Forced Induction Superchager Help

[nickstone302]

I have been informed that I have to run a fmu from one well know mustang technician and from another guy( my car tuner) that I don't need one but with a good tune an fmu isn't needed. Any help is appreciated

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[Noobz347]

If you install injectors capable of supplying the fuel necessary for your engine combo and have it digitally tuned to operate those injectors, then you do NOT need an FMU.

An FMU is a device that is used to increase fuel pressure by a LOT in order to force undersized injectors to operate outside of the fuel pressure ranges that they were initially designed to operate in. It is a band-aid (more or less).

The better of the two options is to replace the injectors with units that are specifically designed to provide the fuel necessary to feed your engine combo at STOCK fuel pressures and a tune that ensures your air fuel ratio is perfect throughout the power band.

Both methods are tried and true (so don't get me wrong). The FMU option will be the cheaper option. The injector and tune will be the most accurate and least likely to screw-up option.

 

[nickstone302]

I've got 38 lb injectors and car will soon b supercharged prob 375rwhp

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[Noobz347]

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
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.

You're WELL WITHIN the capability of a 38 lb injector to supply fuel to that combo. 36 lb injectors will support 576 naturally aspirated HP at 100% duty cycle. At the power level you're suggesting, you would be at roughly 65% duty cycle normally aspirated. Add a ballpark 5% for boost and you'll be at 70% duty cycle. You still have another 15% duty cycle left before reaching the maximum suggested duty cycle of 85% and that is by no means the limit of the injector. The 85% is rule of thumb to follow when selecting injector size for an application.

By the way... there's no FMU that I've ever seen or heard about that will run a 38 lb injector for that size engine combo. Any FMU you could find would make that injector squeeze WAY more fuel than what is required for 375 HP.

One last edit: The chart above assumes crank HP. Your crank HP will be roughly 430 ish if you make 375 at the wheels. You're still more than 100 HP away from maxing out those injectors.

 

[nickstone302]

Thanks for ur help bud I appreciate it.

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[Noobz347]

Re-accomplished with 430 flywheel HP.

75% duty cycle for a 36 lb injector N/A.
5% added for boosted app = 80% duty cycle.

Still 5% below the sweet spot and 20% duty cycle margin of safety at stock fuel pressures.