Base ignition timing offset on a ViPEC i88, what does the offset number physically relate to?

I've been helping a university FSAE team with some electrical items on their car for 2015, and I have a question about the base timing setting for the ViPEC i88 ECU they are using. The procedure outlined in the help file has been followed (quite a process, as the engine being used had no timing reference marks, so they had to be created, and a window made to observe them), and the engine now runs. This is great. I'm wondering what the offset number physically refers to though?

The engine is using the factory crank ref sensor, with a custom added cam sync sensor.

Is the ignition offset number the difference between the position of the engine when the cam sync signal occurs, and TDC on cyl 1's compression stroke?

It depends on which trigger pattern do you have. If your crank trigger has no missing teeth, then yes, the ignition offset is the difference between cam sync signal and TDC of 1st cylinder. If you have crank trigger pattern with missed teeth, then the offset is the angle between the window, which occured after cam sync signal and the TDC of 1st cylinder.

Anyway what this information do you need for? )

Cheers for the reply. It's the first setup you mentioned with a regular ref signal pattern from a crank mounted wheel and sensor, with a single sync signal pulse from a cam mounted wheel and sensor.

Just wanted to know for next time, as I've constructed a couple of custom cam sync systems in the past (not the one on this motor however), and getting the base timing offset calibrated correctly usually takes a few iterations. Next time, I'll measure the angle difference when fabricating the setup, and can hopefully get it really close first try :-). More knowledge is always a good thing!

Believe me, you will waste much more time if you would try to calculate something, than shooting 2-3 times with timing gun)

But I admit that few years ago, I tried to make the same things as you)

The physical location of the sync position relative to TDC #1 firing on the Link/Vipec is irrelevant and this is simply accounted for in the base timing value. I wouldn't lose too much sleep over where you've got it provided there is no conflict between a ref and sync pulse. Just use a timing light to find out where the base needs to be set.

Be aware that it is possible to have the base timing 360 degrees out too. It will look like everything is lined up correctly but the spark will occur on the exhaust stroke. Simply add (or subtract) 360 from the base timing to fix this.