QuestionQUESTION: Most of the Triumph cars, small or large, Pi or carbs, all have
different static engine timing degs for their respective 'groups'
The Pi's have a low number of degrees,and the carb car engines have a higher number.
Question.-
Is this given number of degrees directly dependant on the distributor workings that has its own mech' advance and vacuum codes and operating conditions?
Many thanks.Nick.
ANSWER: Hi Nick,
Ignition timing is set to match the flame speed in the combustion chamber. Many things affect flame speed. Grade of gasoline. High octane gas burns slower then low octane gas. High compression speeds up flame speed. A lean mixture speeds up flame speed.
Octane rating is a rating that the fuel will self detonate under heat and pressure. The max horse power is just before the point of detonation. The problem is that detonation (abnormal combustion) is extremely destructive to the engine so car manufactures set the timing a little below point of detonation due to the differences between each of the same kind of engines and taking into account what gas will be available to the owner. Barometric pressure also affects the point of detonation.
Even though Fuel injection meters the fuel more accurately then most carburetors it in it's self has little to do with timing.
The advance curve (Mechanical and vacuum control) sets the ignition timing at the lower RPM range and under the different load conditions.
ANY changes to the engine will upset the factory specs necessary for good performance and safety of the engine.
Low RPM detonation is easy to spot and is commonly called "Pinging", "Valve rattle"(nothing to do with the valves) and "Spark Knock". It is very destructive to the engine and will in a very short time burn a piston, break rings and destroy rod bearings.
Retard the ignition timing and it will stop the noise and destruction. However, high RPM detonation is much more difficult to hear and is many times more destructive quicker. Many of the newer cars today have a knock sensor system built in that will automatically retard the timing and some even richen the fuel mixture up to control detonation.
Older cars don't have this system but there are accessory systems that can be purchased ranging from aftermarket injection systems and electronic ignition systems but they are all expensive. However, a less expensive system is a knock display system (MSD)that can be added to any car that gives the driver a visual display of detonation, thus warning the driver that there is detonation at a specific RPM range so the driver can take steps to correct the problem before damage is done. The detonation can be anywhere in the RPM range especially under load. Also especially under heated conditions.
Many old mechanics use to advance the ignition timing until they get detonation under load and then retard the timing by about 5 degrees and if the detonation stopped they called that their correct timing.
Howard
---------- FOLLOW-UP ----------
QUESTION: Thanks for the theory Howard.
Do I take it that you are saying YES in answer to my question?
Cheers, Nick.
AnswerNick, the amount of advance at any RPM, load and condition is only dependent on all that I listed above, compression ratio, octane rating of the fuel, condition of the engine, load applied, air temp and pressure.
The method used to get the correct advance at any time and condition is achieved by use of a certain distributor with correct mechanical and vacuum advance unites.
As long as your engine is stock, you should depend on which distributor was designed for your engine by the car factory.
Only if you alter the engine do you need to know all the methods of getting the correct advance curve by altering the mechanical and vacuum advance unites.
The "advance curve" is all that the working parts of the distributor can control. "Total advance" is adjustable and set by moving the distributor. Both of these are designed by the car manufacture for a stock engine.
Your question is not a "Yes-No" question because if you change the amount of manifold vacuum the engine produces, it alters the advance degrees etc with the same distributor. Thus changing the engine vacuum changed the advance degrees not the distributor.
Getting the correct advance curve in a distributor is a complex testing procedure and many race engine builders don't know how to do that very well.
Howard
