Given the same motor, same fuel (high enough octane to be compatible with the higher C/R) and different compression ratios (let's say one is 8.5:1 and the other is 10.5:1). Will there be any difference in exhaust gas temperature?

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Let me see if I can muddy up the waters a little here. . .

Actually, its not increasing the compression ratio that increases power output and decreases exhaust temperature. Its increasing the expansion ratio that does the trick. Now for most engines the expansion ratio is the inverse of the compression ratio -- an engine with a 10:1 compression ratio will have a 1:10 expansion ratio.

However -- with an engine with variable valve timing its possible to have, for instance, a 5:1 compression stroke and a 1:10 expansion (combustion) stroke.

How much of a difference do the different expansion ratios make?

Assuming a combustion temperature of 3500 deg F, here are some brief simplified calculations from the standard gas tables that are not completely accurate (neglects efficiencies, friction, doesn't model combustion right, etc) but should give you a rough idea of what's going on:

For a 10:1 compression/expansion ratio: Compression work: 137.9 BTU/lb gas: Output 441.09 BTU/lb, Net output: 303.18 BTU/lb, Fuel energy: 756.05 BTU/lb, Efficiency 40.1%, Exhaust gas temp 1520 deg F

for a 12:1 compression/expansion ratio: Compression work 155.4 BTU/lb gas, Output 465.1 BTU/lb, Net output 309.7 BTU/lb, Fuel energy 732.8 BTU/lb, Efficiency 42.26%, exhaust gas temp 1403 deg F

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