By Brent Covey

Gary Zingle writes;

Maybe it would be better to set up an earlier HEI distributor with aftermarket detonation control and quite a bit of mechanical advance and have it all in quite early, say 2000 rpm.
The combination could serve to provide easy starting with lots of advance once running...
Gary, thats good thinking , but isnt going to really work in practice-

The main thing you have to remember is electronic knock sensor devices dont operate with enough precision to adjust ignition timing 'on the fly'. You cant set your engine up to be slightly advanced beyond the point where ping starts and then expect the knock sensor to pull it back a little and 'adjust' timing, they are much too crude to do this.

Also, more advance is not always better for performance, although within a certain window its desirable. Theres usually a 'sweet spot' beyond which more advance doesnt accomplish anything.

The centrifugal advance systems are set up to work with a given engines cam timing more than anything. The cam will determine the pressures in the cylinders at different speeds, and advance is generally restricted during periods of high cylinder pressures, as knocking will occur easier at high cylinder pressures. Cylinder pressures rise and fall pretty closely with the torque curve. At the point of the torque peak, you're at the point the cam is most effectively trapping fuel in the cylinders, and hence the point of highest pressures and combustion temperatures. As you pass the torque peak, the pressures will fall as engine speed rises and more advance is possible. Some advance is necessary also just for the increased engine speed of course.

Vacuum advance is important because it maintains appropriate timing when the engine is operated at less than full-throttle and hence lower cylinder pressures.

Centrifugal advance curves are not especially effected by altitude. If a distributor has a good curve at sea level, namely just short of ping all the way to 5000 RPM at WOT, it will still be a good match at 6500' altitude, however it may be able to tolerate significantly greater initial lead. The shape of the spark curve that works at full throttle will remain constant regardless of altitude. You may be able to start the curve at a greater initial setting at high altitude however.

Temperature will also effect this, most cast iron head V8 engines will begin to ping suddenly approximately at the 225F water temp zone.

Modern cars have this all taken into account. The basic spark curves for engine speed are essentially identical to what mechanical distributors would use, but using atmospheric pressure, manifold pressure, and water temperature sensors they can tack on extra advance or remove it in direct proportion to temperature, altitude, and load.

The 1977-78 Toronado has an early electronic control system, called MISAR which was a temperature sensing spark advance. It allowed a little more precision in spark control by tailoring timing to match engine temperatures primarily. Later systems like Cadillac DFI in 1980-on went the whole nine yards and added pressure and temp sensors and could tailor a spark for a specific environment and engine load.

Generally the advantage of the electronic control systems is you dont have to set the engine up to run 'worst case' and suffer slightly less than optimum performance the rest of the time. If you're GM, you have to make a distributor with timing that will run at the lowest altitude, in the hottest weather, on the worst fuel, leaned out, with 250F water. This naturally means if you're doing something besides driving 80 mph thru Death Valley at high noon with bad gas and a radiator plugged with bugs you'll be getting somewhat less performance than you could have availible.

Knock sensors were added as a last ditch safety valve. They brutally reduce timing to kill knock to prevent severe engine damage if everything else fails. A properly set up engine will never turn one on in normal driving however. The knock sensor is just insurance, it will ruin performance and economy and spoil everything if it is called into play more than very rarely. Adding them just allows you to fudge the worst case settings a little closer to optimum, without risking a broken piston etc.

Some aftermarket systems exist to permit a person to program thier own spark curve- if these have enough sensors they'd work very well I think. Also, the GM factory installed systems like DFI Cadillac can be reprogrammed to work with othe engines as well, which may be an idea.

If a person is using a old HEI or breaker point ignition system, and makes a habit of driving between destinations of very large differences in altitude, perhaps setting up the engine to run well at the higher altitude on 87 gas and just increasing fuel octane as you come down may be a solution, depending on the distances involved. Vacuum advances on older mechanical units should be calibrated to flatter the lowest altitude environment you usually drive in in any event.

If you want to have your cake and eat it too, using 87 octane fuel at any altitude with agressive spark timing that adjusts on the fly, you're looking at a computer programmed to adjust timing with sensors for temperature, pressure and manifold vacuum that can compensate for changes. A knock sensor is not needed in any scenario except as an insurance policy if you had a serious problem.

Hope this helps make sense of some of the options availible-
Brent Covey
Vancouver BC