thermal effects of combustion chamber

[Edward Hernandez R]

"Looking for somebody to bite  on the discussion of aluminum pistons, 
valves, ceramic piston tops or coatings with respect to fuel and 
ignition requirements....500 Caddy vs '84 Jag V12"

There are lots of reasons why the Caddy and the Jag react differently 
to the same fuel inspite of the Jag's higher compression:

1) The Jag is a 5.3L V12 while the Caddy is a whopping 8.2L V8. That 
means that each cylinder of the Jag is less than HALF the size of the 
Caddy. At equal burn rates, the Jag is much more likely to finish 
combustion before the end gas detonates than the Caddy. That said, I 
would venture an educated guess that the Jag chambers burn faster than
the Caddy's, giving it another advantage.

2) Al has 3 three time the thermal conductivity of Fe. Therefore, you 
can run higher compression since Al heads will absorb more heat of 
compression than Fe. Incidentally, this heat gets transferred to the 
coolant, sent to the radiator and dumped to atmosphere INSTEAD of
turning the wheels, so the only way to win with Al is to run higher 
compression.

3) Al has about twice the specific heat of Fe. Therefore, Fe heads are
  more susceptible to developing and maintaining hot spots than Al. If
you'd like, you can polish the Caddy's combustion chambers to help, 
but I'm not sure you'd notice the difference.

4) You didn't mention the use of an intercooler, so assuming you don't
have one, you've just raised the charge air temp before any 
compression takes place in the cylinders. See how this all stacks up 
against the Caddy?

One way to take advantage of Fe is to run lower coolant temps. I 
wouldn't play with improving coolant circualtion in the heads unless 
you know where the problems are and how to fix them. It's easy to make
 it worse if you're guessing. 

Ed Hernandez
Ford Motor Company
ehernan3 at ed8719.pto.ford.com