<VV> Jets for power

[Padgett]

>My understanding is that richer runs cooler - less (chance of) knock.

OK first you need to understand the theoretical concept of Stoichiometric 
or "perfect" combustion where all of the fuel and all of the air is used in 
the reaction. See 
http://home.earthlink.net/~graham1/MyToyotaPrius/SideBars/Stoichiometric.htm 
for a quick look with some numbers.

For maximum power from gasoline you want something like 11:1 and this is 
why many systems provide for "Power Enrichment" but obviously this is not 
good for economy where you really want 15-17:1 since you can operate quite 
lean at a high vacuum cruise. With a lean high vacuum mixture the flame 
front propagation also slows down and you need more advance (why you have a 
vacuum advance). It is all a juggling act.

In general a leaner mixture will result in higher exhaust gas temperatures, 
too lean and pipes can glow. This is because the excess oxygen is still 
reacting during the exhaust cycle. Modern cars use Exhaust Gas Recycling 
(EGR) to control heat and prevent the formation of NOX which occurs when 
there is an excess of Oxygen at high temperatures during very lean 
operation. Originally this was somewhat of a kludge but modern systems rely 
on EGR to enable a very lean burn.

The whole issue is quite complicated but in general a rich mixture will run 
cooler but at a cost in MPG. Water injection is cheaper (if you do not use 
imported water).

Padgett