Kip,

What your thermodymanics teacher should have told you is that the optimum firing point varies according to engine speed, load, octane rating of fuel, inlet air mass flow, engine temperature and just about anything else you care to think of. So the position sensor tells the computer each time the engine passes TDC on cylinder No 1, other sensors relay the other information and the computer calculates the firing point accordingly.

I can promise you Steve knows this, he was just simplifying it for ease of explanation.

__________________
Mick J
'08 Chrysler 300CRD (MB OM 642 engine)
'95 E220 estate
'89 230TE (R.I.P.)