Tuning an ignition, part 2
So now it’s off to the track. The procedure is described in section Testing. We cannot emphasize how important consistency is, both in where you drive, how you drive, what the weather is like (if it starts raining, give up and go home). Great effort must be made to ensure the only variable is the one we’re trying to optimize. The beautiful curve in the section Testing will never happen in real life, but if it goes up and down too much, you can’t use it to conclude anything.
Be prepared, a few items can make tuning ignition timing a lot easier. The first is that you have to be able to see the timing mark on the damper or equivalent. If you have an American V8 to tune you can buy timing tape to put on the damper, it goes all the way around that has clear indications of degrees. Even easier is to buy a kit for the distributor that has a degree plate mounted to the intake manifold and a pointer mounted to the distributor. So you can adjust ignition timing without a timing light. Of course you have to set the plate and pointer in the first place using a timing light, but from there on you don’t need it. Remembering that the rotor in the distributor turns at half speed, make sure the degrees on your plate show crankshaft degrees, and if not, make a mental note so you know what you’re doing.
What needs to be done here is only adjust the initial timing at idle and then test. Here’s a repeat of the relevant text from section Testing:
Go to the track, test the car, write down the result. Now the car is warm. Test again, write down the result. This is your base line. Then increase timing to 3 degrees earlier. Test again, write down the result. If it’s faster, do additional 3 degrees earlier. If slower, do 3 degrees later timing than baseline. Test again, write down the result. Continue until you get slower both earlier and later timing. Then set back the timing to the original setting and test, write down the result.
Now you’ll have a set of data points, plug them into a table in the sequence you did them. The reason to do this is that the car might have changed performance simply from the repeated use and heat build up, we want to eliminate that from the results.
Results
We see that the 0 result has improved by 1.5 MPH, that has to be eliminated because it must be attributable to other factors than the one we’re trying to test for. So we extrapolate the 1.5 over time and get to this result:
Results adjusted for outside factors
Then we sort the data by the degrees:
Then we plot it into a graph. If you’re into Excel you can get the program to calculate a polynomial trend line, as seen in graph below.
Based on this, we would set the ignition at 7 degrees early, that’s where the curve would be the highest if we had tested all settings.
There are two factors that could force you to not go for the optimum 7 degrees. One is if you hear detonation, either when doing the testing or when driving home normally, maybe up a long hill. That has to fixed immediately and that’s done by reducing the advance to maybe 5 degrees early or even less, until the detonation is no more. Detonation can kill your engine and ruin your day. The second is that some engines have so much initial advance that they have a hard time starting, the air/fuel mixture ignites so early that it is before the piston has reached Top Dead Center (TDC), so it actually works against the engine turning. This can be fixed by either less advanced timing (not as early) but then you lose performance, or it can be fixed by having a switch on the dashboard that cuts the current to the ignition coil. Thereby the engine is given a chance to get up to decent rpm on the starter, then the ignition is switched on and the engine starts willingly. Keine hexerei…