DISTRIBUTOR AND TIMING MODIFICATIONS
Posted: 06 Apr 2001 19:55DIZZY MODS
If you want the best possible performance and throttle response from your car then the ignition timing curve is an area where great improvements can be found for next to nothing as far as cost is concerned.
I will mention here only stuff that applies to street or street/strip vehicles. If you have a race ONLY car then you need to consider things like locking the advance, phasing the distributor and grinding the rotor.
First of all you must make sure that your distributor is not worn. Obviously, for high performance, an electronic distributor is best but there is no need for an expensive aftermarket unit with roller bearings. The stock electronic unit with ordinary bushings will be fine up to 10,000 rpm.
Another thing you can do is to check that there is a ventilation hole in the top of the cap. If not, then drill a 3/8" hole directly in line between the coil wire terminal and the No.1 cylinder terminal, as close to the No.1 terminal as possible. If you shine a light into this hole you can line up the rotor to get the engine started if you have had the distributor removed. This hole will also vent the cap. Venting lets the ozone out which builds up wherever there is high voltage. Ozone is not as good a conductor as air. I might not do this on a slant six engine as the dizzy is in a good position to collect dirt and water.
CUSTOMISING THE ADVANCE CURVE
If you have a lot of money to burn you can buy an ignition timing computer which will electronically set one of a number of timing curves for your car or can be programmed and adjusted with a lap top computer. Neat and cool!
The rest of us poor folk have to fiddle with the centrifugal weights and vacuum advance. You will need an adjustable (dial back) timing light and or a timing tape on the crankshaft balancer and an accurate meter to measure the engine rpm.
Total advance in a distributor as described here means the initial advance plus the centrifugal advance. Vacuum advance is measured separately and does not apply at wide open throttle.
CENTRIFUGAL ADVANCE
First of all it is essential that the dwell is set as accurately as possible if you`re using points as this will have an effect on your results. The vacuum advance must also be disconnected and plugged to stop air being sucked into the inlet manifold.
Next the initial advance at idle must be measured and noted. Then increase the engine rpm noting the amount of advance in 500 rpm increments up to 4,000 rpm or until the advance reaches a maximum.
Now slowly decrease the rpm noting the same 500 rpm increments. If there is more than 2 degrees difference between the timing as rpm increased and that as it decreased then the centrifugal weights are sticking and need attention. You will then need to do these measurements again.
Subtract the initial advance from each of your readings to obtain the amount of centrifugal advance at each rpm.
You can buy kits with different weights and springs to modify both the amount and the rate of advance. Usually one spring will be lighter than the other.
It is difficult to generalise about the correct curve for any engine but I will give you pointers to get you in the ball park.
First try to get the total amount of centrifugal advance down to 20-26 degrees max. in order for a higher initial advance to be set. This is more important on a modified car with a hot cam and big gears. You can do this by filling the slots for the weights with epoxy resin in order to limit the amount they can swing out.
Make sure you are only on initial advance from idle up to 1200-1500 rpm.
You want half your centrifugal advance in by 2300-2400 rpm and the rest in by 3200-3500 rpm.
Now set your total advance by revving above 3500 rpm and moving the distributor until you read 34(min) to 38 (max) degrees.
Make a note of the advance at idle now as this is your new initial setting. It should be between 10-16 degrees.
A real world test of your ignition curve is to set a slightly higher initial or total advance so that the engine is just starting to "ping". If you get a ping at all rpm then you have done a good job and you just need to set it back to where you had it. If though, for example, it only pings at high rpm then it needs more advance at low rpm.
VACUUM ADVANCE
You need to use a vacuum advance on the street in order to get good mileage and to keep the plugs clean. But because you are now running much more advance in the distributor you need less vacuum advance. In fact you only need about 10 degrees.
I have made the slot smaller in mine in order to limit the amount of travel and I ended up with about 11 degrees which is close enough and it seems to work well.
It is possible to buy a vacuum advance with only 10 deg. built in and you can also get adjustable units that you can change both the amount of advance and the rate it comes in at. I have a set of instructions for setting one of these up someplace.
If you want the best possible performance and throttle response from your car then the ignition timing curve is an area where great improvements can be found for next to nothing as far as cost is concerned.
I will mention here only stuff that applies to street or street/strip vehicles. If you have a race ONLY car then you need to consider things like locking the advance, phasing the distributor and grinding the rotor.
First of all you must make sure that your distributor is not worn. Obviously, for high performance, an electronic distributor is best but there is no need for an expensive aftermarket unit with roller bearings. The stock electronic unit with ordinary bushings will be fine up to 10,000 rpm.
Another thing you can do is to check that there is a ventilation hole in the top of the cap. If not, then drill a 3/8" hole directly in line between the coil wire terminal and the No.1 cylinder terminal, as close to the No.1 terminal as possible. If you shine a light into this hole you can line up the rotor to get the engine started if you have had the distributor removed. This hole will also vent the cap. Venting lets the ozone out which builds up wherever there is high voltage. Ozone is not as good a conductor as air. I might not do this on a slant six engine as the dizzy is in a good position to collect dirt and water.
CUSTOMISING THE ADVANCE CURVE
If you have a lot of money to burn you can buy an ignition timing computer which will electronically set one of a number of timing curves for your car or can be programmed and adjusted with a lap top computer. Neat and cool!
The rest of us poor folk have to fiddle with the centrifugal weights and vacuum advance. You will need an adjustable (dial back) timing light and or a timing tape on the crankshaft balancer and an accurate meter to measure the engine rpm.
Total advance in a distributor as described here means the initial advance plus the centrifugal advance. Vacuum advance is measured separately and does not apply at wide open throttle.
CENTRIFUGAL ADVANCE
First of all it is essential that the dwell is set as accurately as possible if you`re using points as this will have an effect on your results. The vacuum advance must also be disconnected and plugged to stop air being sucked into the inlet manifold.
Next the initial advance at idle must be measured and noted. Then increase the engine rpm noting the amount of advance in 500 rpm increments up to 4,000 rpm or until the advance reaches a maximum.
Now slowly decrease the rpm noting the same 500 rpm increments. If there is more than 2 degrees difference between the timing as rpm increased and that as it decreased then the centrifugal weights are sticking and need attention. You will then need to do these measurements again.
Subtract the initial advance from each of your readings to obtain the amount of centrifugal advance at each rpm.
You can buy kits with different weights and springs to modify both the amount and the rate of advance. Usually one spring will be lighter than the other.
It is difficult to generalise about the correct curve for any engine but I will give you pointers to get you in the ball park.
First try to get the total amount of centrifugal advance down to 20-26 degrees max. in order for a higher initial advance to be set. This is more important on a modified car with a hot cam and big gears. You can do this by filling the slots for the weights with epoxy resin in order to limit the amount they can swing out.
Make sure you are only on initial advance from idle up to 1200-1500 rpm.
You want half your centrifugal advance in by 2300-2400 rpm and the rest in by 3200-3500 rpm.
Now set your total advance by revving above 3500 rpm and moving the distributor until you read 34(min) to 38 (max) degrees.
Make a note of the advance at idle now as this is your new initial setting. It should be between 10-16 degrees.
A real world test of your ignition curve is to set a slightly higher initial or total advance so that the engine is just starting to "ping". If you get a ping at all rpm then you have done a good job and you just need to set it back to where you had it. If though, for example, it only pings at high rpm then it needs more advance at low rpm.
VACUUM ADVANCE
You need to use a vacuum advance on the street in order to get good mileage and to keep the plugs clean. But because you are now running much more advance in the distributor you need less vacuum advance. In fact you only need about 10 degrees.
I have made the slot smaller in mine in order to limit the amount of travel and I ended up with about 11 degrees which is close enough and it seems to work well.
It is possible to buy a vacuum advance with only 10 deg. built in and you can also get adjustable units that you can change both the amount of advance and the rate it comes in at. I have a set of instructions for setting one of these up someplace.