Ignition Timing

Ignition timing plays a huge role in how an engine runs, and not just from a power standpoint. Timing also effects fuel economy, ease of starting, engine life, and even the temperature at which the engine runs.

The Method:
So how is timing set? The best way to set the timing is by using a timing light. You can get close by ear or by using the timing tab for the harmonic balancer in conjunction with a test light, but a timing light is the best way.

The Setup:
First, unhook and plug the vacuum advance line, which lets you see just the initial advance without the effects of the vacuum advance. Hook the inductive pickup for the timing light to the #1 cylinder plug wire (front cylinder on the driver's side on a small and big block Chevy). Be sure that it is fully clipped around the wire, then hook the power leads to the positive and negative posts of the battery.

Checking the Timing:
With the engine idling (in gear with the parking brake set in an auto tranny car (BE CAREFUL!)), aim the light at the timing tab on the front of the motor (down behind the crank pulley) and pull the trigger. The light will pulse in time with the firing of the #1 cylinder and let you see the timing mark on the harmonic balancer. The numbers on the timing tab tell you what the ignition advance or retard is (you need advance) depending on which number the mark lines up with.

Adjusting the Timing:
The initial advance is adjusted by loosening the lock bolt on the distributor hold down tab. Rotating the distributor clockwise retards the timing and rotating it counterclockwise gives you more initial advance. Most cars will have the stock timing setting on a sticker somewhere in the engine compartment. Be careful when turning the distributor with the engine running, you can get quite a shock through deteriorated plug wires. Once you have set the timing, lock down the distributor, recheck the timing to make sure you didn't change it while tightening down the distributor clamp, and plug the vacuum advance back in.

Vacuum Advance:
Vacuum advance is controlled by the canister that's on the side of the distributor. Its function is to provide extra advance at high vacuum to increase fuel economy. The amount of advance can be adjusted by swapping on a different vacuum canister or by putting on an aftermarket adjustable canister. Also, where you have the vacuum line hooked up affects the vacuum advance. Ported vacuum (above the throttle blades, usually on the side of the carburetor) gives you no advance at idle and increasing advance (to a point) as the throttle opens. As the throttle approaches wide open, the vacuum goes away and so does the vacuum advance. Full manifold vacuum takes its signal from below the throttle blades and is highest at idle and gradually drops to zero at WOT. Full manifold vacuum generally isn't used except for some factory stock applications, and even then it's often not a good idea.

Centrifugal Advance:
Centrifugal advance is produced by weights in the distributor that are under the rotor. As rpm increases, the weights swing out and change the relation between the rotor and distributor cap. The amount of centrifugal advance is affected by the shape of the weights and a bushing that sits in a slot. The fatter the bushing and/or weights, the less advance possible. The rate of advance is effected by the weights and the springs that are attached to them. Heavy weights and light springs give you the advance sooner (at a lower rpm). Lighter weights and heavier springs delay the centrifugal advance. The purpose of centrifugal advance is to light off the air/fuel mixture sooner as rpm increases so that there is sufficient time for proper combustion. You can buy kits to tailor your centrifugal advance to your vehicles needs.

Timing Recommendations:
How do you know where to set your timing?

On a drag car it's fairly simple. You don't need vacuum advance and you typically want all the centrifugal advance in by your torque converter stall speed and the initial is set to whatever yields the lowest ETs without detonation.

On most street cars you want some vacuum advance to help your mileage, but you don't want so much that you get part throttle detonation (often called pinging or rattling valves). Most stock centrifugal advance mechanisms come in too late for a performance car. 2500-3000 rpm is a pretty good starting point for having all the centrifugal advance in. If you are racing a car with a 2000 stall converter, it can be difficult to get all your advance in my the stall speed without having detonation, so some compromise is necessary. The amount of centrifugal advance is determined by what total advance (initial + centrifugal) is necessary to have the best power without causing hard starting. Too much initial advance makes the engine hard to start, so you can have the same total advance by reducing the initial and allowing more centrifugal. More initial advance can improve off-idle throttle response, but as mentioned above there's a point at which the engine becomes difficult to start because of too much initial advance.

To sum up:
The proper amount of total advance for your vehicle is best determined by making lots of dragstrip or G-Tech runs and trying for the lowest ET while NOT experiencing detonation. Just watch your spark plugs for flecks of aluminum (piston material that has been deposited on the plugs) which would indicate detonation.

If you have any questions or comments please click the "Contact Craig" link and let him know.

Copyright 2009 Bruce Johnson and Craig Watson