Maximum ignition advance compared to offset angle?

[loki1342]

Hi there,

I have what may be a very silly question, but I can't find any definitive answer to it relating specifically to the Megasquirt ECU. I'm looking to set up a (Microsquirt based) wasted spark ignition system for a 2 cylinder motorcycle (Honda CB250) and I want to run ignition advance values greater than the stock setup allows. The OEM VR sensor is offset about 45 degrees from TDC, and something like 20 degrees from the timing mark on the flywheel, making for a total ignition advance offset of 25 degrees. If possible, I want to run anywhere up to about 35-40 degrees advance based on engine load, but I don't know whether I need to move the VR sensor to do so. So, to cut a long story short: can I run ignition advance angles greater than the offset angle, or do I need to move the VR sensor such that its offset is greater than the maximum ignition advance the engine will see?

Thanks in advance folks.

Lachlan

[Matt Cramer]

It depends; what is the pattern of the trigger wheel that triggers this sensor?

[loki1342]

No trigger wheel I'm afraid. The stock ignition system is a CDI ignition system triggered by a single tooth on the flywheel set at the maximum ignition angle (25 degrees). I haven't had much of a think about how I'd fit a trigger wheel on/behind the flywheel, but I think I might have to...!

[Matt Cramer]

That is not very well supported in the current code. It would treat this like a distributor, and this is in a range where it doesn't get a good amount of advance. You would be able to run from 0 to 20 degrees of timing without either moving the crank sensor or converting to a multitoothed wheel.

[loki1342]

I thought that might be the case, though I was hoping for a pleasant surprise. As it turns out there's a variant of this engine made in China that comes with an integrated missing tooth trigger wheel, with one missing tooth at 90 degrees. That ought to give me a little more wiggle room, right?