Ignition timing/ relative compression

Hey everyone. This is just a question I have about doing a relative comoression and using a Ignition coils control wire (3wire coil) for checking timing. Every time i try and simulate a no start and check relative compression with Ignition timing my Ignition coil control is way off before TDC. Like very advanced. I've tried this on many cars and I keep getting the same results. It always looks like the timing is off. What am I doing wrong?? Thanks!!!
I've only ever had luck with this when checking with a 2 wire coil and checking current ramp.
Any suggestions please? Thanks

It is normal to see a firing event occur BTDC. Typically on most vehicles I have done syncing of relative compression and ignition firing events I usually see anywhere from 0 degrees to 20 degrees of advancement during cranking, but this will differ from vehicle to vehicle. The importance is HOW FAR off of TDC the event is taking place, not necessarily that it is perfectly at TDC. There are variables to this of course, as you should pretty much never see a firing event take place after TDC. Setting cursors to determine actual degrees off of TDC is useful to see if ignition timing is in a normal range.



This is a bit of interesting capture using secondary ignition and relative compression to identify a timing problem on a Nissan Frontier 3.3 engine. Another technician performed a timing belt replacement and the vehicle came to me after a severe shake was found at idle following the job. As you can see, every other secondary sync in the capture is actually taking place after TDC, with the other bank firing pretty much right on. This vehicle has one camshaft one tooth off.

ThanKS for the reply!
I understand firing event takes place before TDC. What I was saying is when I do a relative compression and a trigger on ignition control wire , it always looks like the timing is off. The ignition event is close to BDC, always looks way off. And this is always on a know good vehicle with nothing wrong with the timing. I will see if I have any patterns saved to post what I mean

That would be great if you could! Quick question though, you didn't accidentally invert the capture did you? If you did then your timing would look wayyyyy off, appearing to be at BDC vs TDC.

FYI, moved this out of Welcome Mat and into Diagnostic Tools.

Welcome, podhajnya! I like where SniperDan is going with his thinking about a possible inverted trace. I'd love to see your captures, too, if possible.

Yes indeed nice thinking SniperDan.
Now we're all curious to see some captures podhajnya

Thanks everyone. I will get you guys a pic tomorrow!