Tuning an ignition, part 1
The most important variable is initial timing. But there are other things to consider: dwell angle, vacuum advance, centrifugal advance, advance curve. Let’s discuss these before starting tuning. In this section we’ll deal with the systems in this sequence:
- Dwell angle
- Vacuum advance
- Centrifugal advance
- Advance curve
Dwell angle
A term from when ignition systems had points, it’s a measure of how many degrees the points are closed. On electronic ignitions it’s non adjustable
Vacuum advance
An engine generally needs to have timing advance variable in relation to the throttle. This is obtained by having vacuum advance on the distributor
Centrifugal advance
An engine simply stated need the same advance timing all through the rpm band. But…here we mean the same advance timing measure in time, not degrees. If we didn’t have centrifugal advance, the timing would constantly be for example 10 degrees before Top Dead Center (TDC). But the combustion process in the cylinders doesn’t care about degrees, it cares about time, meaning that at higher rpm the ignition should deliver the spark several degrees earlier to obtain approximately the same time for the air/fuel mixture to ignite properly. If it sounds complicated, just remember this, an engine with maybe 10 degrees early ignition ad 1000 rpm would need 30-35 degrees early timing at 5000 rpm. This is obtained by centrifugal weights in the distributor. The total amount of centrifugal advance is depending on the weights and springs that control this
Advance curve
This is the curve with which the centrifugal advance works, it’s plotted in a graph as seen below
Note one thing. In a 4-cycle engine like we’re dealing with here, the pistons go up and down two times per ignition spark, once to ignite air/fuel ratio and make power, the second time to get rid of the burnt gasses (exhaust). This means that the distributor turns at half the speed of the crankshaft. All the “degrees” mentioned on these pages are crankshaft degrees, not distributor degrees. But if you buy a new distributor, the data on it may be in distributor degrees.
Tuning
First we need to set the dwell angle. Every time that is changed, the timing is automatically changed and need to be readjusted. If you have an electronic ignition, forget about this step. Some distributors like Chevrolet can adjust the dwell angle from outside with the correct size bacho tool, or id you don’t have that, you can probably find/fabricate a screwdriver that’ll fit. If this is the case, connect a multimeter set on dwell, note it has to be set at the correct number of cylinders. If you don’t know how many cylinders you have, you should probably close the hood and do something else ;-). With the multimeter connected and engine idling, turn the tool until you hit 30 degrees, this will be OK for most cars. If you find that you have to do this adjustment every few months, then your points are being worn too fast, buy new ones. If the dwell can’t be adjusted from externally, note the angle on multimeter, stop engine. If the angle is too big, it means that the points are too close to the axle, and needs to be moved slightly (0.1mm means a lot) so that the points open more. If the angle is too small, the points need to open less. Try a setting, install the distributor cap, start the engine and see the result on the multimeter. The engine might run worse than before, don’t worry about that, it’s because any change affects the timing as well, we’ll adjust that later.
Next is the vacuum advance. Normally it’s not adjustable, but you can buy parts to make it adjustable. For our purpose it’s not that important because when we push the throttle to the floor, vacuum becomes very low, and the vacuum advance goes to zero. So you can probably leave the vacuum advance as it is. Only thing to note is that when we later adjust the timing the vacuum advance hose must be disconnected and the carburetor port providing the vacuum must be plugged. To that end, but a collection of caps or find a hose that fits and insert a bolt in the other end. Note also that the hose has to go to ported vacuum (above the throttle plates), there’s a special port for that on the carburetor, it should not go to intake manifold vacuum.
Centrifugal advance/advance curve is a complicated topic. You can either leave it alone or do the full tuning. As a rule of thumb, if you’ve changed many parts on your engine but still has the original distributor, then you need to change the advance curve. If you also have a performance distributor they will most often incorporate a more suitable advance curve, and there is very little to gain from spending time on it. To find out how your current advance curve looks do the following: With the vacuum advance disconnected and plugged, use a timing light to measure advance at 1000 rpm, 1500 rpm, 2000 rpm, 2500 rpm, 3000 rpm, 3500 rpm, 4000 rpm, 4500 rpm and 5000 rpm (you neighbors are going to love you…). Put that data in the graph, it’ll look something like this:
You can buy kits with weights, springs and bushings that allows you to adjust this curve. When you increase the power of your engine, the curve generally requires the following: Less total centrifugal advance, earlier centrifugal advance. When we say less total centrifugal advance, note that we don’t mean a lower total advance, but a higher initial advance. Total advance rarely needs to change. So a better curve for a performance engine would be:
To obtain that curve you would need to first insert a bushing that limits the total centrifugal advance from 24 degrees (34-10) to 20 degrees (34-14). Secondly softer spring or heavier weights will make the centrifugal advance come at a lower rpm. Most kits tell you which springs/weights to use to get a certain curve, but of course you have to test when installed, again measuring advance at several different rpms as describe above.
Now it’s time to set the ignition timing. Some people do this by listening to the engine, where does it sound like it’s having a good steady idle. Some use the tach. We always use the vacuum gauge, see section Vacuum gauge, the wonder tool. And in theory it’s quite simple, turn the distributor until the maximum steady vacuum is obtained. That’s going to be very close to optimal. However, note that there are some natural limitations on how early timing/high vacuum you can use:
-First, we're talking steady vacuum. If the needle starts to be more unstable, that's probably too early timing
-Second, you need to be able to start the car(!). The earlier the timing, the more the engine refuses to turn because ignition happens when the piston is on the way up. This can be avoided by either getting a better starter, or by getting an ignition cut off switch in the car, where you cut off ignition until the starter has gotten the engine turning at a high enough rpm, then switch on the ignition and it starts. Or the more cumbersome way, keep the accelerator to the floor until the intake empties itself of fuel, then revs come up, lift the accelerator a bit and it starts
-Third, but not least, if your engine detonates, reduce the early timing immediately
In conclusion, you want as high vacuum as possible, but the three limitations above must be taken into consideration.
We’ll settle for this adjustment now, we’ll fine tune the ignition later in section Tuning an ignition, part 2, after having tuned the carb.