An EGT gauge is used to monitor the exhaust combustion temperature coming out of the engine.  By keeping track of EGT's, we will instantly know if the engine is running in a dangerous condition.  Lean mixtures would result in higher-than-normal EGT's.  Overly advanced ignition timing would also result in higher EGT's.  Rich mixtures would display lower EGT's, but excessively rich mixtures would result in fuel being ignited in the exhaust system which easily elevates EGT's also!

The EGT gauge can be used for tuning AFR mixtures if you have means to control fuel delivery.  This will work with fuel controllers to full-blown stand-alone EMS units.  If you've got a good idea of what EGT's you're supposed to be running at under certain conditions, then you can easily tune and adjust fuel delivery solely with an EGT gauge.

Due to the higher heat levels a rotar engine can attain, this eliminates a lot of the EGT gauges available, since many were designed for use on piston engines that run much cooler.  Rotary engines can run as hot as 1000C which is about 1830F; most piston engine EGT's run only up to 1600F to 1700F.

How to use an EGT gauge?  I've written this up recently, and I've decided to incorporate it into this page...

EGT gauges will read slightly differently for different cars EVEN WITH THE SAME 

If you want to make absolute sure what YOUR safe temps should be, get the car 
on a dyno with a wide-band.  This will verify what is safe and what is not.

The EGT gauge is one of the most misunderstood tuning tools, as interpreting 
it's output is not very straight-forward.  Wide-bands have always been popular, 
because you shoot for a target number, and reaction time is quicker (versus EGT).

EGT probe placement is also paramount in how the EGT gauge reacts and what kinda 
temps it displays.  Funky exhaust gas resonances can mess up readings off the 
EGT gauge!  I've tuned a big turbo FC running a divided turbine set-up that has a 
GReddy EGT installed with the probe in the downpipe; EGT's have always read low.  
Leaning cruise fuel mixtures to the verge of lean surging will only show 700C; I 
could do this to my car, and my exact same EGT model would easily show 800C+!  
That's a different of over 100C!  This is why you should "calibrate" the EGT 
gauge readings on a dyno.

Now, once you know what your safe EGT number is, we need to explain the relationship 
between lean and rich.  The engine has a narrow window to which the EGT is pretty 
stable.  We are primarily concerned with WOT (i.e. best power) EGT numbers, so 
we'll concentrate on that.  At "best power", the EGT number should be pretty 
stable.  Going leaner than best power will raise EGT - the dyno should also 
show the engine LOSING POWER.  This is due to inaudible detonation.  If we go 
leaner, EGT will start to skyrocket and detonation is pretty evident.  From best 
power, if with richen it up, the EGT will actually go DOWN for slightly richer 
conditions.  You still get almost complete combustion, but the extra fuel cools 
everything down.  If we richen it up substantially, it'll come to a point where 
all the extra fuel will start to ignite out the exhaust port, and this will cause 
the EGT to rise (this looks exactly like lean!) and sound like detonation, because 
the excess fuel is igniting in the exhaust system; this is when too rich will also
raise EGT.

Now, on top of everything, there is a delay in the response of the EGT.  REMEMBER 
THAT!  O2 sensors (narrow and wide) will react pretty quickly, but the EGT reacts 
significantly slower...and you need to keep that in mind, especially when tuning 
fuel controllers and stand-alones.

With that in mind, these are the SAFE numbers I target until I can verify (with a 
dyno) that you can go hotter...

EGT probe in downpipe, 700°C
EGT probe in turbo exhaust manifold, 740°C

Farenheit Celcius
800F 426C
900F 482C
1000F 537C
1100F 593C
1200F 649C
1300F 704C
1400F 760C
1500F 815C
1600F 871C
1700F 927C
1800F 982C
1900F 1038C
2000F 1093C
2100F 1149C
2200F 1204C

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