Alternative Fuels

Hi all
Just checking what the optimum ignition timing should be on a 93 gmc 3500 automatic with an impco 300 mixer and propane fuel
Thx
Darren

That is a loaded question and there is no optimum for such a general question simply because all engines are different. If you start with 14 degrees at idle, 14 degrees centrifugal advance (CA), and 14 degrees vacuum advance (VA) you'll be able to work from there.

If you want optimum get a fully adjustable ignition computer (if the GMC already has EFI and the ignition can be adjusted you are already half way there) and get a dyno tune. Make sure the tuner tunes the ignition to obtain MBT (Minimum timing for Best Torque) or as close to it as possible at all available timing points. This is the only way you will get optimum timing for your combination, anything else that doesn't measure the effects of timing change in real time as you tune will take alot of time and effort.

Ok thanks
I'm just interested in what I should set my initial timing nothing fancy it's a work truck and I want some fuel economy and power no racing
Darren

Alot of people don't realise that optimum timing provides the most power and the best fuel economy.

I'm a little late jumping into this one. The 1993 GM's actually benefited very little from timing advancers. We ran many dyno tests back then on brand new engines and the HP difference between Gas and propane was negligible. We attributed this to the knock sensors used on the 350 which would advance the timing as far as the GM program would allow no matter the fuel. The one area that concerned us was at WOT the timing would retard to about 6 or 8 degrees of advance. We felt the propane could take more advance then this. On a straight propane unit many of the dealers would bump the timing between 2 to 4 degrees. We advised caution here as the advance was exponential. A 2 degree bump could turn into 6 to 10 degrees at the full load top end. We were not worried about the mid range as the knock sensor would control that but at full load ECU's go into open loop and use a programmed algorithm.