TimingShortcut | Long Version | Real Long Version | Vaccum Advance Set your timing at 12 to 14 initial and 36 to 38 total. Use advance springs that have all the advance in at 2500 RPM. Run whatever octane fuel you need to avoid ping. Timing affects performance. Pinging, or detonation can cause damage. Many variables affect detonation, and some of them are beyond the control of the design engineer, so to preclude damage over a wide range of loads and fuels, the engineer needs to be conservative in defining the "factory specs". In older cars, the timing was retarded excessively to help reduce emissions before adequate emissions controls were available. Once you understand that the "factory specs" are not meant to make your engine perform at its optimum level then you will understand why there can be tremendous gains in power, or mileage, or both if you are willing to optimize the timing. "Tuning" is the word used to describe this optimization and it is different than just looking up the specs in a book and setting them. Generally, the more advance the better, up to the point where you hear pinging. Initial advance "at idle with vacuum advance disconnected" should be in the 12 to 14 degree range. You can try more, and when the engine is hard to start hot then you know you have gone too far. If you get pinging at high rpm it means you have too much total advance. If it pings a little and then quits as rpm continues to increase, then the advance may be occurring at too low RPM. Generally all the advance should be in by 2500 to 3000 rpm. Many times in older cars that had low initial advance, the amount of mechanical advance in the distributor must be reduced to get the right total advance with adequate initial advance as well as using lighter springs to get the advance in faster. This "recurving" a distributor is an old hot rod trick, but don’t be mislead that only hot rodders benefit from these modifications. The extra power comes from efficiency, and that can translate to improved mileage and drivability even if your driving style is very conservative. These modifications work on modern electronic cars too, as exemplified by all the performance "chips" on the aftermarket. Engineers still must make allowances for lousy gas and a wide range of operating conditions. Higher octane fuel will eliminate pinging. If you are willing to pay for the extra octane, you should be able to advance the timing more, and get significant additional performance. In many cases, the extra efficiency will increase mileage enough to offset the additional cost of premium fuel! Make sure you optimize the timing with the fuel you plan to use all the time. The real long version (or probably more than you really wanted to know): Gasoline burns slowly. In order to get the maximum efficiency, the pressure on the piston needs to be highest when the connecting rod has the highest mechanical advantage. The spark has to start the burn ahead of time, so that the pressure is right at the right time, like leading a rabbit when you shoot the arrow. To really get the right answer, you would have to add up all the pressures at all the crankshaft positions, and try and optimize the collective total. When you figure that compression ratio, fuel mixture, and octane all change the rate at which the fuel burns, you realize why the engineer has to be conservative. He can’t guess the flame speed any better than you can guess the precise average speed of the rabbit! Then what if you change the situation with a different cam, or Nitrous, or letting the combustion chambers fill up with carbon? So we have to optimize the timing experimentally for the way we drive, the modifications we make, the deterioration of our engine, and the octane of fuel we choose. Procedure is the same as above. Try more until you don’t get any benefit or you find a limit. There are a lot of things that affect detonation or pinging. If you suspect that any of these things could be contributing, then you will need to address them before you conclude that you have optimized the timing. Compression ratio, hot spark plugs, and lean mixtures all increase octane requirement. You can mask the effects by retarding the timing. Oil consumption and carbon buildup also can cause pinging. The converse is also true. Low compression, cold plugs, and rich mixtures can help, and so can performance cams that close the intake valve later and shorten up the compression stroke. With all these variables, it is easy to see why tuning is a long and often iterative process. At low load, high vacuum, conditions the flame speed is slower. At these conditions, the engine can stand or require additional advance. Vacuum advance can improve mileage and reduce the heat load on the engine. The performance at maximum power should be optimized first, as outlined above. Add vacuum advance to optimize low speed driveability. When you have too much the symptom will be a surging or bucking at low rpm cruise that goes away if you drive slightly faster. Ralph Winslow |