[Diy_efi] Why I asked about the GM V-6 MPFI

Joe Boucher boucherj
Sun Aug 13 12:27:37 UTC 2006

I have a '89 325i.  I also came across a turbo just right for a 2.5 liter
engine.  Turboing the 325i is a big temptation plus playing with the fuel
and spark maps.

BUT, as best I can tell, no one has hacked the Motronic 1.1 FI computers to
any kind of detail like the GM ECU's.  Which surprises me, as the size of
the memory chip used in the Motronic is really small.  2k or 4k?  And that
is the size of the maps and programming.  But I could be wrong.

So, the voices in my head keep telling me to piggy back a GM system onto the
2.5.  The list of road blocks to do this are short.

Engine timing.

The MPFI system would require some kind of crank trigger.  The spacing of
the disk slots on the crank could be copied on to a computer controlled,
laser cut disk and the disk sandwiched between crank pulley and the harmonic
balancer.  The BMW crank position sensing system (60 minus two knobs) is on
the harmonic balancer and the bracket could be useable for mounting the GM
sensor or maybe the BMW sensor would work.

Or, use a TBI system.  There is a lot of room in the BMW distributor cap.  I
haven't looked at the details, but mounting a GM HEI reluctor ring (the
pointy thingies) might be easy.  But turboing would be easier with 749 ECM

Speed input to the ECM.

The hardest part of converting my carbed '81 Suburban to an '89 TBI system
was the speed sensor situation.  The 700R4 on the back of the TBI engine I
bought had the electronic sensor and not the cable drive.  After installing
everything, successfully running the car down the road, THEN figuring out
the output of the tranny speed sensor needed conversion to a step wave, I
should have pulled the tranny and had the output shaft converted to a cable
system and bought the in-line speedometer cable sensor.

The BMW uses a sensor on the differential which detects the pinion ring
teeth.  I doubt the number of teeth will equal 2000 or 4000 pulses a mile.
Some kind of converter circuit is possible, but I can't do that.

Knock sensing.

BMW didn't start using knock sensors till 1991.  Mounting a GM knock sensor
on the bimmer block might be a challenge.  I have a Bosch knock sensor off a
Volvo, but am not sure it operates the same as a GM sensor.  I don't HAVE to
have a knock sensor, but it sure would be nice.

Any thoughts would be appreciated.

Joe B.

More information about the Diy_efi mailing list