Serge and Dan You are correct Serge, the retard I am referring to is the retard AFTER the engine is running, especially at rpm. Virtually all systems start at some "base" setting, and if this is too high, hard start. If it is just base timing low, then easy starting. When cranking a computer controlled engine with the scanner hooked up, you can watch the computer actually retard the timing to 20 degrees ATDC to allow for second rotation start. As soon as the engine gets over 400-600 rpm, depending on design the computer will advance to around 28 to 34 degrees usually, just like the Great Plains, and will hold back the timing from there until closed loop or hot running mode, at which time it will utilize full spark advance programming from there. Why mention all this? What does this have to do with a KR or Corvair?
Well, for every process that we now know to be electronic, it once was performed mechanically. Therefore, everything that I have mentioned that the computer does now, was handled by a mechanical device in the Corvair engines of the past. What my particular installation is attempting to do is to update with modern parts, the mechanical processes that Chevrolet developed for the engine, that were so successful, so that my airplane engine will run as well as the car engine did, just spinning a prop. The distributor was equipped with mechanical spark advance through the use of weights attached to the breaker plate under the points. These centrifugal weights would sling out causing the plate to "advance" the timing, usually between 12 and 14 degrees, depending on the strength of the springs and the actual weights. This advance is rpm dependent, and will occur every time at a given rpm. Base timing was usually set somewhere between 4 and 8 degrees which gives a total MECHANICAL advance of 16 on the low side to 22 degrees on the high side. Vacuum advance was used to advance the engine the final 10 to 14 degrees for best performance. This advance only occurs when the rpms are more constant, deceleration, and at idle. Just like the mechanical advance, it is not present when cranking, so the engine only cranks on base timing. When accelerating vacuum advance is lost, and the timing retards to mechanical advance only to prevent detonation, since the combustion chamber temperature will spike due to the sudden introduction of all that air. You will notice that fixed timing systems try to compromise between best performance, and detonation protection with settings around 26 to 28 degrees max. Problem: aviation carbs have no hookups for vacuum signals on their carbs. This is the reason I am using a down draft Rochester Monojet carb. It has both ported (vacuum on acceleration only) and manifold vacuum (vacuum at constant rpm). If you do not use vacuum advance , then you will have to recurve your distributor like WW does, and watch your temps closely. You will not be able to allow your engine to get as hot as the car did due to the effect on pre-ignition and detonation that this has, and you will almost assuredly have to use 100LL. Premium fuel will not give enough detonation protection. Remember, these engines were designed when 100 octane fuel (with lead) was common at the pump, and were designed for its use. When the lead went away, timing had to better controlled, some cars even equipped with spark retard modules, and compression ratios dropped until advancements were made it overall computer/engine control. Oh that "relay" is probably a simple transistor module triggering the coil, which is what most electronic ignitions use. The other parts of the ignition module handle ignition impulse modification if needed for computer use, and advance if desired by the designer. Since it is set up as an on/off system, it is perfectly compatible with the computer, where the trigger from say a point set tends to linger to a computer which makes it more like an AC wave, instead of the needed digital wave (square wave), but that is all another story... Back to sanding my cowling.... Colin Rainey brokerpilot9...@earthlink.net
