My '96 Blazer has been running bi-fuel on CNG and gasoline for about 1 year now just fine. Here's the problem: Natural gas has a slower flame speed than gasoline AND has a higher octane of 130, and therefore needs to have about + 13-15 more degrees of ignition advance at any given point than the gasoline counterpart. Well I installed the kit last year and all went well, but I did not know how to solve the issue with ignition advance until now. I've always noticed a POWER LOSS when running on natural gas due to not running sufficient ignition advance, but just lived with it since natural gas is only $0.95/gal equivalent. The power loss feels about like the difference when you turn ON or OFF the A/C on a 4 cylinder economy car. That's about how profound it is. I know I can do better than this. I've always wanted to fix the ignition problem ever since. So NOW I finally found a dual curve ignition box by MSD that is made for CNG or LPG fuels, and it allows you to run 2 timing curves. If you apply power to the trigger lead, it runs the CNG curve which by default will add 13 degrees of advance to the stock curve at any given point. With trigger lead unpowered it will leave it at the factory curve. Well, in order for this to work, they REQUIRE that you ADVANCE the ignition timing to ADD 15 more degrees on top of the factory's base timing spec. This way, the MSD box will RETARD that same 15 degrees you added while running on gasoline, then when you switch to CNG it will revert back to 13 degrees of that incorporated 15 degrees you added in. Make sense? Only problem is, I don't know how to manipulate this into the 96 4.3L Vortec engine. Either I would need to program it into the PCM, or somehow make the crank sensor adjustable I would assume? Which would seem near impossible to move looking at this timing chain cover (looks pretty fixed). Unless I could mount a new 3rd party crank sensor (plate between pulley and harmonic balancer) with custom sensor bracket, but I would have to find an aftermarket sensor kit that provides the same waveform as the OEM sensor creates so that the PCM wont complain. Lots of work? Yes, it would be, that is why I'm hoping to figure out how to program the PCM with a manual timing advance parameter. I do have a Crane Powertuner now, and the only setting it gives you is a setting for octane (normal or high), and timing control (adjustable from +/- 10% from baseline). The manual includes this following statement regarding the timing tuning:

"To adjust timing, a shift of the spark curve up or
down by ten percent can be selected. Use the 0%,
or no change at first and work your way towards
plus or minus ten percent. A minus 10 percent shift
can help overcome engine ping from poor quality
gasoline. If a low restriction cat back system has
been installed, a plus-10-percent-in-timing adjustement
can be beneficial.
Use caution when
adjusting timing, over
advance can cause detonation
and engine

The problem is that I don't think the 10% is anywhere near the 15 degrees that I need. I DID set the Crane Powertuner to octane level=high, full 10% advance curve over factory curve, cam perf 2 (which corresponded to the bigger Crane Compucam that I installed when I built the motor), and with all this change, it runs nice on 92 octane gasoline, and doesn't ping at all. It has about 9 or 9.5 to 1 compression now. Natural gas has 130 octane so I believe it can tolerate this other 13 degrees more than what I have. Does anybody here have any ideas on how to manipulate another 15 degrees into the base timing spec? Turning the distributor (even IF it was modified to turn), wouldn't have any effect on the base timing anyway, would it? the cam pulse on the 4.3L Vortec is only the cylinder #1 reference signal only, right? The actual TDC calculations come from the CKP sensor on this engine, right? Any help would be greatly appreciated.