fms injectors

[Bob Reynolds]

I think I asked this question before but I didn't get a response. Has anyone here used the Ford Motorsport 24lb/hr injectors in an obd1 lt1 without having the computer tuned for them? There flow is rated at 38 psi fuel pressure and stock gm injectors are rated at 43.5. I've read where lt1 fuel pressure can run anywhere fron 41 to 47 psi static and 2 to 10 lbs less engine running.
Thats a big spread and I wonder if the stock computer can adjust pulse width enough to make up the difference in injectors.

[Injuneer]

Stock GM RP injectors are rated at 3bar (43.5psi). The specs on the fuel system are 43.5psi +/-2.5psi, so operating pressures from 41-46psi are acceptable. The fact that the pressure is "less engine running" is irrelevant. That is the vacuum compensation reducing the rail pressure so the net DIFFERENTIAL pressure across the injecter (rail pressure minus manifold pressure) stays constant at 43.5psi.

FMS injectors are rated at 2.7bar (39.15psi). Since flow is proportional to the square root of the pressure, this would theoretically cause the FMS injectors to flow 5.4% more fuel. But... "24#" is a "nominal" rating. The GM 24# RP's are actually programmed in the PCM as 24.896#. So, unless you know the exact number for the FMS injectors, you can't really evaluate the EXACT flow difference. You also need to consider the time required to open and close the injector, and a pintle style injector (FMS) is generally slower than the ball and seat RP style injectors.

Since its fair to say the flow difference is probably less than 5%, this is well within the ability of the PCM's long term fuel corrections (-15% to +25%) to handle. I found when I installed the FMS 24# injectors, with no flow constant changes, the BLM's went up, partiularly at lower flow rates, indicating they didn't flow as much as the RP's. I also didn't like the fact that they didn't fit correctly in the rails, with regard to the clip location. But they will stand up to the kind of pressures required by a dry nitrous system, so I used them.