How many teeth needed on trigger wheel

[Andy_Stprbeck]

I am in the process of microsquirting a WR450, the stock crank trigger is one large tooth machined into the flywheel, it would be difficult to add a trigger wheel with more teeth, so I'm wondering if this is enough?

It's going to be fuel and spark.

How do you determine how many teeth are needed? As far as the accuracy of the timing of the spart, I think the stock single "tooth" is positioned so it is just before the spark (so the tooth is something like 20deg btdc, leaving only what <10 degrees before the actual spark?), so there is little error in crank position at the time of the spark, but it can only calculate rpm once per revolution, so it seams like if rpm changes very quickly (which is probably will in this application) that would be the limiting factor???

[SQLGUY]

You can run with that, though you'll need to move the pickup or tooth to get your tach signal somewhere ATDC so you'll have sufficient dwell time. With one tooth, though, you're probably going to see some fairly erratic timing at lower RPM's. The fact that your engine has a flywheel will probably help a lot, though.

For my SECA 750 I originally had a one tooth setup, but I had about +/- 20 degrees fuzziness on timing at idle. I made an 8-1 wheel instead, and that improved idle accuracy to +/- 5 degrees or so. Above 2K RPM, the accuracy tightened right up with either setup.

Can you post a picture of the current trigger setup?

[kiwibum]

For my SECA 750 I originally had a one tooth setup, but I had about +/- 20 degrees fuzziness on timing at idle. I made an 8-1 wheel instead, and that improved idle accuracy to +/- 5 degrees or so. Above 2K RPM, the accuracy tightened right up with either setup.

This is interesting as I have a similar issue with a V-twin (only one tooth for each cylinder). How often do we ride under 2K RPM? Idling at traffic lights waiting for them to change is the main time I can think of. My bike idles at 1300, normally ride 4-8K and redlines at 10K. How much difference does this fuzziness make, just a rough idle?

[SQLGUY]

The result on mine was a very rough and inconsistent idle.

The thing is, the stock ignition system is much more accurate with just a single tooth, because it's not waiting 270 degrees or more to fire; it's firing pretty much when it gets its tach pulse.

Of course, with a lot of V-Twins you actually want some huffa-chuffa to the idle. An I4, though, should idle smoothly, IMO.

[kiwibum]

I guess I'll just have to try it and find out. I'm in the same boat that there isn't really any way to fit a toothed wheel without doing a huge amount of work which I'm trying to avoid initially. My V-twin is only a 550 and revs quite hight so there isn't a lot of huffa-chuffa to the idle at 1200 rpm compare to larger twins.

[Andy_Stprbeck]

Thanks for the response guys.

SQLGuy: I'm not really sure where the single tooth is in relation to tdc, I'll have to check that, I was just repeating the explanation that I had heard for the one tooth thing from somebody else on this forum that kind of made sense.

Why would the stock ignition be more accurate with one tooth than the microsquirt? Stock setup is some sort of "black box" computer controlled deal, I would be using the same "coil on plug" coil and the same sensor with the microsquirt as is stock, so pretty much the same dwell time I think ??

I guess I'll have to go check where the "tooth" is relative to tdc. It would be difficult to change the orientation of it because it is machined into the flywheel which is keyed onto the crankshaft.

The "flywheel" I'm refering to is the stock thing with the magnets that spins around the stator, that may not be technically called a flywheel I guess.

I'll post a picture when I get a chance

thanks
Andy

[Andy_Stprbeck]

Alright this lets see if I can figure out the picture thing.

This is the "tooth" hard to see in my fuzzy pic but it's a very long raised portion machined on the outside of the flywheel. In this picture the piston is near tdc.

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This is the mag cover, the hole on the top is the "window" you look through to line up a mark on the flywheel with a mark on the cover that line up at tdc. On the right is the VR sensor that reads the "tooth" on the flywheel. My halfassed measurements with a protractor put these at 90degrees from each other.

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This is the flywheel. The top of the protractor is lined up with the "tdc" mark, and the bottom is lined up with the beginning of the "tooth", so they are about 165 degrees from each other, so the tooth hits the sensor about 75 degrees before tdc. The end of the tooth is about 10 degrees before tdc.

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So the rising edge of the tooth should come well before the spark. Assuming spark will be 20-30 ish degrees before tdc, that leaves 45-55 degrees of rotation between rising edge of signal and spark.

I'm not sure how Microsquirt works for this, but if my dwell time is around 2msec, which I think would be relatively low (?) that means that at anything over about 4000rpms 45deg of crank revolution is not enough time for the dwell, so it seems like if the signal from the crank position sensor comes with not enough time to start charging the coil for this revolution, it would wait until the appropriate time for the next revolution, so that the spark is always based on the conditions of the last revolution. Not sure if things change enough in one revolution of the engine for it to matter?? I could see how it could be a problem at very low rpms/idle when rpm is probably not very consistant, but then there is enough time for the dwell to happen on the same revolution so???

Guess I'll just have to try it and see if it works, not much I can do about it.