im running 25lbs ls1 injectors at 37psi but going to be mounting on a supercharger with in a week or so and didn't feel like running with the fmu so should i jump up to bigger injectors to be safe?

Hell yes. The size of the new fuel injector depends on how much Hp you intend to make. Do a search to find the appropriate injector equation.

Yeah like he said above. FMU's are just a "band aid". Do it right and run bigger injectors.

You have three choices (or a combination of them)

1) use the 25#/hr injectors with an FMU (little or no ECM tuning)
2) get bigger injectors and ECM tune
3) run a water/alky kit with a good percentage of alky (progressive controller)

Is the FMU good to have even if you have decent size injectors and tuned properly?
I had always thought it was better to put on there anyways.

I need to find that formula to calculate injector size for projected power.
At one point I was considering a blower setup and was hoping 24lb'rs would be sufficient to run ~350 @ the wheels.

No way… extra pressure means extra load on the pump, and extra load on everything in between, FPR diaphragms tend to rupture more often… it’s also more difficult to tune when burning chips since the delivery rate of the injectors is going to be modified by the FMU.

350hp = about 435bhp = 32pph @ 100% DC with 8 injectors/.60 BSFC = 38pph at a more reasonable 85% DC.

I’d probably go with some 42’s or bigger to give you some room to grow.

Christ, I was totally off, thanks.
42's seem huge though!

So the fuel delivery can all be done in the tune and ditch the FMU huh? Interesting.

83 Crossfire TA is correct about the injectors. 42#/hr are not huge for a boosted v8 rig. I run 80 to 90% duty cycles with 42#/hrs on a v8 355ci at 7PSI of boost with a 216* duration cam at a WBO2 11.9 AFR and stock fuel pressure.

Ford 42#/hr were cheap but they raised the prices. The Ford injectors work great with the $8D code, but take a fair amout of tuning with the $58 code. Take a look at racetronix - index for some siemens deka injectors. Good deals are found on ebay every so often.

I actually just picked up a set of new Ford 24lb blue tops off eBay for $225.
I've heard great things about Racetronix too and will be getting the high pressure pump + harness.

Was planning on running a blower with minimal boost, 6-7psi, but put that on hold for now.

I'm using $6E and I see guys pushing more power than I would be with 24#'rs, but just wasn't sure if I should make it work with that FMU.

The Walbro 255 LPH pump is a good buy for $100. If you plan to keep the 165 MAF setup then using the FMU is your best choice in my opinion. You will be fine with 24#/hr and a good adjustable FMU.

Just an FYI:

For a turbo or supercharger with 42#/hr injectors, we have the following equation:

engine Hp = [42 x 8 x 90%] / 0.60 = 504 Hp


For an NA 3rd gen motor, we have:

engine Hp = [42 x 8 x 90%] / 0.50 = 605 Hp


The Hp values above represent the max Hp that you can make which is supported by the 42 #/hr injectors. Because a boosted engine is actually less fuel efficient, that is why the NA motor can deliver more Hp per same size injectors.

well im make 300rwhp with my 25lb at 37psi so im thinking that with the charger the 42lb injectors should be right on the money but one other thing that has my attention well ok two things is i still have a stock mass air flow sensor and stock throttle body do you think they will be fine? or upgrade to something else?

I run a MAP Sensor and report the calculated airflow. From memory I hit 255 gm/sec at around 4300 RPM at around 4-8PSI of boost (can't remember). The stock MAF will only read up to 255 gm/sec of air. At 6000 RPM and 7 PSI I read about 400 gm/sec. So yeah, the MAF will be WAY maxed out.

I run a stock LT1 intake with a stock LT1 throttle body (48mm). It works fine for my purposes. I would rather spend the $300 on other stuff. If you have an extra $300 left over after the S/C install then you might as well get a 58mm.

On our stock 355 TPI (with mild blower cam) with a P1SC I took everybodys advise and bought flowmatched green top 42's

Really? What’s the problem? The only ford injectors that I’ve run under multiple codes were some navy top FMS 36’s, and I’ve run those under the original ’87 165 stuff, both later 165 codes (forgot the broadcast codes), then under a 730 wired like a 749 and then under a 749… I’m not sure but I might have noticed a little difference betweenthe 730 and 749 running the same code, but otherwise I don’t think I saw a big difference no matter what ECM/code I was running