The Nu-Rex centrifugal advance unit is a reproduction copy of the original era accessory Phillips Automatic Spark Control. This unit is placed inside the valve chamber in place of the upper distributor shaft, and has the effect of automatically changing the ignition distributor spark timing position as a function of engine RPM, making the stock manual advance lever unnecessary.
This unit uses an imbalanced rotating ring which is gimbaled and aligns like a gyroscope as it spins. As the rotating ring aligns in the horizontal plane, a helix joint between the upper and lower shafts changes the relative angular position of the shafts, thus advancing the upper shaft relative to the lower shaft.
The geometry of the helix mechanically controls the maximum possible advance to about 30 crankshaft degrees, which is more than sufficient.
The stock Model A distributor had 40 crankshaft degrees of possible manual advance designed in, so that the initial timing could be retarded to 0 crankshaft degrees (TDC) for enabling starting with hand cranking, cold weather, and poor fuels.
The pics and data below were collected by Jim Kellett who ran the unit on a Sun distributor test machine and collected the advance curve data.
Here is the spark advance mounted in the test machine. This shows the unit which behaves like a gyroscope and also changes the relation between the upper shaft and lower shaft as the rotating ring changes attitude in response to changing RPM.
This chart also shows the advance curve data recorded for the Model B distributor. The Model B distributor has an internal centrifugal advance unit. The initial spark timing position for the Model B is set 19 crankshaft degrees before TDC which is why the data is shifted 9.5 distributor degrees (distributor turns at half engine speed).
Distributor advance degrees versus distributor RPM.
|Advance Degrees (distributor)|
|Run 1||Run 2||Run 3|
|flat with values 14.5 to 15.0|
The table above shows the distributor advance degrees versus distributor RPM of the Nu-Rex unit as read on the Sun machine.
|Advance Degrees (crankshaft)|
|Run 1||Run 2||Run 3|
|flat with values 29 to 30|
The table above has been converted to show the equivalent crankshaft advance degrees versus engine RPM based on the distributor data measured on the Sun machine.
Thanks to Jim Kellett for sharing the data and charts and pics!