Ask Away with Jeff Smith: A Basic Guide to Ignition Advance

I hate to admit that I’ve been working on cars for more years that I care to admit and yet I still don’t understand all the different things about ignition advance. Every time I ask my friends who say they know all about that – I get a different answer. Can you make it simple enough that anybody can understand?

S.S.

Jeff Smith: Ignition advance isn’t too complex if you break it down into separate ideas and then combine them to come up with a total. It’s not difficult to understand and oftentimes minor changes to the ignition curve can result in a stronger running engine. Let’s start by stating that this is another example where too much timing can be downright destructive while too little will result in poor performance and even worse drivability.

Let’s see where this takes us.

We’ll break ignition timing into three categories: initial, mechanical, and vacuum advance. Initial is the easiest and you can consider this as the base timing. This is the amount of timing in degrees before top dead center (BTDC). Engines need advanced timing because the explosion that is the common description for combustion is in reality more like a prairie fire that burns across the top of the combustion space, radiating out from the spark plug location. This requires time to burn and the faster the engine spins, the more time (in terms of degrees of crankshaft rotation) is required to allow the combustion process to occur. The ideal ignition timing occurs (at any given rpm) when maximum cylinder pressure is achieved at roughly 15 to 20 degrees after top dead center (ATDC). This is when the piston and rod combination create maximum leverage on the crankshaft.

MSD’s timing tape is a simple and quick way to turn a stock balancer in a degreed unit that makes it easy to set total timing. And you don’t need an expensive dial-back timing light.

With the engine running and the timing light flashing on the crankshaft, let’s create an initial timing figure of 10 degrees BTDC at an idle speed of 850 rpm. This is always checked with the vacuum advance disconnected. Now if we rev the engine while watching the timing marks with the light, we’ll see the advance increase. If the harmonic balancer is degreed, this is a simple case of reading the numbers. If it is not, then you might need a timing tape. MSD sells a sheet of several timing tapes. Each tape is designed to work on a specific diameter balancer. This is because the spacing between the individual marks will vary depending upon the circumference of the balancer.

If you’re in a hurry and don’t have a timing tape, you can make one really very easily. First you will have to do some simple math. The circumference of a circle is determined by multiplying pi (3.1417) times diameter. So in the case of an 8-inch balancer, this is 25.13 inches. Now divide that figure by 180 to come up with a distance 2 degrees. This number is 0.139. We use a dial caliper to make marks on a length of masking tape starting with 0 – or TDC. This means that at 10 degrees, the distance will be .695 inches and so on. You don’t have to make marks every two degrees. Just at the important numbers like 10, 15, 20, 25, 30, 36, and 40 degrees. Carefully place the masking tape on the balancer with TDC overlapping the TDC mark on the balancer. Make sure you put the tape so that the numbers fall before TDC, not after.

So now let’s say we have 10 degrees of initial timing. Rev the engine and you can read on the timing tape the amount of timing at each rpm point. You can then record these numbers on a piece of paper. Remember, the vacuum advance must be disconnected for this test to be accurate. Let’s use these numbers as an example:

Engine RPM                          Initial + Mechanical Advance

900                                                10

1,500                                             14

2,000                                             20

2,500                                             30

3,000                                             30

This means that our total mechanical advance is 20 degrees (10 initial + 20 mechanical = 30 degrees). But let’s say that we know our engine will run best at 36 degrees of total advance. The simplest fix is to add 6 degrees more initial timing to 16 degrees. Then the total will be 36 degrees. The advance is determined by the combination of mechanical advance weights and springs usually located directly under the rotor on most GM distributors. Some Ford and Chrysler distributors place the mechanical advance components under the main plate, which is much more difficult to access. Virtually all aftermarket distributors place the mechanical advance on top, under the rotor.

Now let’s add the vacuum advance. This device adds timing to the engine at part throttle.

Many enthusiasts think they should eliminate the vacuum advance for a hot street engine because their hero drag racer doesn’t use vacuum advance on his race engine. It is true that vacuum advance plays no part in a pure drag race engine, but for the street, it is still a very good idea. This is because at part throttle, the throttle valves are nearly closed and a very limited amount of air and fuel enters the cylinders. This is far less air and fuel than would be present if the engine were operating at wide open throttle (WOT). A less-dense mixture burns much slower than a dense mixture. This means the engine can make more power with that less dense mixture if more timing is added. This greater lead time allows the burn to occur at the proper 15 to 20 degrees ATDC. This improves fuel mileage and also improves throttle response at light throttle.

The amount of timing added depends upon the engine’s load. By connecting the vacuum advanced canister to manifold vacuum, we can now vary the amount of timing added with engine vacuum. The lighter the load on the engine, the higher the manifold vacuum and the more advance the engine will need to operate at its highest efficiency.

To check this number, reconnect the vacuum advance line to the distributor and rev the engine up to 1,500 rpm. The total advance will be higher because now the vacuum advance can is adding more timing.

With 10 degrees initial, our first mechanical advance test gave us 14 degrees of initial plus mechanical advance at 1,500. If we now read 29 degrees of timing, that means that we have 15 degrees of vacuum advance (29 – 14 = 15 degrees). This will be roughly the same 15 degrees of additional advance at all the different engine speeds because the engine vacuum will be the same.

However, in the car, when you begin to open the throttle more, load increases and the engine vacuum will diminish. This will reduce the amount of advance until you get to WOT, at which time no vacuum advance will be present. It’s not unusual to generate 20 degrees or more of vacuum advance timing. Each engine will want a certain amount of timing. The only real way to determine this number is through experimentation and accurately measuring fuel mileage.

There’s much more to this topic because we have not really touched on ways to create the best timing curve and why engines want a specific timing advance number. It’s also important to point out that just adding more timing doesn’t necessarily increase power.

At each given engine speed, the engine will want a particular amount of timing. This is determined by hundreds of different factors, including compression ratio, the fuel’s octane rating, inlet air temperature, a many more. Too much timing can create detonation and can also cause a loss in power, so it’s important to properly set up the entire initial, mechanical, and vacuum advance curve combination. That’s really a separate story, but certainly worth covering.

The best part is you can come up with an ideal timing curve for your engine without giving money to a tuner. You can do it yourself as long as you understand the basics.

Your engine will tell you what it wants.