Spin doctors are the public-relations people who put a spin on a situation to project the image they desire. Hot rodders think of ignition systems that way. They look at all the information and end up confused. While there is nonstop voodoo out there concerning ignition systems and what they do, there is plenty of simple information concerning timing curves and how to dial in an ignition curve for your car. Its actually very simple and fun to do.
But (theres always a qualifier, isnt there?) before we can get into dialing in a custom ignition curve on your street runner, we should review a few basics. The whole point of ignition timing is to accurately initiate spark-plug firing in the cylinder to create maximum cylinder pressure. Since a street engine operates through a very wide band of rpm, ignition-timing requirements change as the rpm vary. For purposes of this discussion, we will use crankshaft degrees in reference to timing. You should know that distributor degrees (as seen on a distributor machine) are half of the crankshaft degrees because the distributor spins at half of the crankshaft speed.
For older hot rods with stand-alone ignition systems, there are three basic components to ignition timing. The first is initial timing, which is the amount of timing you check with your timing light on the damper at idle. Most stock timing specs require 6 to 10 degrees of advance before top dead center (BTDC). This means the spark for each cylinder will fire, at idle, 6 to 10 degrees before the piston reaches the top of its travel. Theres no reason to ever set timing to occur after top dead center (ATDC). However, we have seen situations where enthusiasts have set the timing after TDC rather than before because they thought ATDC meant advanced.
The second component of ignition timing is vacuum advance. Most distributors come equipped with a vacuum-advance canister attached to the distributor housing. This canister uses engine vacuum to advance ignition timing during part-throttle operation when engine vacuum is relatively high. With the throttle mostly closed, there is very little cylinder pressure so the engine can tolerate more ignition timing. The vacuum-advance unit pulls a lever that moves a plate inside the distributor, advancing the timing. There are literally hundreds of different vacuum cans offering various amounts and rates of advance. On the older point-type distributors, the vacuum cans added between 10 to 20 degrees of advance. The later 70s HEI distributors offered more vacuum advance with anywhere from 10 to 26 degrees.
The final ignition-curve segment is also the most important. The mechanical advance begins just above idle. As engine and distributor speed increase, centrifugal force acting on the weights and springs just underneath the distributor rotor moves a plate that advances the ignition timing. The speed when advance begins and the rate at which it advances the ignition timing is determined by these weights and springs. Mechanical advance is limited by a small pin that travels through a slot in the plate (see photo on page 104). The good news is its relatively easy to change the rate of advance with a simple spring or weight change. With a little more effort, you can also increase the amount of mechanical advance by lengthening the slot or reduce the total advance by brazing up the slot.
Lets take an example to show you how it works. Lets say we have a mild, 9:1 compression 350 Camaro with a basic point distributor. The initial timing is set at 8-degrees BTDC. By testing the distributor, we discover we have 12 degrees of vacuum advance and an additional 20 degrees of mechanical advance thats all in by 3,000 rpm. By adding the initial advance to the mechanical advance, we find we have a total of 28 degrees of total advance. This is the total we will have with the engine at wide-open throttle at or above 3,000 rpm. Below that speed, there is less timing.
The only time vacuum advance comes into play is at part-throttle when manifold vacuum is present. Lets say we have 8-degrees initial and 20 degrees of mechanical at a 3,000-rpm cruise with 14 inches of manifold vacuum that gives us another 10 degrees of advance. Now we have 38 degrees of total advance, but only at part-throttle. As soon as you mash the throttle, that 10 degrees of vacuum advance disappears. Vacuum for this advance originates from a ported vacuum-advance outlet on the carburetor. Ported manifold vacuum means that at idle with the throttle blades closed, there is no vacuum present at the port. But as soon as the throttle is opened slightly, a port is uncovered in the carburetor venturi and vacuum is routed to the vacuum can. You could connect straight manifold vacuum to the vacuum can, but this tends to add more ignition timing at idle than the engine can use. However, in certain applications with low manifold vacuum and a big camshaft, there might be an advantage to experimenting with this option.
Another part of the ignition-timing equation is the curve, or rate of advance generated by the mechanical-advance mechanism. Note that in our example above, the rate is fully advanced by 3,000 rpm but it doesnt start until 1,800 rpm. This means that at wide-open throttle, the engine only has that small amount (8 degrees) of initial advance up until 1,800 rpm. Then, as engine speed increases toward 3,000 rpm, some amount of mechanical advance is added to the initial timing until we reach the total at 3,000 rpm. For a performance engine, we want that curve to be more aggressive. Lets change the mechanical-advance curve to make it more aggressive as well as increase total ignition timing. These changes will increase torque at lower engine speeds as well as mileage and improve horsepower at peak rpm.
Our first move should be to bump the initial timing from 8 to 12 degrees by advancing (turning) the distributor. Next, lets modify the slot to now offer 24 degrees of total mechanical advance. Finally, well add some lighter advance springs to the mechanical-advance mechanism to start the mechanical advance sooner at 1,200 rpm, having it all in by 2,600 rpm. Now we have a total of 36 degrees of total ignition timing at wide-open throttle thats all in by 2,600 rpm. This kind of curve will make the engine much more responsive and will increase power throughout the entire rpm band.
This is a simplistic example, but for most normally aspirated street engines running pump gas, this curve will offer 90 percent of what you need from a customized advance curve. Other variables that will affect this curve are an engine with a more aggressive camshaft and a little more compression. For example, a 350 or 454 with 10.5:1 compression, good heads, a 230-degree duration at 0.050-lift camshaft, a single-plane intake manifold, and a big carburetor and headers would want a more aggressive ignition curve. This combo could benefit from as much as 16 to 18 degrees of initial advance. This means you must limit the mechanical curve because you still only want 34 to 36 degrees of total timing above 2,600 rpm. With 18 degrees of initial timing, you only need another 18 degrees in the distributor to come up with 36 degrees total.
Ignition curves are fairly simple once you understand how they work. Do a little investigative work on your street car. If the curve is a little slow, you can tune it up with very little effort and really feel the difference. The best news is that this costs virtually nothing but a little bit of your time. You cant beat free horsepower!