Rage

If one would want to convert a Mercruiser 496HO from the Merc PCM555 ECU to the GM MEFI4B ECU will the GM MEFI4B ECU plug into and work with the existing Merc PCM555 ECU wiring harness and sensors or what of these items will need to be replaced?

bobl

You will need the harness also. Did you talk to Tyler? He sells the ECU/Harness as does Whipple. If I were doing my personal engine that's the way I would go. I think the cost is around $1200, but I'm sure that would include a custom calibration. Plus, I can program the MEFI computers.

Rage

Thanks! I am not really serious about changing to GM ECU at this point. Actually that is the last thing I want to do. I am just checking out options and their cost.

I did talk to Tyler about the fuel pressure regulators. You are right, he has them on the shelf but suggested I try to see what I can get out of the stock unit first since there is some adjustment available. Thanks for the suggestion!

Dave_N

The PCM555 can (and does) use sequential port fuel injection, the MEFI4 can only do batch fire. The other thing that comes to mind is the 555 uses a pulse width modulated (Ford) IAC motor and the MEFI uses a stepper motor (GM) IAC.

Dave

Rage

Dave,

Really! The more I learn the dumber I feel.

If you would please describe 'batch fire' that the MEFI4 uses.

If you would please describe the 'stepper motor' approach that the MEFI4 uses.

Dave_N

Batch fire refers to firing more than one injector at a time. The MEFI4 ECM only has two injector driver circuits, each one firing four injectors simutainiously on each engine revolution. The PCM555 has eight injector driver circuits so it can time the injection of each cylinder to the intake stroke. However, sequential port injection is really only mariginally better than batch fire as far as producing power. Maybe thats why Merc rates the 496 HO at 425HP, while Volvos 8.1 HO (MEFI4) is rated at 420HP. But thats just a guess on my part.

I believe that due to the fact that it has eight injector drivers the PCM555 can also do individual cylinder fuel trim (knock prevention), although there seems to be some disagreement about that (as you know).

A stepper motor IAC has two or more windings that receive a signal of alternating polarity to move the motor armeture (and pintle) a predetermined amount (one step). The ECM never really knows where the IAC is, it only knows where it should be based on the number of signals it has sent to it since it was last reset (it resets every time you shutdown the engine or do a IAC reset with a scan tool).

Dave

tomcat

Hi Dave:

Have you ever taken a look at the crank trigger on the 525EFI? There are teeth missing and not in a symmetrical pattern. What's up with that?

My theory on the individual knock control is that there are two knock sensors so the computer knows which side is detonating and enriches one cylinder at a time until the knock stops, then backs off on the previous ones. If sequential fire does not add HP that's the only reason I can think of for having it. Chances are the computer learns which cylinder was knocking and enriches that one first the next time it happens.

That's my theory and what it is too.

Raylar

Tom & Dave
The sensor wheel on the front of the HP525 is a 60-2 reluctor type wheel that a MEFI4 ECM won't read. The MEFI uses a split sided 24X reluctor The two missing teeth (60-2) let the ECM find #1 cylinder for spark and fuel timing. The knock sensor system in the PCM 555 is triggered strickly by the knock sensor outputs and the PCM 555 also uses a Batch A - Batch B injector firing sequence just like the MEFI.
thats why there are two knock sensors one on the right, one on the left.

Regards,
Ray @ Raylar

Rage

Dave,

Thanks for the explaination.

I suspect I am missing something major here, not unusual. My layman's undestanding of a V8 IC engine is that there are eight individual power strokes none of which happen simultaneouisly to any other ie. 12345678 firing order. If four injectors fire simultaneously(??) every engine revolution (again I assume at the appropriate time for each individual cylinders power stroke) that would seem to mean that each cylinder gets three additional shots of fuel before the appropriate time for its power stroke. Where did I go wrong here?

I assume that the IAC is the fuel injector?
The amount of movement of the IAC stepper motor armature I assume determines the amount of fuel that will be delivered when the injecter fires or is the armature movement itself in fact the firing of the injector? Does this armature movement effect the time the injector is open and flowing fuel when signaled to fire or other? My understanding of the PCM555 is that it adjusts fuel per shot by the 'pulse width' or time the ECU electronically holds the injector open and flowing fuel.
It sounds like two different types of injectors each of which would not work with both ECU's. Again where did I go wrong here because no one has ever mentioned having to change injectors when switching between these two ECU's?

Thanks Dave.

Pat McPherson

The IAC is the idle air control. It adjust the amount of air that bypasses the throttle plates in the throttle body.

The fuel injectors are an electrical solenoid driven valve; just open and close...