Megasquirt - Trigger Wheel Question

[kev_the_mole]

Thought I'd ask here before straying onto the MS forums. You may be kinder to my stupid ideas.

I'm collecting bits to convert a Healey 3000 6-cylinder to EFI as a bit of a practice before buggering up my V8. I didn't much like farting around fitting a 36-1 wheel to the front of the Chevy and it struck me that the MS goes down to a 12-1, coupled with the fact that the dizzy turns at half engine speed my crackpot theory is to fit a 24-2 (each missing tooth 180 degrees apart) to the dizzy spindle as I can make these easily.

Advantages : The MS gets a 12-1 signal each rev of the crank, I don't have to take the rad out and everything is much easier to fit.

Disadvantages : Less accurate than being on the crank? Will the VR sensor be able to pick up the much smaller teeth and gaps?

Over to you guys.

Cheers,

Ian

[katanaman]

don't know the answer but I was thinking about this as well. Not to use as the main trigger but in place of the cam trigger for sequential stuff.

[kev_the_mole]

you could have two or more completely different no's of teeth on the same spindle controlling ignition, sequential and even fuelling for variable valve timing

Who wants to go first?

[ChrisJC]

The VR sensors are amazingly good. I happen to have a 36-1 wheel & VR sensor set up as a stimulator, and although I've used a specific chip to interface to the VR sensor (NCV7001), and I don't know what the MS uses, I can't see any reason why it won't work with a large 24-2 wheel.

http://www.trigger-wheels.com have some 'scope traces on their website which give some idea of the engine rev range that it works with.

I guess if the wheel will be tiny it might be an issue, maybe it's worth making the wheel and trying it on an electric drill with VR sensor first....

Chris.

[bill shurvinton]

24-2 is used in a number of applications as jap cars use that. If I ever get my westfield back on the road then it will be using that config. You will not get quite the rock solid spark timing of a crank wheel, but it will certainly work. As long as there is at least 1 tooth per ignition event then you will be fine. It gets upset trying to time off a gap.

[kev_the_mole]

Chris,

what I'm trying to do is put this "petite" trigger wheel inside something not much bigger than the the dizzy cap. Typically a 10 mm pitch timing pulley of 24 teeth is no greater than 75mm diameter. I'll ask a mate with a scope to see if I get an acceptable output from the VR sensor

[ian.stewart]

If I am reading this right, fit the trigger wheel to the dissy shaft, the advantage of the trigger wheel is there is no inherent timing movement from Chain stretch and gear backlash or once its fixed to the crank, thats it, it does not move and is absoloutly constant to the crank,
But when I was at Emeralds last time dave had a laser cut timing wheel to go in a dissy, it was a running optical trigger, the wheel was about 0.4mm thick and 50mm dia, and weighed nothing.

[ChrisJC]

Chris,

what I'm trying to do is put this "petite" trigger wheel inside something not much bigger than the the dizzy cap. Typically a 10 mm pitch timing pulley of 24 teeth is no greater than 75mm diameter. I'll ask a mate with a scope to see if I get an acceptable output from the VR sensor

I would expect that as long as the teeth are of a similar size to the larger wheels, then the wheel diameter and overall no. of teeth would be irrelevant.

Chris.

[Eliot]

If I am reading this right, fit the trigger wheel to the dissy shaft, the advantage of the trigger wheel is there is no inherent timing movement from Chain stretch and gear backlash or once its fixed to the crank, thats it, it does not move and is absoloutly constant to the crank,
But when I was at Emeralds last time dave had a laser cut timing wheel to go in a dissy, it was a running optical trigger, the wheel was about 0.4mm thick and 50mm dia, and weighed nothing.

indeed, aim to put it on the crank.
can you not sandwhich the thin ford wheel on?

http://www.mez.co.uk/ms11.html#mattstrigger

[bill shurvinton]

I can't seem to find the pics on the web of the mazda CAS, so If I remember I will toddle up to the garage and photograph the one I have.

Although I don't recommend a dizzy mount solution it is easy to get going and nothing stops you going to a crank mounted wheel later.

[kev_the_mole]

Actually I think that a plain vanilla dizzy install on the standard Healey is going to be about 100 times better than what it's got at the moment. I've been talked into a series of articles on this conversion for the Healey club magazine and the easier it is for the men in cardys, slippers and flat caps to copy it the better. I'll get a before power and emissions figure on a rolling road and an after with a dizzy install and then maybe a crank trigger.

Thanks for all your help and good advice.

Cheers,

Ian

[bill shurvinton]

By vanilla, do you mean a locked distributor and MS triggering off that? In which case that is fine.

BTW I have just had a cancellation so I can convert a built unit for you.

[kev_the_mole]

By vanilla, do you mean a locked distributor and MS triggering off that?

Yes

BTW I have just had a cancellation so I can convert a built unit for you.

Great