Determining a centrifugal advance curve

April 1, 2016 | By Staff

Question:

I want to verify that the centrifugal advance is correct from 700 to 4800 rpm on my 110 HP 1965 Corvair. Documents show the correct beginning and end of the advance but how do I know where that is by shining a timing light at my pulley? A friend offered his timing light with retard knob but I don’t understand it. Also I have heard of using tape on the pulley. How does one do these things?

Answer:

To accurately determine the advance curve on your Corvair you will need a timing light and a tachometer. There are timing lights available that have tachs built into them and they make the task more convenient and easier. I have not seen a timing light with a retard button, but I have seen them with an advance knob that lets you change when the strobe light fires, making it possible to determine the parameters of the advance curve, but I have never bothered with one of those.

To adjust the beginning and end of your advance curve you merely loosen the distributor clamp and turn it while you shine the timing light on the crankshaft pulley with the engine running. Sometimes it is difficult to see the marks, so wipe them clean, and then mark them with a magic marker to make them more apparent.

You attach the timing light to the number one sparkplug wire before starting the engine. You would then attach your tachometer to the distributor.

To determine the total advance curve, you can use a disc that has the degrees already marked on it, available for degreeing in a cam that attaches to the front of the pulley, or you can just use tape and mark off your pulley yourself. There will be a few timing marks on your crankshaft pulley or your harmonic balancer that will give you the spacing between degrees for that diameter pulley, and then you can measure those and mark them on tape that you then wrap around it so you can have the range you need marked off.

Degree
A degree wheel for installing a cam can be used to determine the advance curve.

The prescribed range of advance from start to all-out for your engine is a conservative compromise between easy start-up and maximum advance for maximum power. You can fiddle with these settings to get more power and quicker acceleration by advancing the spark as far as it will go without hard-starting and pinging on the bottom end, and total advance at the top merely by turning the distributor to advance the spark a little more until you start to have problems, but that is getting into performance tuning which is a whole new area. Short of using a dynamometer, the only way to determine if you have made a positive difference is to time yourself and make a few runs over a fixed distance.

There are kits available that make it possible for you to change the springs in your mechanical advance mechanism to allow the timing to advance quicker, and/or increase the range of the advance curve, but keep in mind that the engineers who designed your Corvair’s engine did so with an eye toward making it easy starting, smooth, dependable and long-lived, so think hard about what you want to accomplish before taking things too far.