Advance Curve Requirements
by Marcel Chichak
The mechanical system, having a fixed geometry, works the same regardless of operating conditions. The same cannot be said for the chemical reaction it harnesses. Combustion speed of an air/fuel mixture is dependent upon numerous factors, some set during engine design time, a lot that vary during operation and some that vary even under steady state conditions. Indeed, just like snowflakes, no two combustion cycles are the same. Fortunately, the cycle-to-cycle variations can be safely ignored, as can some other factors, but most cannot. It is important to identify the physical controlling factors when changes are made to a stock engine so that an initial advance curve can be determined. The other factors, let's call them environmental factors, have to be accommodated in operational controls or at least safeguards provided to ensure the engine does not operate outside safe ignition limits.
What this means is that to ensure the peak cylinder pressure occurs when it can do the most work, the flame front has to be initiated at the correct time. Unfortunately, this time varies, and because we are talking about a rotating machine, so does the angle. Because a cylinder charge takes a particular amount of time to burn and the engine speed varies, so then must the point at which combustion is initiated in advance. A typical ignition advance curve is shown in figure 1.
The major factors affecting ignition timing requirement are volumetric efficiency, engine speed and burn rate. Volumetric efficiency, or VE, is an expression of how much of a lung full a cylinder gets for each breath. As it turns out, a full cylinder burns faster than one that is only partially full so any improvements to VE also affect timing. Improvements to cylinder head ports, inlet manifold or camshaft improve VE at certain rev ranges so the ignition timing must also be changed to suit.
Lets look at a bunch of factors that affect
timing requirements and discuss a few:
As you can see, there are a lot of factors that decide what a particular engine’s advance curve looks like. If you look at advance curves for a dozen different engines you will have a dozen different advance curves, but, they all have the same general pattern. This has a lot to do with the physics of an internal combustion engine. The fine tuning of exactly what the advance curve looks like for a particular engine is not so much designed in as pulled out of a running engine. The advance curve for a similar engine can safely be used as a starting point and in fact, the engine may run just fine. This is good news for the home tuner, however, until the optimum timing point is found there is horsepower going unharnessed.
If you are planning an engine modification, or have an already modified engine the above table will not help pinpoint where to push and pull the advance curve, or by how much. Some combination of factors cancel each other out, some amplify and some are so subtle they can be ignored. But how each of these factors affect an engine can only be determined by testing. When an engine has been changed in many areas, typically cylinder head, camshaft, inlet manifold and compression ratio all at once, any advance curve that will allow the engine to run should be considered nothing more than a starting point.
There are a few suppliers that offer ‘performance’ distributors, notably Aldon, Piper and Accel. Although they have sexy names like "Red", "Rally" and "Dominator" they are in fact no different from your original distributor except for the advance curve. As far as the engine is concerned it couldn't care less if it were a distributor made by Lada or a gold plated one with a sexy name as long as the spark hits at the right instant. Considering the long list of factors affecting ignition timing, how is it possible for a third party to supply an advance curve that optimizes an engine they've never seen? They can't; so to avoid pre-ignition problems they supply a generic conservative advance curve that may be just as conservative as your original. The actual difference between the ‘performance’ distributor and your own is 2 tiny springs. That's it. And the chances of those springs delivering the optimum advance curve your engine needs is very low indeed. At the price of a new distributor, that's an awful high price to pay for a shot in the dark.
Save your money. Don't trust your ignition advance curve to anybody, but take your car to a rolling road dynamometer with an operator that knows what he's doing. It will be cheaper in the end and you'll maximize the return on all the mechanical modifications.
About the author - Marcel Chichak is a long time Miniac, engine builder and automotive restorer who can be credited with saving at least 30 British cars, including a Jaguar MK 2, from the scrap heap. When he's not restoring cars he is exploring and writing about the subtle nuances of motor vehicle operation. In his spare time he's a Bridge Hydrotechnical Engineer in Alberta, Canada.
© 2002 JAGUAR CLUBS OF NORTH AMERICA, INC.