Old motronic BTDC conversion formula

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hk665
Posts: 2
Joined: Tue Nov 28, 2017 1:43 am

Old motronic BTDC conversion formula

Post by hk665 »

Hi, everyone
I'm creating xdf for my old euro e28 535 (motronic 1.0 061) and found most of maps (including injector lag\voltage, eng temp\idle speed - i didn't see it in other old motronic xdfs)

The only question i've got - what is the conversion formula for ignition advance?

i saw (X*.6977)-9.0 and (X*.6977)-13.75, but i think it's copypasted from old porsche motronic conversions

and (X*0.75)-22.5 for e34 535 (179 dme)

So can somebody tell where's the truth? ))
Mykk
Posts: 99
Joined: Sat Sep 05, 2015 8:28 am

Post by Mykk »

The best explanation I've heard yet is in the 413 thread.

bosch 60-2 trigger wheels - 4 data points per tooth, 60 teeth counting both leading and trailing edge = 120 edges.
so 120 edges * 4 datas points means 480 points per rotation.
360/480 = 0.75 deg per data point.

Cut out is offset by 45 degrees, thus offset by 60 data points. (45/0.75=60)

Factor for ignition is then (X-60)*0.75

Just remember it's 60 teeth minus 2 for the cut out. Perhaps measure your own crank trigger wheel to verify cut out offset.

Image
Last edited by Mykk on Wed Nov 29, 2017 3:20 pm, edited 1 time in total.
hk665
Posts: 2
Joined: Tue Nov 28, 2017 1:43 am

Post by hk665 »

i've got 116 teeth flywheel with crank sensor reading info from it
i just don't understand where to get "Cut out is offset by 45 degrees" - or is it a constant value for each motronic (unrealistic version)

and why each tooth = 4 data points?
Hairyscreech
Posts: 196
Joined: Tue Jun 20, 2017 3:19 am

Post by Hairyscreech »

It's not a constant, that -60/45 degrees only applies to the M50tu and onwards.

The M20 and M50Nv has a different offset as it is calculated from the position of the missing teeth on the cam trigger pulley.

Braking down the equation:

Each tooth is read as 4 data points on the rising side and 4 data points on the falling side. 1 rise = 4 points and 1 fall = 4 Points. Each tooth is 8 pulses.

60 teeth on the pulley making for 480 pulses.

480 pulses in 360 degrees = 0.75 degrees per pulse.

With a missing tooth to indicate TDC at a -45 degree angle to the sensor when the crank is set at true TDC you have:
-45/0.75 = -60



For your engine you have 116*8 = 928 (assuming the very early motronic is the same number of data points per tooth. You have M1.1 right? Early E30?)
360/928=0.3879 degrees per pulse

You need to know the angle or how many teeth before TDC the crank trigger is to work out the offset bit.

Assuming M1.1 still counts that way as I feel like that is too many pulses per revolution for an 8 bit system.
LeiseyJr
Posts: 3
Joined: Fri Jun 18, 2021 7:32 am

Re: Old motronic BTDC conversion formula

Post by LeiseyJr »

https://web.archive.org/web/20150731232 ... dvance.htm

So basically it is relative to the flywheel teeth. Since early motronic has 2 reference sensors, one for tdc and one for engine speed and it uses the starter teeth. There is a one off chip in the DME to process this.

I believe the US DME 0261200059 DME does use the 20 degree offset and I think the euro 0261200061 13.75 degree offset. Both of those will result in 9 degrees at idle and around 30 WOT.

But yes the TunerPro Definition is wrong.

BMW
116 teeth

0.7759°

I'm sure that offset is stated somewhere in the .bin but I am not sure where that might be.
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