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RPM fluctuation issue


Turbohead
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Hello,

We noticed that RPM is fluctuating in our Thunder ECU during all conditions. Attached are a log and the calibration as well as a trigger scope capture. Quick help with this issue would be very much appreciated as we soon have a race and would like to get the calibration done in time! 

Regards,

Ernest

 

sep27.pclr

Log 2018-09-26 6;14;32 pm.llg

Trigger Scope Log 2018-09-27 1;20;12 pm.llg

screen capture scope.jpg

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Thank you for the reply.

Concerning your comment on n=38 tooth pulley - I assumed that engine position arithmetics was done with at least 32 bit fixed or even floating point numbers, making any precision loss by non-integer divide of 360 by n insignificant... At the end, 360 degrees in a full circle is just a number chosen by convention, not really any physical background to it... Am I missing something here?

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Why has the cam moved between your two trigger scopes?  In the image above your cam rising edge is not is a good place, however in the attached triggerscope llg file the camshaft tooth is about 40 degs retarded (further to the right).  Did you move this on purpose or is there something lose mechanically?

 

6 hours ago, Turbohead said:

At the end, 360 degrees in a full circle is just a number chosen by convention, not really any physical background to it... Am I missing something here?

My understanding is it wont affect your displayed/logged RPM parameter - but it will affect the accuracy of the ignition timing due to rounding error per tooth.

Here is a quote from an older post on the same subject:

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 in your case 360/38 = 9.47deg.  You are going to have 0.47deg timing error for every tooth.  After 37 teeth that is about 17degrees error, so quite significant.

 

hkQN8cp.png

 

UwRxrO9.png

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