still running rich--30 lb injectors

Discussion in 'Fox 5.0 Mustang Tech' started by redstang50, May 26, 2010.


  1. redstang50

    redstang50 New Member

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    ok, so last summer i finished a project on my '93 5.0 GT40 heads, e303 cam, GT40 intake, 30 lb injectors, 73mm MAF, and 65mm TB. Ever since i finished it, its been running really rich (smell gas bad) and gets only 12 mpg. a few days ago installed MSD 6AL ignition and that didnt help much either. Im just looking for any help to how i could fix this. i know the 30 lb injectors are overkill for a n/a motor, but i wanted to leave room for improvement. any help would be appreciated!

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  2. Pontisteve

    Pontisteve New Member

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    You need to tune for the injectors. A MAF calibrated for the 30# injectors would certainly get you closer than an uncalibrated MAF, but is not the ideal way to go.

    If you open the injector for the same amount of time, but have 30/19ths of the original amount of fuel entering during that time, you've increased the amount of fuel by 158%. Stock O2 sensor authority is around 25% up or down, so an increase of 58% (over your original 100%) is way out of range of what the O2's can "fix" at part throttle. At full throttle, the O2s are ignored anyway.

    With the heads, cam, injectors, and MAF swaps you've done, each of those things ought to be tuned for. All of them together definitely require dyno tuning. The good news is that power will certainly go up once you're correctly tuned.
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  3. redstang50

    redstang50 New Member

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  4. Pontisteve

    Pontisteve New Member

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    If you've got an SCT dyno tuner near you, I would go that route. While a lot of hype has been made about buying mail-order tuning or chips, the truth is that putting a car on the dyno and measuring the actual air/fuel ratio with good equipment is the only real way to do it. When you change a MAF meter, air intake piping, camshaft, or heads, you really can't accurately predict what the motor needs. You have to measure and analyze, which is done on a dyno. Remember, air/fuel ratio is only half the battle. There's timing requirements, idle airflow and cranking characteristics, and several other tuning issues to consider.

    Now if you have a bone stock motor, and wanted to change one simple thing, like fan on/off speed, or injector size, or something like that, then a mail order chip would be fine. But start putting several non-matching parts together, and you quickly lose the ability to mail order tune.

    It would be like buying an expensive suit, but instead of having it tailored locally, you just describe to some guy over the phone what you look like, and without using a measuring tape. A guess at best.
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  5. redstang50

    redstang50 New Member

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    okay, that makes perfect sense. :rlaugh: now i just gotta find a dyno shop close to me. thanks again for all the help! :flag:
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  6. Pontisteve

    Pontisteve New Member

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    Try the dealer locator at sctflash.com, if you want an SCT dealer.
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  7. Pratice

    Pratice New Member

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    Reinstall the 19# injectors. They're good for 300hp..As stated earlier 30# injectors are over kill for your combo unless you get a tune.
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  8. redstang50

    redstang50 New Member

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    i found an sct dealer near me, but they dont have a dyno. i'll call them tomorrow and see what they can do.


    i would go back to my 19's but i dont wanna go back to my stock 58mm MAF (and i dont wanna buy a bigger one calibrated for 19s) i might as well just get the tune. that way i'll be ready for a supercharger :lol:
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  9. Pontisteve

    Pontisteve New Member

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    That depends on the fuel pressure, duty cycle and the brake specific fuel consumption. I tuned a 94 GT with bolt-ons and 24# injectors, which made 245 at the wheels (about 300 at the flywheel), and the 24's were all but done. Anything over 300 really required 30# injectors in his case.

    If you push the injectors to a higher duty cycle than the optimum .80, and assume a very fuel efficient motor (.45 bsfc), then you could maybe get away with 280 flywheel.

    Fuel Injector Max Flywheel HP = (Injector Lb/hr Flow Rate x # Injectors x .80) / BSFC

    Required Injector = ((Flywheel HP * BSFC) / # cylinders) / Desired duty cycle

    Assuming 85% duty cycle max and bsfc of .50, your 19's would only provide 258 flywheel HP. You could turn up the fuel pressure to provide a higher flow rate (assuming you have adequate pump supply), but you would have to use the Bernoulli equation to find the new lo and hi slopes, and enter that into the tune. Otherwise, she'll be rich down low.
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