Ignition Timing Less Than Zero

37Pickup · Post by **37Pickup** » Sun Jun 10, 2012 4:11 am

Hi
I am running a super charged 351 with LPG/Propane. On boost I'm down to one's and zero's for ignition timing and the engine is performing well. I want to go negative! Can this be set or do I trick it by setting the crank trigger offset positve? or would that be negative?

cng1 · Post by **cng1** » Sun Jun 10, 2012 10:16 am

You can only achieve "negative" timing by moving the trigger wheel itself, you can't just adjust it in the software.

I would ask why you want to go negative though, LPG typically needs 10 degrees _more_ advance than petrol so even running 1 bar of boost you'd still only end up roughly back where you would be on petrol. A 351 is an old-style engine so needs quite a lot of advance, 30 degrees peak is often where they end up so working back from that 0 degrees even with the addition of boost just sounds wrong to me. Is your trigger wheel offset already perhaps?

37Pickup · Post by **37Pickup** » Sun Jun 10, 2012 1:58 pm

Yes typically 10deg more initial and max of 28 to 30 all in by 2800rpm. Then a normal petrol motor needs minus x deg per pound of boost. So with 5 lbs of boost you take off a far bit of advance, but then you add in the fact that LPG does everything weirder than petrol. People I've spoken to with SCs all say you just keep taking off advance till it works.

I'll investigate moving the cog, thanks thats a great idea.

cng1 · Post by **cng1** » Sun Jun 10, 2012 4:10 pm

The usual rule of thumb is to take of half a degree per psi, so if you're only running 5psi of boost then I'd not normally expect more than a handful of degrees come off. If specialist advice for your engine type when running both LPG and forced induction suggests that you should be running close to zero advance on boost then I guess that is just what that combination wants.