|The MGA With An Attitude
MGA Twin Cam Ignition Curve - TC-304
Later Issue Non-Vacuum Distributor
At 03:06 PM 5/9/05 -0400, Fred Emig wrote:
"The auto advance spring set for the Twin Cam distributor is part number 37H5272. It was replaced by part number 57H5134 beginning with (E) 2223. The part number for the weights changed at the same time. It went from 17H5095 to 57H5049. There were a bunch of parts that changed at that time. That is the point when they changed from the vacuum advance distributor to the traditional distributor without a vacuum advance. See The Twin Cam Service Parts List."
The 23D4 Model distributor from an early Mini Cooper S is the same model used for the MGA Twin Cam. Keep in mind that although they are the same model number, they are different part numbers. The Mini used part number 40819H and the Twin Cam used 40718A. The 40819 can be used on the Twin Cam, but you should try to check the advance curve before relying on it in a Twin Cam. That caveat would apply to any unknown distributor, even if it was a 40718A, just to confirm that modifications or wear have not thrown out the timing. There is very likely a difference in both the cams and springs used and furthermore, modifications are not unheard of.
Below is the MGA Twin Cam ignition advance curve for the later distributor with no vacuum advance. The diagram is taken from the MGA Twin Cam Engineering Technical Data book, as used at the factory during production.
The earlier distributor with vacuum advance was superseded by the non-vacuum unit, and the factory recommended fitting of the later unit to the earlier cars. The factory also covered the cost of the retrofit at the MG dealer (free to the customer), so it is intended that all Twin Cam engines should be equipped with the non-vacuum distributor. Refer to Confidential Service Memorandums MG/296 dated 28 January 1960 and MG/301 dated 24 February 1960.
Specifications for Twin Cam distributor without the vacuum advance is:
Model number 23D 4
Part number 40718A
The cam has 14 degrees stamped on it.
Measurements from a NOS spring set Lucas part no. 54412530 are:
Spring no.1 Spring no. 2
Diameter of wire 0.020 0.045
Number of turns 8 5
Spring diameter 0.170 0.195
Coil length 0.325 0.275
Overall length 0.650 0.775
Centrifugal advance test data for the Twin Cam distributor compared
with one of the many Cooper 'S' distributors:
From "Tuning Lucas Ignition Systems" by Speedsport Motobooks 1972
Advance Advance Advance No adv
Rev/min degrees Rev/min degrees Rev/min degrees below
40718A 3500 14max. 1800 9-11 700 6.5-8.5 250
40819A 3800 14-16 800 5-7 300 0-1.5 225
The date code stamped on Lucas parts was month and year, then changed to week and year in early 1970's, so for the MGA 1 63 is January 1963, not week 1 1963.
Notice: The data points listed above for the Twin Cam distributor 40718A are for the earlier distributor, before the advance curve was moved to the right. See diagram below. The red and blue dots represent the data points listed above for the earlier unit. I believe that "Speedsport Motorbooks 1972" got it wrong by posting the advance spe'cs for the earlier Twin Cam distributor (prior to 28 Jan 1960).
On 6/30/2015, Mark Hester wrote:
"Distributors Doctor spring for £22 arrived next day, so great service from Martin.
Small Spring looks virtually identical but larger spring is more different. See attached spreadsheet. Will the large thick spring be OK to use"?