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Dynodom

Custom trigger suggestions please

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Hello

I need to fabricate / install a custom trigger disk on  a inline 4 engine, naturally aspirated.
The ECU is a Atom II.


What trigger pattern gives best resolution and is easiest for the ECU to work with. Numbers of teeth
and missing ones?

Sync will be used from the existing ignition distributor.

Is it better to use a Hall sensor or a inductive reluctor type sensor ?


Thank you for your advice

 

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My suggestion for the best balance of ease of setup, with good resolution, is a missing tooth crank wheel, 36-2 or 24-1 is my personal favourite.  The extra resolution that is possible in theory from say a 60-2 is barely noticeable in most situations.  For a 4 cylinder, aim to have the missing teeth pass the sensor around 90deg before or after TDC.

Reluctor or hall is up to you, reluctor gives slightly better accuracy and much more favourable signal to noise ratio at higher RPM but can be a little more fussy in terms of tooth design.  For a reluctor sensor the teeth need to be about the same size as the pole piece inside the sensor (you can use iron filings to see its size if it doesnt poke out the end of the sensor).  Also with higher tooth count wheels, the design of the "gap" becomes important - not usually important with 36-2 but if you are drawing from scratch you might as well do the proper "half depth" design in the gap like the image below.

8hRjglp.png

 

For the sync in the distributor, almost any single tooth event will work, reluctor or hall is fine.  Often you can just use the stock distributor internals and grind all except one tooth off.  The only real rule is you dont want the cam tooth to occur close to the gap on the crank.  

However if you are starting from scratch a nice option to consider (when coupled with a missing tooth crank) is what we call a "cam level" sync.  This needs a hall or optical sensor to work but is where you have a "half moon" type tooth/slot.  The advantage of the "cam level" sync is the engine only need to turn a maximum of 1 revolution to achieve full 720 sync.  Compared with a more typical single small tooth, the engine possibly has to turn 2 full revolutions before it can sync.

jDgDaze.png

 

Another important consideration for some users on the choice between a hall or reluctor sensor is you really need an oscilloscope (and know how to work it) to confirm the polarity of the reluctor sensor is correct.  Obviously this is less of a consideration with our ECU's that have a built-in triggerscope but unfortunately the Atom doesn't.

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The TWL_2 is too wide in diameter. I will have to go with the 24 tooth wheel.

Do you advise to remove one tooth to get a 24-1 or run with the full 24 teeth and why (I assume) the 24-1 is better than full 24?

Great support here. Thank you indeed!

Screenshot_2018-08-30-20-14-24.png

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What engine ?

 

Presumably it has no crank trigger, or another model year of same platform has any you can use ?

 

24-1, 36-1, or whatever is always better than no missing/reference teeth...simply because of that.

 

24 teeth is almost meaningless other than for a rotational speed reference...which would be a waste of effort really.

 

A missing tooth or other similar marker then also gives a specific crank position reference...typically cyl1 TDC ( in case of inline 4 that would be shared 1/4 hence if you want full sequential you then need a phase reference to ID cyl 1 from 1 or 4 )

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7 hours ago, Dynodom said:

The TWL_2 is too wide in diameter. I will have to go with the 24 tooth wheel.

Do you advise to remove one tooth to get a 24-1 or run with the full 24 teeth and why (I assume) the 24-1 is better than full 24?

Great support here. Thank you indeed!

 

 

Yes you can just remove one tooth from our 24 tooth wheel to make it into a 24-1.  I have done that before and it still provides a good waveform.  The main advantage of using a missing tooth crank wheel in your situation is the position of the cam tooth is much less critical than it is when you use a non-missing tooth crank.

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