Crank sensor offset - getting it right

Give input on tuning as well as any tips and tricks you may have. Also feel free to share base mapping files for various engine types.
SMR
Posts: 137
Joined: Tue Jan 16, 2018 9:38 am
ECU Model: S100 Pro
Distributor: DTA
Location: Aberdeen

Crank sensor offset - getting it right

Post by SMR »

Hi All,
I'm in the process of re-assembling my car and have been checking various settings in my S100.

The crank sensor offset is set at 72 degrees in the software (roughly setup after the engine was originally built). I've now cut a hole in (a spare) engine side casing and glued some clear perspex over the hole so I can check the timing with a strobe without getting sprayed with oil.
Modified trigger wheel cover
Modified trigger wheel cover
Sorry - damn photo has rotated........

I also carefully measured the actual installed position in relation to TDC which measures at 76 degrees BTDC.

With the 72 degrees BTDC setting, the strobe light shows a bit less than 1/2 a tooth out - so around 4 to 5 degrees.
With the setting changed to 76, the strobe light nicely lines up the sensor & TDC mark.
This is with the main ignition map all set to zero and temp compensations set to zero, and only at cranking speed (the engine isn't up and running yet as only partially installed).

So, in order for me to put in the correct offset of 76 degrees BTDC and NOT screwup my ignition map, am I correct in thinking I need to subtract 4 degrees across the board in my main ignition map?
Would I have to change anything else that may be affected by this 4 degree error?

I know it's not necessary, but I'd like to get it right!

And related to this, My "Firing tooth on startup" is set to 0 - which I understand means it fires at TDC during cranking until it starts - does this sound right?

Once I've got the engine fully installed again, I plan to try "1" (i.e 10 degrees BTDC with my 36-1 trigger) and see if it's any better, but trying this whilst cranking the strobe shows the 10 degree move. The engine has always started really quickly and easily - helped no doubt with the large off-board starter motor that spins it over at 1100rpm so not a problem, just a surprise!

Cheers
Steve
stevieturbo
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Location: Norn Iron

Re: Crank sensor offset - getting it right

Post by stevieturbo »

At cranking speeds....it does not use the main map ?

If you have a visual on it, can you not just count teeth from TDC to the trailing edge of the missing tooth, and use that as a starting point ?

it'd certainly be enough to allow you to start the engine then check properly with a light.

And although I cant think of any way to do it on DTA itself..

If you stuck a pressure transducer into cyl 1 and then took a reference of spark, all set to fire at zero deg, you could log the two ( scope ideal ) and see if the spark does indeed coincide with peak pressure in cyl 1.

If you could log cyl pressure and get an actual crank scope trace, you could correlate the correct tooth with TDC accurately too, without the need for the window
SMR
Posts: 137
Joined: Tue Jan 16, 2018 9:38 am
ECU Model: S100 Pro
Distributor: DTA
Location: Aberdeen

Re: Crank sensor offset - getting it right

Post by SMR »

Hi Stevieturbo,

I believe you're right about cranking speed - using the "0" or "1" etc in the "firing tooth at startup" box, so me zeroing everything else was likely over-cautious!

The complete installation has been working great for a few years - this was just a winter offseason 19/20 dismantle to investigate a newer engine option. So it's just going back together again. I do plan to test this all again once running, but am impatient and wanted to try whilst the engine is still out and accessible.

Fast logging did show the zero adcance, and then the 10 degrees advance when I changed the tooth-on-startup number so happy the software is doing what it should.

Cylinder pressure logging would be cool - and it appears sensors/logging systems are becoming (almost) affordable......

But getting back to my original query, do I subtract or add the 4 degrees to my ignition map if I change the 72 to 76?
stevieturbo
Posts: 3577
Joined: Tue Aug 02, 2011 12:08 pm
ECU Model: No ECU
Location: Norn Iron

Re: Crank sensor offset - getting it right

Post by stevieturbo »

SMR wrote:Hi Stevieturbo,

I believe you're right about cranking speed - using the "0" or "1" etc in the "firing tooth at startup" box, so me zeroing everything else was likely over-cautious!

The complete installation has been working great for a few years - this was just a winter offseason 19/20 dismantle to investigate a newer engine option. So it's just going back together again. I do plan to test this all again once running, but am impatient and wanted to try whilst the engine is still out and accessible.

Fast logging did show the zero adcance, and then the 10 degrees advance when I changed the tooth-on-startup number so happy the software is doing what it should.

Cylinder pressure logging would be cool - and it appears sensors/logging systems are becoming (almost) affordable......

But getting back to my original query, do I subtract or add the 4 degrees to my ignition map if I change the 72 to 76?
For what I proposed ( and have used ), simple £15 pressure sensor off ebay, £15 compression tester to get some fittings....and cheapo pressure transducer.
Think I used a 350psi absolute sensor.

And don't know about the other lol I try to think about it, then confuse myself going back and forward.
Alex DTA
Posts: 1622
Joined: Fri Feb 28, 2014 1:03 pm
ECU Model: S40 Pro
Distributor: DTA
Firmware Version: 79

Re: Crank sensor offset - getting it right

Post by Alex DTA »

TL;DR - Increasing the crank sensor angle retards the engine, so add the equivalent timing to the main ignition table. See warning below!!

Explanation
Assume you have an engine that where the correct crank sensor angle is 72*. To fire the spark plug at TDC, the ECU will see the first tooth after the gap, then wait 72 degrees to fire the ignition at TDC.
If you now increase the crank sensor angle to 76 degrees, the ECU will see tooth 1, then wait 76 degrees, and fire the spark plug.
As the correct angle was 72 degrees, the ignition will now fire 4* ATDC.
So increasing crank sensor angle retards the timing. Decreasing the crank sensor angle advances the timing.
This also affects injection angle, and probably other stuff I haven't thought about.

Warning: If you change the crank angle, your main ignition map is not guaranteed to be correct.
For example, if the engine was mapped with ignition compensations being applied, and one of the sensors is calibrated incorrectly, the error will have been mapped in.
Only if you are sure there were no ignition compensations being applied during mapping, can the above process be reliably applied.

One other thing to note:
Firing angle when the crank sensor angle is set incorrectly by less than a tooth will give a cranking timing that cannot be replicated with the correct crank sensor angle.
In the above example, if the firing tooth is set to 1 on a 36 tooth wheel, the cranking timing will be 10 degrees when the crank sensor angle is correct, and 6 degrees when it is at 76*.
Probably irrelevant for most engines, but worth noting.
SMR
Posts: 137
Joined: Tue Jan 16, 2018 9:38 am
ECU Model: S100 Pro
Distributor: DTA
Location: Aberdeen

Re: Crank sensor offset - getting it right

Post by SMR »

Thanks for the explanation Alex - I think I understand it now.

Though i think I'm the other way around; the 72 setting is wrong as the correct position is 76.

Either way, I won't mess with it now - I'm down to Daytuner at the end of the month for a dyno session so that's the place to do it before we start the mapping; I don't believe any compensations were on during the original mapping.

In terms of injection angle, the number is greyed out in the "engine config / sequential" menu and the table in "essential maps / injector angle" is all at zero. Is the single greyed out number over-written if you use the table? Is either applicable if I'm not running sequential? What angle is used for batch-firing - is it zero as the table is all zero'd, or is the greyed-out -70?
Does this mean at the moment that i'm running it 4 degrees out from what the ECU thinks? (Not that the angle has ever been played with - I'm hoping we can try this on the dyno).

In terms of the cranking angle, that was my thinking aswell - I'll try it here in the garage once the engine is installed and see what difference it makes, although again, it's going to be out by 4 degrees either way?

Cheers
Steve
Alex DTA
Posts: 1622
Joined: Fri Feb 28, 2014 1:03 pm
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Re: Crank sensor offset - getting it right

Post by Alex DTA »

No problem.

Yes, yours is the other way around. It was the principle I was explaining.

Injection angle can't be changed as you don't have a cam sensor.
SMR
Posts: 137
Joined: Tue Jan 16, 2018 9:38 am
ECU Model: S100 Pro
Distributor: DTA
Location: Aberdeen

Re: Crank sensor offset - getting it right

Post by SMR »

Excellent, thanks Alex!
Rob Stevens
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Re: Crank sensor offset - getting it right

Post by Rob Stevens »

Steve, sequential is worth looking at if you have big injectors, on my road car it made it idle beautifully and I'm sure it feels sharper. Dont expect big power gains however.
SMR
Posts: 137
Joined: Tue Jan 16, 2018 9:38 am
ECU Model: S100 Pro
Distributor: DTA
Location: Aberdeen

Re: Crank sensor offset - getting it right

Post by SMR »

Rob,
I've made a new cam trigger wheel with a much deeper tooth for the Honeywell HAL sensor to read. I've also remounted the sensor to just under 1mm clearance to the trigger. It works on the lathe (trigger wheel in the chuck and sensor on the toolpost) using a PC scope & bench PSU to test the output, but doesn't appear to be captured by the DTA's scope capture during cranking. I suspect the one tooth per revolution isn't enough at cranking speed....

The engine (or to be more accurate, the compete back-end of the car) is now back on the tub , fluids in and ready to fire-up tomorrow. I'll see if the DTA scope captures the cam signal and if so I'll definitely be trying sequential. My injectors are 440cc (Brown PICO) IWP043's running at 4.5Bar and max duty cycle I see is around 74% so yes, quite big injectors which may benefit from playing with the injection angle in sequential.

I've also tracked down deadtime numbers for the IWP043 injectors so will try the test and see how it stacks up.
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