To Jim Scales:

Jim, you didn't say just how much larger the diameter of your new system is
than your old so this is just a guess, but I'd say that an increase in the
diameter of the exhaust pipes would allow a bit more gas expansion as it
leaves the cylinder, thereby producing some additional temperature drop over
what you had been getting. A relatively small increase in diameter produces
quite a bit of additional area (or in this case, volume) so the cooling due
to expansion could be more than expected.

Also a guess, but it's probable that your engine is breathing somewhat
better and producing more power than before, resulting in the slightly
higher fuel burn. Changing any of the parameters in the intake/exhaust
dynamics likely changes the flow characteristics in some subtle ways. Trying
to predict or explain the results, numerically, could be difficult to say
the least.

Since it takes a significant increase in HP to see much cruise speed
increase, you probably don't see much change there but you might see better
rate-of-climb figures (if you are getting more power) since ROC responds
more directly to HP changes.

Again, just a guess. You'll probably get a better answer from some of the
engine gurus on the list.

Dan Schaefer