EFI 351W won't rev past 6000 - HELP!

I have an EFI 351W in a '90 coupe which will not rev past about 6000 RPM. When it gets to that point, it falls flat and burbles and pops as if it was having an ignition misfire. This problem happens at WOT and part throttle, with the stock A9L program, a modified Tweecer program with higher rev limiter, and with a Ford RPM Extender with a higher rev limit. Datalogs indicate that the air/fuel ratio is fine, even at the point where it falls over, and this coupled with the fact that the problem happens at part throttle too seem to point away from a fuel pump or injector capacity problem. The ignition is a new Mallory Hyfire VI-A and MSD HVC-II coil, and even with this new ignition the problem remains and is exactly as it was with the stock ignition. The cap and rotor are extremely low milage and look brand new. The distributor is also new, but is a truck distributor for an EFI truck.

The engine configuration is as follows:

- 351W, 11:1 compression, .030" over, balanced, square decked, torque plate honed, etc.
- Comp XR292R solid roller cam, 254/260 (@.050) duration, 621/627 lift.
- Victor 5.8 intake, ported
- TFS Track Heat heads, ported, 2.02" intake, 1.60" exhaust
- K-Motion K-800 springs, installed 1.78" high. About 190# seat pressure and about 450# open pressure
- 1.75" long tubes, 3.5" dual exhaust.
- Mallory Hyfire VI-A ignition, MSD HVC-II coil, Ford EFI truck distributor and TFI module.
- A9L EEC-IV engine control, with Tweecer
- 90 mm throttle body, 95 mm MAF, 30# injectors, 190 LPH in tank fuel pump
- Engine is naturally aspirated.

I am looking for any considered opinions on why this engine cannot rev past 6000 RPM. Mechanically, I don't see any reason why it shouldn't, but I am starting to run out of ideas. What do you think?

-Matthew

Thanks for your input - I didn't get a chance to work on it today because I had other cars in the shop all day, but I plan on running a cylinder balance test, then I guess I'll replace all the plugs on spec if that doesn't reveal anything.

RE: PIP value - I have it at 300 in the Tweecer program, but remember that the problem occurs with the tweecer, with the stock program, AND with an SVO Extender. This leads me to believe that it is not a control system rev limit that I am running into.

RE: A/F Ratio - I obtained data on a Mustang MD-600-HP dyno with wideband and the A/F ratio looks steady throughout either a power run or a step test, even when the car starts to stutter. If I was running out of fuel, I would expect to see a lean trend when it was stuttering. The car is making about 360 HP at the tires, corrected, however the correction factor in Calgary is around 1.13 most of the time. The injector duty cycle appears to be about 80% maximum, or about 16.5 ms pulse width under full load at the point that it experiences the problem. Also, the problem occurs on a part throttle step test, which really points away from fuel delivery.

I'm sure that there is a simple explanation for this, but it sure seems like a weird problem. The car should rev to around 7000.

-Matthew

Stang951 said:

Sounds like ignition to me. You may also want to check and make sure that you are getting enough voltage to the distributor and the ignition box at around that RPM.

Definitely soudns like ignition though.

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I thought that it had to be ignition, but I've begun to doubt that since putting in a way more powerful and higher voltage ignition system did nothing. I like your idea about the plugs though, and I have them gapped fairly wide - .043 or so - so I will try it from that angle. The wires are pretty new as well - about 3000 km on them is all - but I guess they could be a problem too.

I'll also log the battery voltage during a pull to ensure that there are no drop outs, but everything looks good on the gauges.

-Matthew

Not yet - I've had a bit of the flu and when I haven't been fighting that off, the shop's been tied up with customers. Plus, it snowed! Thanks for asking, I'll keep after it. -Matthew