Quote:
Originally Posted by Cliff R
How could our race car SLOW DOWN adding a very expensive billet distributor vs a plain old $5 junkyard variety points unit. All I did to the points unit was to brase down the center plate and "lock-out" the advance. The MSD was also locked-out, and with both distributors we set the timing at idle speed at 36 degrees exactly.
Hum? Well it turns out the day we tested the new MSD at the track on "old timer" was pitted next to us...
...He just smiled and told us that you lost your natural timing retard at high RPM's when the dwell falls off slightly. Me and my good friend Mike looked at each other with a "duh" expression on our faces!.......
|
Not that simple. (but keep in mind that I quit dealing with points distributors about forty years ago, and have zero interest in going back there at least until the EMP blast destroys all of our electronic devices.)
Any magnetic-pulse-triggered ignition will
also have "high speed timing retard" of ABOUT 1 degree per thousand RPM (round numbers.) When I measured a particular HEI distributor, it was more like 2/3 or 3/4 degree per thousand RPM. The GM service manuals used to advertise this "feature" in regard to the old, 1960's electronic ignition (Option K66.)
This RPM-related retard is covered-up by the centrifugal advance mechanism. If your mag-pulse distributor gives you an indicated 20 degrees of centrifugal advance at 3000 rpm, if you were to measure the geometry of the thing, you'd find 22-ish degrees of advance, minus a couple of degrees of RPM-related retard. So this RPM-related retard only becomes apparent if the centrifugal advance is either locked-out, or at the limit of it's travel.
Some folks blame this RPM-related retard on the module circuitry--the module reacts at a given, predictable rate, but as RPM increases, that fixed reaction time becomes larger in proportion to the shorter time between firing intervals. Yeah, that may be some tiny part of the overall RPM-related retard. The major factor is the reaction of the pickup coil/reluctor itself. Which is why all mag-pulse triggered ignitions have RPM-related retard, no matter what sort of module they're connected to.
So, in terms of losing performance compared to the points distributor, I'm wondering if the HEI had
MORE "high speed retard" than the Points system, and you then had not-enough advance at the "top end". I don't know how much the points-system dwell varies over the RPM range you were running. I thought dwell was pretty stable, so "high speed retard" based on dwell variation on a GM distributor is kinda new to me.
A distributor machine and an hour would tell that tale...but I suspect it's not worth the effort. Just adjust the initial timing to provide what you need in the important part of the powerband, and call it a day.
But I've been wrong before.