Vacuum Advance versus Vacuum Retard

By Richard Maury

      There has been a lot of misinformation out there about vacuum advance versus vacuum retard and their applications. They each have their purpose and that is what I will try to explain here. I think that a definition of each is in order first. Vacuum advance does just what the name implies. When vacuum is applied to the unit, the timing is advanced. The amount depends on the calibration of the unit and the amount of vacuum. Not all vacuum units are the same and care should be taken before mixing them. The advance unit is usually hooked up to a port that supplies vacuum when the throttle is just opening or at light engine loads. This is because the engine can tolerate more timing advance under light load which improves efficiency and fuel economy. The use of vacuum advance pre dates the Jaguar company.
      Jaguar used the advance unit in the Series 1 XKE 3.8 and 4.2 models. Due to the differences in the two engines, the advance rates of the distributors and vacuum units were different. If you are not sure which you have, check the parts manual for the part number of the distributor. As a rough guide, the 3.8 distributor has a mechanical advance of 13 degrees which translates to 26 crankshaft degrees. Add initial timing of 10 degrees and you get a total of 36.  The 4.2 uses an 11 degree mechanical advance curve for a total of 32 degrees using initial timing of 10. The difference is because the 3.8 cylinders match the chambers in the head. The 4.2 had to have the crank changed so the cylinders are all spaced evenly. Jaguar did not redesign the head so only cylinders 2 & 5 match the head with the others offset. This required the timing to be changed to maximize power without pinging.
      Vacuum retard was not used by Jaguar until emission controls mandated its use. Even then, Jaguar did not use the retard unit until 1970. The 68 & 69 models used a distributor without either unit. The mechanical advance on this unit was 20 degrees which gave 40 at the crank. Add initial of 5 and you get 45 degrees total advance. The extra advance was necessary as the leaner mixture slowed down the flame propagation so combustion took longer. This required the spark to happen sooner. One has to be careful if converting one of these cars to triple SU carburetors that the proper distributor is also used. To much high speed advance can melt the top out of the pistons with expensive results.
      Vacuum retard came on the XKE in 1970 when Jaguar changed the air filter design to a flat style mounted directly on the face of the carburetors. The retard was necessary to control Nitrides of Oxygen or NOx. Peak combustion chamber temperatures caused excessive NOx so retarding the timing kept the combustion from completing and getting to hot. Timing on these cars was usually set with the vacuum line disconnected and plugged. You would notice when you hooked it back up that the engine would slow down. This is because the timing was less than optimal for good running. This also caused the engine to put off more heat and consume more fuel. Even though full throttle allowed the retard unit to be inactive, most of the driving was part throttle and idling. Some owners have put a vacuum advance unit on their Series 2 cars and hooked it up to manifold vacuum. The biggest problem with this is that the timing is over advanced at certain ranges and can cause engine damage unless the mechanical advance is also modified to suit.
      Vacuum retard was used on the V12 XKE also. However, it was a smarter system as it had a thermostat system on the right rear of the engine. This system disabled the retard system when the engine got to hot. This in effect advances the timing allowing the engine to run cooler and at a higher idle. Timing on these engines is 4 ATDC. This is with the retard system hooked up. One of the big problems is not knowing that the vacuum unit is working or not and setting the timing. If the unit has developed a leak and is not working, the timing could be 16 degrees off. Initial timing on the V12 can easily be set by disconnecting the retard unit and setting the timing at 12 BTDC. Hook back up the retard unit if you wish or need to for emissions.
      Putting in a Vacuum advance unit from a later car onto a V12 car can be done. One of the carburetors should be drilled so that the advance unit can have ported vacuum at light throttle, not full time throttle. Jaguar did this on the later XJ6 cars with carburetors if you need a pattern as to where to drill. The hole should be small and not exposed to manifold vacuum until the throttle just starts to open.
      All of the timing curves assume that the distributors are in good working order and the weights not rusted up. This is very common on the V12 engines because of the heat and location of the distributor.