Timing for electronic ignition upgrade
In other words, the engine and operator will not notice any difference to speak of above 950 rpm.
Below 950 rpm the '68 CAP will have less unburned fuel and CO coming out the exhaust in trade for a little less torque.
Do not increase the idle speed with the idle speed screw (throttle stop) unless you have to.
One point of plotting the timing versus rpm is to see when the advance begins.
For example with the distributor for the '68 manual transmission, the specs show advance could begin between 700 and 1000 rpm, and the automatic between 650 and 950 rpm.
Now, lets say you set timing at 950 rpm with the original distributor for an automatic.
That might be OK IF the primary spring had a lot of tension resulting in an advance began at the very latest considered acceptable.
But what if the spring tension was average or a little weaker, allowing advance to begin immediately above idle rpm?
In that case, the advance follows the top curve, so by 950 rpm, 7* have been added to the initial.
In other words, the entire timing curve is shifted 7* down, and the base timing is actually 2*ATDC.
This one reason why initial timing must always be measured at consistance rpm,
Its best to measure the initial at an rpm below the start of timing advance, so a small difference in rpm makes no difference in timing. However with hot rodded engines and mystery distributor guts, this isn't aways possible.
950 rpm is approximately where the timing for CAP distributor becomes the same as the non-smog distributor.
In other words, the engine and operator will not notice any difference to speak of above 950 rpm.
Below 950 rpm the '68 CAP will have less unburned fuel and CO coming out the exhaust in trade for a little less torque.
Do not increase the idle speed with the idle speed screw (throttle stop) unless you have to.
One point of plotting the timing versus rpm is to see when the advance begins.
For example with the distributor for the '68 manual transmission, the specs show advance could begin between 700 and 1000 rpm, and the automatic between 650 and 950 rpm.
Now, lets say you set timing at 950 rpm with the original distributor for an automatic.
That might be OK IF the primary spring had a lot of tension resulting in an advance began at the very latest considered acceptable.
But what if the spring tension was average or a little weaker, allowing advance to begin immediately above idle rpm?
In that case, the advance follows the top curve, so by 950 rpm, 7* have been added to the initial.
In other words, the entire timing curve is shifted 7* down, and the base timing is actually 2*ATDC.
This one reason why initial timing must always be measured at consistance rpm,
Its best to measure the initial at an rpm below the start of timing advance, so a small difference in rpm makes no difference in timing. However with hot rodded engines and mystery distributor guts, this isn't aways possible.