Jim Muller wrote :
> <a very nice explanation of "why vacuum advance">
> The smaller, cheaper cars might even dispense with the vacuum advance
> completely on the grounds that with a light car, you can tolerate
> slightly
> retarded timing most of the time and still optimize the timing for full
> throttle.
Jim :
I presume you're suggesting that such cars would operate at near
full-throttle all the time, and hence not need vacuum advance for best fuel
mileage. In general this is not the case for even the most underpowered of
cars. It only takes roughly 10 horsepower at the wheels for an average car
to maintain 60 mph on a level road. (Obviously this depends somewhat on the
aerodynamics of the car, but it depends surprisingly little on the weight of
the car.) So even the anemic original VW 'bug' with it's 30hp engine spends
much of it's time at less than 1/2 power.
There were of course many cars made for the US market that had no vacuum
advance (later TR6 being an obvious example), but that was for US-mandated
emission reductions. Optimum advance leads to higher cylinder temp and
pressure, which means more NOx production. Thus running with the spark
retarded from optimum means less NOx production. As the US car makers
figured out pretty quick, EGR is a better way of achieving this goal.
Randall
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