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Aug 20, 2012 4:31 pm
Aug 20, 2012 4:31 pm
Subject: Re: Ignition timing ("be a little retarded")
Hi Collin, an interesting subject and the more one learns the more one realizes we know far less than we think!
It is actually a massive subject as you must take in so many variables that affect our engines efficiency.
For example, you are surprised to have to advance the engine in the lower rev range. Think thus.
Our engine runs in a constant variable efficiency range.
Therefore a constantly changing CR must take effect.
The more unstable our efficiency (generally the worse designed older engines) the more we find our "effective" CR to be varied.
Therefore we need to Advance (in general) for our low efficiency and even lower than static CR in order to achieve maximum combustion pressure just past TDC.
As our efficiency gets toward its peak we use the least advance and as it passes this point more advance again.
Hence the Vacuum "advance" unit on 50s to 80s cars that is actually advancing ignition under heavy inlet vacuum (poor engine efficiency) and letting the ignition go back to its normal position when vacuum is released when you press the throttle to accelerate or drive at WOT positions. And then the mech advance adds more again as the engine speeds up.
Heat is also a factor.
People mostly do not realize that Detonation is not necessarily caused by too much CR or spark advance etc., but usually by poorly designed combustion chambers, poor assembly of engines, wrong fuel air ratios etc. etc. Detonation is actually the result not of the fuel/air mixture being ignited on its own before the spark, but of the Spark ignited mixture burning and travelling across the combustion chamber at a speed not fast enough for all the fuel/air mixture to be ignited before the temperature rise in the un-ignited portion spontaneously explodes like a diesel. as you can imagine a flame only travels at a certain speed, and if the combustion chamber is badly designed for instance and the spark plug is at one side of a large chamber, the "flame front" needs to move over a large distance, all the time this is travelling the un-ignited gasses are increasing in temperature dramatically, if the flame front is not fast enough then BANG this portion of gasses go off well in advance of its correct time and not only do you get "effectively" a pre-ignition but you have two flame fronts hitting each other very hard and causing a massive rise in combustion pressure before TDC and with nowhere to go as the space in the combustion chamber above the piston is still shrinking. Bad for bearings! etc.
Obviously a cold engine won't have the same problem as a hot one!
An engine with a poorly designed cooling system may read cool on the gauge but yet be boiling in some areas! Again, bad for the detonation limit. V12 Lagondas for example with std designed engines are terrible for this. All this needs to be considered and about 500 other points like the difference in flame front speed between weak and rich mixtures etc. The list is endless.
When deciding what settings your engine needs, Never be surprised but always be prepared to adjust yourself and your understanding to your findings. Theory is great but practical "reality" is the ultimate teacher.
"If it is, it is! Even if you don't sometimes think it should be!"
Fun subject Collin and I hope people can understand what I’m trying to say and how deep this subject is.
Hi Collin, an interesting subject and the more one learns the more one realizes we know far less than we think!
It is actually a massive subject as you must take in so many variables that affect our engines efficiency.
For example, you are surprised to have to advance the engine in the lower rev range. Think thus.
Our engine runs in a constant variable efficiency range.
Therefore a constantly changing CR must take effect.
The more unstable our efficiency (generally the worse designed older engines) the more we find our "effective" CR to be varied.
Therefore we need to Advance (in general) for our low efficiency and even lower than static CR in order to achieve maximum combustion pressure just past TDC.
As our efficiency gets toward its peak we use the least advance and as it passes this point more advance again.
Hence the Vacuum "advance" unit on 50s to 80s cars that is actually advancing ignition under heavy inlet vacuum (poor engine efficiency) and letting the ignition go back to its normal position when vacuum is released when you press the throttle to accelerate or drive at WOT positions. And then the mech advance adds more again as the engine speeds up.
Heat is also a factor.
People mostly do not realize that Detonation is not necessarily caused by too much CR or spark advance etc., but usually by poorly designed combustion chambers, poor assembly of engines, wrong fuel air ratios etc. etc. Detonation is actually the result not of the fuel/air mixture being ignited on its own before the spark, but of the Spark ignited mixture burning and travelling across the combustion chamber at a speed not fast enough for all the fuel/air mixture to be ignited before the temperature rise in the un-ignited portion spontaneously explodes like a diesel. as you can imagine a flame only travels at a certain speed, and if the combustion chamber is badly designed for instance and the spark plug is at one side of a large chamber, the "flame front" needs to move over a large distance, all the time this is travelling the un-ignited gasses are increasing in temperature dramatically, if the flame front is not fast enough then BANG this portion of gasses go off well in advance of its correct time and not only do you get "effectively" a pre-ignition but you have two flame fronts hitting each other very hard and causing a massive rise in combustion pressure before TDC and with nowhere to go as the space in the combustion chamber above the piston is still shrinking. Bad for bearings! etc.
Obviously a cold engine won't have the same problem as a hot one!
An engine with a poorly designed cooling system may read cool on the gauge but yet be boiling in some areas! Again, bad for the detonation limit. V12 Lagondas for example with std designed engines are terrible for this. All this needs to be considered and about 500 other points like the difference in flame front speed between weak and rich mixtures etc. The list is endless.
When deciding what settings your engine needs, Never be surprised but always be prepared to adjust yourself and your understanding to your findings. Theory is great but practical "reality" is the ultimate teacher.
"If it is, it is! Even if you don't sometimes think it should be!"
Fun subject Collin and I hope people can understand what I’m trying to say and how deep this subject is.