EGT stands for Exhaust Gas Temperature. It is important to measure your EGTs when tuning and to keep them from being too high both while cruising and at heavy load.
Why measure EGTs?
Exhaust gas temperature is a measure of heat in the cylinder during combustion, and is most commonly measured close to the head. Since all metals melt, deform, or undergo transformation under excessive temperatures for that particular metal, it is easy to have too high of EGT and cause damage to engine/turbo components. Thus a tuner must measure the EGTs and keep them in check or suffer the consequences. High EGTs also contribute to knock. You cannot tune your car by EGTs alone, and you should not tune your car without measuring the EGTs.
For example, you may have tuned your car and have perfect AFRs, but you still run into detonation or have suffered engine failure. You can’t figure out why, but it is likely that your timing is too low, thus causing high EGTs, which in turn cause knock. When tuning, you must monitor EGTs and tune your fueling/timing to prevent EGTs from getting too hot.
What causes high EGTs?
Lean burning of fuel (too much air to a unit of fuel) causes high EGTs. Also low ignition timing will cause high EGTs.
When do EGTs become “too high”?
This is a highly debated topic and much was written on it by various sources. EGTs are too high when they begin to pose danger to engine and turbo components. Generally, 1600 degrees Fahrenheit (871 degrees Celsius) is considered to be a good number, borderline high EGTs. It is probably safe to briefly run up to 1650F (899C) degrees. Of course you find people running and various tuners recommending to run 1700F (926C), 1800F (982C) and up to 1900F (1037C) degrees. These temperatures ARE causing damage to turbos and engine components. Here is an excellent article on EGTs by Banks Power, and it pertains to both gasoline and diesel engines.
So, why 1600F (871C) degrees is a good number? Lets look at the melting point of aluminum first – it is only 1220.666F (660.37C) degrees. Your engine has cast iron cylinder liners to help prevent the heat of combustion from melting the engine and the pistons are made out of aluminum alloy, which can withstand temperatures higher than 1220F (660C). But what about your turbo turbine wheel? Turbo exhaust housing is normally cast iron, so it can take a lot of heat, but the turbine wheel is aluminum. Regular turbos can sustain brief inlet temperature spikes of up to about 1600F (871C) degrees. Optimal exhaust gas inlet temperature, however, is about 1200F (660C) for turbos and their components.
On Subaru WRX and STI the length of the equal-length header runners allows for only about 300F (149C) degrees drop in EGT before exhaust gas reaches the turbo. The EGT drop is less in unequal length header runners.
So if you are running 1600F (871C) EGT, then the temperature of exhaust gas entering your turbo is about 1300F (704C) degrees – as you can see this temperature is at the limit of what your turbo can safely take. That’s one good reason to keep your EGTs to 1600F (871C). Keeping in mind that key internal engine components on a road engine are not designed to operate at excessive temperatures (above 1600F (871C)), the failure becomes certain if excessive heat conditions continue for any significant period of time.
Mods can be done that will help safely sustain EGTs above 1600F (871C). Such mods may include superalloy exhaust valves, oil squirters for pistons and ceramic coatings on all engine components subject to high heat, including pistons, and ceramic turbo wheels.
Why are high EGTs bad again?
High EGTs indicate excessive temperatures in the combustion chamber and have the potential to introduce detonation, thus destroying your engine, melt your exhaust valves, pistons and damage the turbo itself. Simply put, engine components and turbos on road engines are not able to withstand excessive temperatures.
Some may say that Formula 1 and other race cars run EGTs in excess of 2000F (1093C) degrees. What they fail to say is that F1 and other race engines are built with unlimited budgets, built from different materials than road engines, with special coatings to increase thermal efficiency – those engines are built to withstand high temperatures. They are not reliable for daily driving and are usually built for one race only, then scrapped or completely rebuilt. Same goes for turbos used in racecars – race teams can afford to regularly replace their high end, ceramic-coated turbos with ceramic wheels that can withstand high temperatures, albeit for one race only.
Where should I put the EGT sensor?
EGT sensor probe should be installed into a header runner, as close to the head as possible. Generally, EGT probes are installed within the first 3 inches from the head. Read below about equal vs. unequal length headers to see where you should place the EGT sensor probe on your car.
Equal length vs. Unequal length headers
While this information is not specific to Subarus, we will explain the difference using the Subaru example.
When modifying WRX/STIs, it is important to plan for the type of the aftermarket exhaust manifold (header) your will be getting. Unequal length header will allow you to keep the Subaru Boxer rumble sound, yet unequal length header is a poor choice if you want to maintain equal EGTs among all cylinders and safe turbo inlet temperatures. Since header runner for cylinder 4 is the longest on unequal length headers (on WRX/STI), you have the highest EGT in that runner and on cylinder 4. This is because hot exhaust gases travel longer in the longer runner. Cylinder 2 runner is the second longest. Since you are continuously subjecting cylinders 2 and 4 to higher heat than other cylinders it is very common to see cylinder 4 and cylinder 2 failures in Subarus.
Shorter runners of cylinders 1 and 3 provide for lower EGT on those cylinders. However, since the temperature drop is less than in cylinder 2 and 4 runners, higher-temperature exhaust gases get transferred to the turbo, subjecting the turbo to additional heat stress.
Equal length manifold (header) smoothes exhaust pulses of the Boxer engine, thus changing the exhaust note to smooth vs. burpy. Equal-length header is optimal for maintaining equal EGT between all cylinders and delivering a constant, lower-temperature exhaust gas to the turbo.
TIP: on unequal-length manifold, install the EGT sensor probe within 3 inches from the head on the longest runner. (cylinder 4 runner for Subarus). On equal-length manifold, you can install the EGT probe on any cylinder runner, still within 3 inches from the head.
How can I lower the EGTs?
EGTs can be lowered by adding more fuel for richer mixture, by adding alcohol/water injection and by increasing timing. A combination of these measures will provide best results in keeping your EGTs at or below 1600 F at load.
TIP: Subarus can run up to 32 degrees of timing near redline at WOT. Running high timing (given you do not have any detonation) will also result in a substantial increase in power. Running up to 34 degrees of timing near redline was experimented with, however adding more than 32 degrees of timing does not result in power gains, but in power loss.