking at. What does your A/F look like on the dyno? Just how many RPM's are you turning that make you think 24's wouldn't be enough?

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21lb injectors?

I turn 6k...like any other daily driven FTI 302 combo...

Never been on a dyno...I drive my car everyday back and forth to school and work and get the same gas mileage as I did before the h/c/i swap...

The original thread maker is going to have a mild h/c/i which is very similar to a 93 Cobra's setup with a touch more than the Cobra and the Cobra has 24lb injectors...

I use this table as a reference for horsepower level and factory style tune...

225hp = 19lb injectors (stock 87-93 5.0)
265hp = 24lb injectors (actual output of 93 Cobra)
300hp = 30 lb injector (if you follow the same pattern)...
330-340hp = 36lb injector? (average AFR/FTI combo)

That's about the most conservative chart I've ever seen. But that's not surprising since I don't refer to very many charts. I prefer to do the math specifically for each application. I know for fact that 21's will support 280rwhp/~325fwhp (See 'Old SlowGT' in sig for the combo). Run the numbers:

Fuel injector sizing 325 fwhp requires 21.51 lb/hr fuel with the injector at 85% duty cycle.

Determine max HP for a given fuel injector size At 40psi, 30's are good to around 450fwhp. At 50psi, they're reaching toward 500. At 40psi, 21's will support around 320fwhp @ 85% and 335fwhp @ 90%.

Fuel injector flow change calculator

air flow/fuel flow requirement calculator (may require login)

EDIT: that 280rwhp was running A/F right around 13.0

SlowGT said:

That's about the most conservative chart I've ever seen. But that's not surprising since I don't refer to very many charts. I prefer to do the math specifically for each application. I know for fact that 21's will support 280rwhp/~325fwhp (See 'Old SlowGT' in sig for the combo). Run the numbers:

Fuel injector sizing 325 fwhp requires 21.51 lb/hr fuel with the injector at 85% duty cycle.

Determine max HP for a given fuel injector size At 40psi, 30's are good to around 450fwhp. At 50psi, they're reaching toward 500. At 40psi, 21's will support around 320fwhp @ 85% and 335fwhp @ 90%.

Fuel injector flow change calculator

air flow/fuel flow requirement calculator (may require login)

EDIT: that 280rwhp was running A/F right around 13.0

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There are other factors to determine the proper injector size...fuel pump/fuel pressure/computer...the calculators kind of have a bitter/sweet ending to them...you can't always go by the calculators...Calculators don't equal real world...or vice versa...

I said "21lb injectors?" because that isn't a common injector from FRPP

With a tune/chip the 21lb injectors won't cut it for full driveability benefits...I listened to Ed Curtis on what injector size to get... ...he said the 30lbers had a greater benefit over the 24's...

And I said I "might" have lean issues with the 24's...

5spd GT said:

There are other factors to determine the proper injector size...fuel pump/fuel pressure/computer...the calculators kind of have a bitter/sweet ending to them...you can't always go by the calculators...Calculators don't equal real world...or vice versa...

With a tune/chip the 21lb injectors won't cut it for full driveability benefits...

Click to expand...

Define - 'full drivebility benefits'.

I didn't have any issues at all. And my dyno data showed that my setup was right on the money. And that was no chip/no tune. Actually that was a 'seat of the pants/mph' tune . At the dyno I asked Chris Winters (CrazyHorse) what I could adjust to make my setup better and he replied that he wouldn't adjust anything since it was primarily a street car.


Tthe injector calculations do include fuel pressure. And the airflow/fuelflow calculator tells you how much fuel flow is required-->that way you can size the pump accordingly. Yes, of course the injector calculator assumes you've got enough flow to support the injector. And the computer has nothing to do with it. The computer has no idea how much fuel is really flowing. It can only determine if it's too much or too little fuel based on input from the O2 sensors, MAF meter and TPS. Then it gets to decide to increase or decrease pulse width to compensate for the last input it received. That's why MAF calibration is so important. The only time the computer's a factor is when you consider the drivers needed to run high or low impedence injectors. But that's not really an issue with most daily driven H/C/I combos.

If you don't like the calculators, maybe you'll like the longhand fuel injector calculations, compliments of MSD.

and here's how to calculate change in flow for a given change in pressure (longhand), also compliments of MSD.

I like Accel's fuel injection data better, but that catalog won't come up at the moment.

Next time you get an injector recommendation, ask them how they reached their decision. You can put enough pressure to 19's to make them support what a 24 will at lower pressures, but the spray pattern will not be very efficient and at 100% duty cycle, they probably won't last very long in a daily driver. That's where 21's come in. I'm guessing your 30's are probably only running at around 65-70%. You could figure it out pretty accurately with some dyno data.

You put "And the computer has nothing to do with it." and then you put..."It can only determine if it's too much or too little fuel based on input from the O2 sensors, MAF meter and TPS."...

Those above two quotes kind of contradict each other...those O2 sensors/MAF/TPS are kind of important to help with fuel deliever ...the computer still has a determining factor...

The 30lb injectors give more play for a safe tune...Ed told me he sees (with the 302 FTI combo) a better tuneability with the 30lbs compared to the smaller injectors like the 24lbers...

Again if you look at the chart I gave...it shows how the injectors give you play...and how they come from the factory...

Talk to Grn92lx on here and ask him what he thinks about 24lb injectors on his combo (ours are very similar)...he had to bump the FP up to over 50lbs to get a good A/F ratio...and the stock computer likes to be around 38-40psi on these cars...and the 30lb injectors allow that FP fairly easily...

A dyno tune would further my cars abilities...but I figure a 24/30lb injector are fine do to personal experience by doing a "home tune"...but on a dyno/chip run a 30lb injector would be better between the two...