Quote:
Originally Posted by Walt Fricke
And a few degrees of movement of the cap wouldn't make any difference in spark timing or intensity, would it? The cap posts inside at the rotor are what - maybe 5 or more degrees wide? And the end of the rotor maybe three times that?
Although compatibility is needed, isn't the advance determined by (other than by the ECU) the rotation (centrifugal and vacuum) due to where the points or reluctor bits are? And change in position at firing of the rotor itself kind of immaterial?
In designing an in-car cam measurement system using a replacement cap, my 944 friends warned me that the 944 cap retention system was rather loose rotationally, and they said it didn't need to be tight because the ECU did the timing and the rotor tip system was broad enough to handle the entire advance/retard range there.
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The new cap was never on a variable reluctance system, only DME (3.2L and later). Two degrees at the cap is 4 degrees at the crank, which matters. DME has a wide timing range of authority at all speeds. In fact, the DME distributor has a small, very stiff centrifugal advance mechanism to keep the rotor in the range of the cap posts at full advance. Isn't that interesting?
A vacuum push system just costs more than making the distributor rotate the opposite way. It's not technically impossible to make a push type vacuum capsule and a clockwise distributor, but why do it when there are better options?
Here is a cutaway of the dual action vacuum capsule. You can see how the annular diaphram allows for a push/pull system.
