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Number of teeth for Trigger 1

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Hi everybody 
In software help, it is recommend to use number of teeth must be divide evenly into 360 such as: 60, 30, 24, 12.
But my engine is one cylinder with 33 crank teeth (360/33 = 10.909).
Is it Ecu (fury) will working correctly with high RPM (10000+)? 
And in help, it recommends like what 32 teeth is an only exception case?

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It is a requirement for 360 to divide evenly (in whole numbers) by the number of teeth on the crank trigger wheel (including any missing teeth). 32 tooth wheels are an example of what will not work. 48 tooth wheels will not work either.

Common tooth counts that work are 12, 24, 36 and 60.

33 teeth will not give stable control and will cause problems.

Scott

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 Can you please expand on this topic of the no. of trigger teeth. Why is it necessary for the tooth count to be a whole no. factor of 360. The fact that we have 360 degrees to a circle is just a convention. In Germany they used to (and maybe still do) divide a circle into 400 divisions and called them grads.What is likely to happen if the tooth count does not divide into 360. What would you notice. On my parallel twin cylinder motorcycle engine I have 16-2 trigger teeth. The teeth are milled on one of the crankshaft webs. The gap is opposite the crankpins. This was done to give clearance for the piston skirt when the piston is right down. At the time I drew this up I was not aware of the need to have the tooth count  divide into 360. Now  360 divided by 16 = 22.5 , not a whole no. but the engine seems to run OK. Could this be because the engine has evenly spaced firing intervals, that is once every turn. Maybe I was just lucky. Suppose the engine was a V twin with uneven firing intervals, what would happen.

 John

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What it will cause is uneven firing events due to rounding errors in the ignition calculation. It can compound for each tooth so can result in up to a 30deg error worst case.

 

 

 

 

Edited by Simon

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Just to add to what Simon has said. The engine position calculation is performed by dividing 360 by the number of teeth. The result is truncated to a whole number. This means that the error increments as more teeth go by the sensor. When the gap in the teeth appears (or the trig 2 sync on a crank wheel with no gap) the error is reset to zero. 

So depending on the amount of error you have, the incremented total error could be large or small. For John I would expect an error of up to 8 degrees (16 teeth x 0.5 degree error). For powerboy I would expect an error up to 30 degrees (33 teeth x 0.909 degree error).

Scott

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Thank you Scott for that clarification. I have not heard it explained that way before. I understand it better now. On my engine if I have 30 degrees spark advance it takes between 7 and 8 teeth (after the reference tooth) to pass the sensor to reach the spark point. The rounding error per tooth will be 1/2 degree so the accumulated error would be about 3  1/2 degrees. My expected advance at full power will be about 30 degrees.  What if I put a degree wheel on the crank and use a timing light and set the timing correct at 30 degrees by adjusting the trigger offset value entered into the Link.The normal advance range would be no more than one tooth (22.5 degrees) either side of the 30 degree figure so the error would be no more than 1/2 degree. A consequence would be that the trigger offset value entered into the Link would not match the exact geometric position of the sensor in the crankcase but that would not matter 

 Do you think this would work or am I missing something.

 John

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Hi John, 

I think this could work. Just trying to get my mind around your suggestion. On you engine I think you're running a crank sensor only and wasted spark ignition?

Scott

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Hi Scott

 I am currently running with a synch tooth but I want to try it without the synch  and use waste spark but I don't think this will make any difference. Because either way  there will still be only one spark pulse  per crank turn coming from the Link ( but the pulse will go to both coils at the same time ). The crank position is set back to zero after every turn so there will be no difference in the timing between the two cylinders. They spark at the same time. Where the error comes in is the 1/2 degree per tooth lost as rounding error. The actual trigger offset is 160 degrees calculated from the position of the trigger relative to the reference tooth at TDC. Between 7 and 8 teeth go by the sensor from the reference tooth to the spark point.( at full advance ). This would give an accumulated rounding error of 3 1/2   to  4 degrees. Could I compensate for this simply by adding the 4 degrees to the trigger offset value entered into the Link so I would enter 164 instead of 160. This means that there would be no more than 1/2 degree error over the full range of spark advance but this would not matter.

 John

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Thanks for that . I will check with the timing light.

 One more question on the trigger tooth topic. I can see how the Link gets crank position every time a tooth passes the sensor but how does it get position in the space between the teeth. With a small tooth count of say 12 these spaces would be quite large.

    John

 

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Hi John,

Our ECU doesn't update the position/speed of the engine until the next tooth arrives.  If the engine rapidly accelerates or de-accelerates between teeth the ECU only finds out about this when the next tooth is read. This can be a problem when the amount of teeth on a trigger wheel is very low. Most OEM trigger tooth counts seem to be in the range of 12 to 60 teeth.

Scott

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Hi Scott

I confuse your words "does not update position/speed.... next tooth arrive".

Example: 4 teeths of crank sensor per rev, ignition @15 BTDC, perfect a crank  tooth and cam signal at 0 TDC.

From your words, the 15 BTDC is not possible to trig the coil because ecu will update positions on 0 90 180 270 360 ... 720 degrees.

Is I missed something.

The 15 BTDC should be calculated/approximated/predicted form time difference of two teeth BTDC (630 and 540 degree crank teeth).

Based on your word and Link ECU ignition resolution , for the most accuracy is 360 pulses on crank?

 

 

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We do forward predict based of the prior trigger events but the updating can only happen at each tooth.

The pick of triggers if fabricating would be a 12 tooth crank with single tooth cam. More then enough resolution to give rock solid timing.

Tooth counts up to 60 can be done with no special firmware required. tooth counts over 60 require custom trigger modes.

Edited by Simon

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Minimum would be a tooth per TDC for 4 cylinder or higher applications. But this gets marginal at cranking speeds.

Edited by Simon

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