Man did you ask a tough one.. to begin with EGT has little to do with
air fuel ratio if the timing is not known..the easy one is the probe
ought to be about 1 1/2 to 2 inches from the port in an area that will
measure the average temp.. avoid the middle of bends especially.. Octane
is compression ratio related so if low compression low octane and if
picked right then the burn speed works out ok. A quick way to see if the
octane is too high is if you have an engine that wants a lot of timing
and the chamber and spark plug is not all that shrouded.. Then you can
run a timing value that is typical for the chamber design. If the timing
is correct a good number to shoot for is 1325 or so and under 1400 for a
NA engine on gas...turbos can go to 1600 or more and nitrous if done
right ought to be 1225 to 1300 but less than the temp without it..the
short version anyway in my mind.
Dahlgren
John Beckett wrote:
>
> Here's a little something I found in "Hot Rod" about exhaust.
>
> "During the combustion process, the leading-edge flame-front
> temperatures may approach or exceed 2,500 degrees F, while exhaust gas
> temperatures measured in the header primary tube register 1,300-1,600
> degrees F. Though many factors impact exhaust gas temperatures (EGT), flame
> speed and compression ratio are the dominant ones. As compression ratio
> diminishes, flame travel slows down and creates higher EGT readings. This
> occurs because the flame burns during blowdown and exits from the exhaust
> valve. Likewise, when ignition timing is retarded, the EGT increases."
>
> So I have two questions: For racing is there one EGT reading to indicate
> the correct A/F ratio? Cause it looks like compression and timing will
> change the reading, but how much? And how far from the cylinder head should
> the probe be placed?
>
> John Beckett, LSR #79, E/FCC
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