[TR] Vacuum advance/retard

[Randall]

> Vacuum advance did 
> not disappear, and is still in use today, albeit usually 
> electronically.

Timing advance at part throttle is used today; but it did mostly disappear
for a decade or so.  I don't believe any emission-controlled Triumph had it
after 1972 or so (unless maybe some of the TR8s had it).

The reason was the same reason that vacuum retard appeared : to reduce
"harmful" emissions.  Having the timing retarded from optimum under cruise
conditions reduces the formation of NOx (by lowering peak combustion
temperature and pressure).  But once more effective means of reducing NOx
emissions were found (like EGR and 3-way catalysts), some timing advance
could be reintroduced to improve fuel mileage.

Randall