Randall Young wrote:
> Jim Muller wrote :
> > <a very nice explanation of "why vacuum advance">
Tanks!
> > 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.
> 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... Optimum advance leads to higher cylinder temp and
> pressure, which means more NOx production... As the US car makers
> figured out pretty quick, EGR is a better way of achieving this goal.
Thanks for the come-back on this; it adds some to my own understanding.
Actually, I wasn't suggesting at all that small cars would operate at full-
throttle most the time. Rather, I was thinking more about cost. For most
mass-produced objects, any small savings in manufacturing or materials ends
up as a substantial reduction in the "suggested" retail price. (My employer
uses a 5:1 price/parts-cost ratio, which is believe is a standard in their
industry.) So it's a matter of tradeoffs - you gain a price advantage in a
highly competitive "bottom-feeder" market, but you lose a little bit in
optimized timing at part-throttle. The driver who wants more power can
always dip into the accelerator well a bit further, and what the heck, he'll
never notice the mileage reduction if the car already does pretty good. At
least, this is the way I've always interpreted it.
The historical fact is that the later (at least) Spitfires never had vacuum
advance, though they did have the vacuum retard at idle. By comparison, the
TR6 had both. Also the Spitfire *did* use EGR to lower NOx, and air-
injection with catalyst to reduce CO and unburned C. The 1500 with O/D
could get in the upper 30 mpg's (if you kept it at legal speeds :-) so a
little lost mileage would still leave it better than most everything else.
One final point, as far as I know (though my memory is fuzzy and I'm
certainly not an expert at the Feds' air regs), the regs didn't worry much
back then about running conditions. Rather, they concentrated on idle since
the air in big cities was the problem and most cars spend a majority of
their urban time idling at stoplights.
On the other hand, what do I know?
Jim Muller
jimmuller@pop.rcn.com
'80 Spitfire (Percy)
'70 GT6+ (Nigel)
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