Distributor Advance Curves

(Roger McWilliams) #1

Does anyone have distributor curves to share? Looking up specifications for the 3 1/2 Mark V engine gives the shop manual numbers, but I cannot find any curve data. Learning what others have could be informative.

From my car, here is a curve set taken with the car stationary in the garage and throttle operated by reaching with hand across back of engine while measuring rpm electrically off the coil and timing light on the crank damper (marked for degrees, pointer mounted nearby on the chain case).

For the Lucas 40149/E measured, the engine was set to idle at 450 rpm with static 5 degrees before top dead center timing with vacuum detached and plugged. The mechanical advance was measured from 550 up to 2300 rpm and progresses consistent with the shop manual specifications (without knowing the overall curve specification). Then vacuum was reattached and total advance was measured from 550 to 2500 rpm. I note the vacuum attached retards the timing 5 degrees at idle (going from 5 BTDC mechanical only to TDC when vacuum attached), this I don’t understand. Taking the total advance (vacuum and mechanical attached) and subtracting the mechanical advance gives a vacuum contribution which has range in line with the shop manual specifications.

(Roger McWilliams) #2

Here is a follow-on for Lucas distributor curve for Mark V.

Attaching and detaching the vacuum line from the engine caused about a 5 degree shift in timing at idle. It also was found that the Sears dwell-rpm meter used did not indicate rpm well at above 2100 rpm, hence data on this second plot are limited.

The plot shows the centrifugal advance versus engine rpm with, and without, static advance.

The plot shows vacuum advance. Vacuum advance was determined by measuring the ported vacuum versus rpm at no load, transmission in neutral, with a mityvac gauge. Then with engine idling, the mityvac pump and gauge were used to provide the vacuum to the distributor with resultant advance change measured.

Idle advance at 450 was set to 5 degrees before top dead center with vacuum line attached. Then total advance of static, centrifugal, and vacuum was measured with engine at no load, transmission in neutral.