EGT Monitoring EGT monitoring can be a very effective tool in
tuning, but "ideal" EGT can vary from engine to engine dependant
upon combination. Usually, 1300 to 1500 degrees F is optimal
for normally aspirated engines on gas, while gas turbos will run
optimally @ 1500 to 1650. An important note: this is
measured before the turbo, not after, as the turbo will reduce EGT
by an average 200 degrees F.
Finding the optimal EGT
signature for your engine is a trial and error (hopefully more trial
than error!) procedure, and other factors such as power output, plug
readings and air fuel ratio equipment should be used to corroborate
the data. Once you have ascertained this ideal EGT, it should
be repeatable regardless of climatic conditions: simply tune
for the same previously determined optimal EGT, and your engine
should perform at full available output under any ambient
conditions.
Easy error for new users of
EGT: high temps indicate lean condition. Not always
true! Excessively rich conditions will result in "after burn",
where the fuel, which was unable to completely combust due to
insufficient oxygen in the cylinder, lights off in the exhaust
system, causing an unusually high temp reading. If all other
indicators still suggest a rich mixture, try leaning in small steps,
and you will likely see the EGT go down. Just be sure that the
power does not also go down from the changes. If you are on
the right track, power should go up noticeably as you lean towards
optimal mixture while EGT drops. As you approach and then pass
the optimal mixture point, the EGT will begin to climb again.
STOP! Richen by one step and you are there! Now, when climatic
conditions worsen (i.e.: hotter temperature, more humidity, less air
density), lean until you get that optimum EGT again. If
conditions improve (colder weather, lower altitude, less humidity),
richen for optimal EGT. Bear in mind: if EGT suddenly changes
for no apparent reason, you may have an aggravating factor (ignition
problem, fuel pressure wrong, clogged air inlet, etc.) which is
unrelated to tuning. Be observant, and the indicators should
guide you to the right tuning decision.
Another caveat: Air
fuel ratios, which are not optimal throughout the entire available
RPM and manifold condition range, will mislead you. In other
words, an optimal EGT signature at high RPM may not show an
incorrect condition at lower RPM or different manifold
pressure. Although perplexing, this problem is truly the
difference between a happy, powerful and long-lived engine and one
that is trying to destroy itself slowly but surely. It is one
reason why the precision of fuel injection is usually superior to
carburetion in both power production and engine life. Just
watch ALL the indicators, and remember: lean is mean, and fuel is
power. Instead of continuously trying to lean it for maximum
HP, try to find ways to get more air to the engine, and thusly
support the combustion of more fuel. There's only so many
BTU's in a gallon of fuel, no matter how you burn it. Just try
to burn more fuel!
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