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 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.
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
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