Ignition Timing 101

Written by: Mike Ervin

In this article I will try and explain to the best of my knowledge what ignition timing is.  I have had a few questions about this.  A lot of people know how to set their initial timing with a timing light and what this does, but to some that's about as far as their knowledge goes on the subject.  I hope this will help to shed some light (no pun intended) on a sometimes confusing part of auto mechanics.

Well let's see, where do I begin.  The timing process isn't really all that complicated.  In order for the internal combustion engine to run it has to have two things, a air/fuel mixture and a spark to ignite it.  I won't go into the fuel part of it here except to use it for examples in the ignition process.  A four cycle engine has well, four cycles.  The intake, compression, power, and exhaust strokes.  Just to make this as simple as I can, I will use the following example.  I know it is not exactly the way it functions, because we have valve timing and other things to consider but I will forgo the valve timing for the moment.  When the intake valve opens as the piston is on it's way down on the intake stroke, the vacuum created by the piston draws in the air/fuel mixture.  When the piston reaches BDC (Bottom Dead Center, or the lowest point it can go before it starts back up), the valve will close and the piston will start on it's way up on the compression stroke.  When the piston gets close to TDC and compresses the fuel charge, the spark plug fires and ignites the mixture.  When the fuel charge ignites it explodes and forces the piston down in the cylinder.  This is the power stroke.  As the piston reaches BDC, the exhaust valve will open and the rising piston will force the burnt gases out through the open exhaust valve.  When the piston reaches TDC, the exhaust valve will close, the intake opens and the process starts all over again with the intake stroke. 

Now on to the ignition timing part.  As all the above happens, it has to have the spark advanced in order to be able to keep up with the speed of the running engine.  This spark is supplied by the distributor and related parts.  The distributor tells the spark when to go to the plugs.  The coil supplies the voltage to create the spark.  If the distributor sent the spark to the plugs at the same time the piston reached TDC, it would be too late because by the time the plugs fired, the piston would already be on it's way down, which would be retarded spark timing, and the engine would be a dog, with no power.

Initial Advance:

Initial advance is just that, initial timing.  This is the initial amount you have the ignition timing advanced.  You set this by loosening the hold down bolt that is used to keep the distributor from moving after you get it set.  You use a timing light which is hooked to the #1 spark plug wire and the battery.  You aim the light at the timing mark on the balancer and turn the distributor body to either raise or lower the amount.  There is a timing pointer attached to the engine in a fixed location that will have degree marks on it, usually in 2 degree increments.  Every time the #1 wire fires it will cause the timing light to flash.  When it flashes it shows the timing mark position at that instant.  You always set the initial advance using #1 plug wire, because the cam and crank are timed using #1 cylinder at TDC.  Once you have it at what you want, you tighten the hold down at the distributor.  All small and big block Chevy's with an vacuum advance distributor will require you to remove the vacuum hose at the distributor and plug it.  More on vacuum advance later.

I have my initial advance set at 14 degrees BTDC.  This means that the distributor will send a spark to the plug when the piston is 14 degrees before top dead center (BTDC) on the compression stroke.  By doing this it can fire the mixture before the piston starts back down on the power stroke.  If you have too much initial advance, it will cause detonation or spark knock.  This is because it is firing too soon or as the piston is still on it's way up.  What you are after is trying to get the piston and spark to coincide at TDC.  Too little initial advance and you are wasting energy.  The most power comes when the gases are in the most compressed state, sort of like dynamite.  The tighter it is packed the more power it will make.  Every engine and combination requires a different setting.  There is no one setting for all cars and trucks.  It requires a lot of trial and error to properly tune the initial setting.

Centrifugal (Mechanical) Advance:

Centrifugal advance and mechanical advance mean the same thing, the amount of timing advance built into the distributor.  This is the total amount that the distributor can rotate internally and thus increase the ignition timing.  This amount is controlled by springs and weights mounted on the top of the distributor shaft, under the rotor.  As the speed of the distributor increases the weights have a tendency to fly out and when this happens the other side of the weights push on a center cam and turn the distributor shaft.  The total amount it can increase the timing is controlled by the shape of the weights.  Usually it is around 10-11 degrees, or 20 to 22 degrees at the crankshaft.  Why is it doubled at the crank?  Because the distributor, which is driven by the engines cam, turns at 1/2 the speed of the crankshaft because of the difference in the size of the timing gears.  If they turned the same speed the amount of advance would be the same at the distributor and crank.  But because the crank rotates twice as fast as the cam it is doubled.  A simple way to explain this is, if you turn the crank one complete revolution, the cam will rotate 1/2 revolution.  So for two complete revolutions of the crank, the cam will rotate 1 full revolution, or 720 degrees of crank rotation equals 360 degrees at the cam.

In order to keep the weights from flying out all the time by themselves they are held closed by two springs.  You can change the amount and speed at which this timing can happen by changing the weights and springs.  On HEI distributor's, it is best to leave the original weights and just change springs.  The amount of advance on these weights is as much as you will need for the street.  Just make sure the old weights are not worn where they fit on the pins they rotate on.  They have a tendency to wear the hole into an oval shape, which will in effect will increase the amount of timing they have, not good.  For some real good reading on how to set up your mechanical advance for performance, check out this article by Damon Nickles.

What does all this mean?  It simple terms it means that the faster the engine turns, the more timing advance you need in order to light the mixture sooner.  If you didn't have this built in advance in the distributor the timing would be so late that the piston would already be down at the bottom on the power stroke when the plugs fired.

Vacuum Advance:

Vacuum advance is for economy and to help engines run cleaner on the street and help reduce emissions.  Drag cars usually won't have any vacuum advance because they are at wide open throttle (WOT) and won't have any manifold vacuum anyway.  The vacuum canister is hooked to the pickup coil in an HEI distributor and a hose is hooked to a manifold vacuum source.  When manifold vacuum is applied to the canister it will rotate the pickup coil and increase the timing.  The amount of vacuum advance you have is built in the canister.  There are different canisters with different amounts of advance built into them.  Also you can buy adjustable canisters so you will be able to adjust when and how much vacuum advance you get.  Again, I recommend reading the above article by Damon for more on this subject.

In ending this article, I hope you find it helpful.  I have tried to explain it so it is easy to understand.  If you are knowledgeable on this subject you will probably disagree with some of the things, and some of the ways I explained them.  Again, let me state, I wrote this for readers who do not fully understand the basics of ignition timing.  I think that this article and the other articles I recommend to read will fulfill the requirements.