hmmm... what's this?

[Moderator]

This snapshot  of an MG Midget vintage racecar apparently shows a Hall effect sensor aimed at a trigger wheel mounted adjacent to the crankshaft pulley... except I don't see any teeth! I'm guessing there's just one tooth on the trigger wheel. Most ignition and fuel injection systems have many more teeth than that (e.g. 35 or 58) to facilitate more precise measurement. Incidentally, this car has a Mallory Unilite distributor and an MSD6AL box. It has a Jones mechanical tach.

So what's this sensor for?  

Normally a four-stroke two-cylinder engine fires twice per crankshaft revolution, right? Every second spark is wasted. My guess is that someone has figured out how to fire their coil just once per revolution... so that the ignition coil has twice as long to build up a charge and therefore will provide a stronger spark at high rpm. Are you guys aware of any commercially available product which would perform this trick?

Do you have a better guess for what this sensor is doing here?

[burner1]

Hard to tell without asking the guy but it could be timed to the teeth in the belt pully. There is plenty of mass in the teethed pully to trigger it. I know someone who addapted a setup for a Contenental 1790 and in experimenting out of curiosity he moved it away from the toothed wheel and it was running fine over 6" away so I would say the proximity would work whith what I see.; but then again tis is nothign but forum speculation.

[Moderator]

You may be right about the pulley teeth and he might even have filed off a tooth or two to give a position reference. I like your idea, but in this case my intuition says the owner has done otherwise. I see a plate outboard of the pulley that's maybe 4 or 5mm thick. (I think maybe it has a spot of red paint on it.) What else would that plate be for? I've been assuming its the trigger wheel. I'll ask the racer, but I'm not hopeful about getting a straight answer. He's both clever and very, very cagey!

Now, about his MSD6AL box... It sure seems fishy to me!  First of all, I don't believe this particular driver would have any want or need for a rev-limiter, so even if he were going to use an MSD he probably wouldn't pick that model. I wish I'd taken better photos of his wiring so I could trace it. I have an idea that maybe he tossed the MSD electronics and put something else inside the MSD case. That would be consistent with some of the other tricks I've discovered on his car.

[flitner]

Infrared tachometer?

[NCtim]

If he's running a distributor and it's cam-driven, there shouldn't be a wasted spark as the cam turns half as fast as the crank. In my Midget days I'm pretty sure my cam drove the distributor and that meant only two sparks per crank rotation to the corresponding cylinder.

Tim