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Setting Your Decoder Offset 
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If you want to statically setup your timing before kicking over the engine for the first time to reduce starter wear or because you're a one man band, then you may need to read this.

I'll use MissingTeeth as an example.

In the MissingTeeth decoder the centre of the first tooth to come past is considered zero degrees. If when you setup the wheel that first tooth isn't at TDC then you'll need an offset. But how to figure out what it is? With FreeEMS to make the timing more advanced you increase the offset and to make it more retarded you decrease it. The offset must be less than the decoder total angle (360 for a crank mounted missing tooth setup), you will get no outputs if it is larger.

If the position that you know is really zero is 60 degrees away from the position of the first tooth post the gap, then your offset is one of two values:

  • 60
  • 300 (360 - 60)

This depends upon whether it is before or after.

Make the offset number larger to advance the base timing, make it smaller to retard the base timing. For example, if you have 10* BTDC in your table, flat, and a timing light shows 5btdc on the engine, then increase this number by 5 degrees. The same applies statically, except you have to think more.

Fred.

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Wed Jan 11, 2012 2:50 pm
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Fred wrote:
Make the offset number larger to advance the base timing, make it smaller to retard the base timing. For example, if you have 10* BTDC in your table, flat, and a timing light shows 5btdc on the engine, then increase this number by 5 degrees. The same applies statically, except you have to think more.

Fred.


Nice hints Fred, I was used to be puzzled on how to figure out my own decoder offset. My engine service manual states vaguely that the offset is 144 degree BTDC without pointing out which tooth to be considered zero degrees. I almost decide to visit a service shop to see the alignment of VR sensor and missing teeth flywheel, fortunately with your hints, I can use my Equus timing light to figure it out, saving me a lot of effort


Tue Jan 17, 2012 9:27 am
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Be careful, reading my post again, I forgot to mention that it's relative to where the sensor is. So if your first tooth post missing is exactly at TDC but your sensor is offset by 70 degrees then you've got 70 or -70 offset. Likewise, if your sensor is offset 87 degrees and your first tooth post missing lines up with it exactly when the engine is at TDC then your offset is zero. Make a little more sense? I think there is a diagram about this. Also, you had me super excited, I thought you'd run your car because it looks like I put this thread in the wrong section! Woops. I'll move it now and then add a photo.

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Tue Jan 17, 2012 9:34 am
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Fred wrote:
I'll move it now and then add a photo.

I think this diagram is what you are looking for, in my understanding, the offset means total delay teeth degrees in the diagram, is that right?
Attachment:
misstoothexample.gif
misstoothexample.gif [ 14.49 KiB | Viewed 2760 times ]


Tue Jan 17, 2012 9:42 am
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I recognise that diagram and know where you stole it from, but that's OK I guess, so long as it wasn't me :-)

Yes, for the above image you've got 360/32 = 11.25 degrees per tooth and 11x that figure = 123.75 of an offset. If you're looking at it from the front then that number of degrees is how much too advanced it would be if you tried to run it. Thus in our scheme you would need to subtract that number from 360 (for a crank based setup as shown) to get your FreeEMS offset. So finally you would have 360 - 123.75 = 236.25 as your desired/required offset. Sadly FreeEMS can only be dialed in down to 0.02 of a degree, so you'd have to round that up to 236.26 or down to 236.24 to use it, however the compiler and/or UI will take care of that for you.

Thanks for posting, it should be pretty clear by now.

Fred.

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FreeEMS.org - the open source engine management system
FreeEMS dev diary and its comments thread and my turbo truck!
n00bs, do NOT PM or email tech questions! Use the forum!
The ever growing list of FreeEMS success stories!


Tue Jan 17, 2012 10:14 am
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