A Look At Exhaust Gas Temperatures

By BRETT WINBERG

Exhaust Gas Temperature (EGT) is a good indicator of your engine's performance. A turbocharger doesn't make heat; it absorbs the heat and uses the exhaust pressure. As the exhaust is blown across the turbine blades, the blades spin at incredible speeds. The shaft of the turbine wheel is slaved to the compressor wheel so they spin together. When you spool up the turbine, the compressor turns as well. Generally speaking, the faster it spins, the more boost the turbo will make.

The problem comes when you're trying to turn the turbine too fast. To do this, it takes a lot of heat and pressure. If you get the turbine too hot, it can damage the turbo. At 1270 degrees F, the tips of the turbine blades begin to blow. If you get them too hot, they'll straighten out or even melt and you'll ruin the turbo. A factory turbocharger will pull all day long at 1250 degrees, however 1270 degrees is the DO NOT EXCEED temp for sustained use, 1300 degrees should be the URGENT point to throttle down and let the engine relax.

There is contradictory opinion and advice about the best place to install the temperature reading device (thermocouple) that supports the pyrometer, but there is no disagreement that monitoring EGT’s with a graduated gauge is a good thing to do.

EGT's will run about 300 degrees F at idle to 1400+ degrees F under an extreme load (such as merging onto the interstate with a 25,000 lb load.) Normal running temps will be between 500 to 900 degrees F. With EGT's at 600 degrees F, (not all diesel engines are equal, so these figures will vary depending on your make of engine and application) complete combustion is being achieved and converted into usable horsepower and torque as efficiently as possible. Maximum fuel mileage is achieved at this point. Above 600 degrees F the engine gets wasteful with the fuel due to load demands. Below 600 degrees F and you lose efficiency (Power and Torque). So when you're out on the road, pay attention to the gauge. If EGT's start to creep up, simply ease out of the throttle if possible.

Some publications dealing with EGT temps favor Turbocharger Inlet Temp (TIT) and not Turbocharger Outlet Temp (TOT). This suggests that the thermocouple should be mounted Pre-Turbo, in the exhaust manifold, not Post-turbo, in the down pipe. In their view, if you choose to mount the thermocouple in the down pipe, EGT readings will be inaccurate. With the thermocouple in the down pipe, the gauge reacts more slowly and is reading the TOT which is lower than the actual temp of the turbocharger. The difference between TIT and TOT can be as little as 50 degrees F to as much as 400 degrees F under a heavy load.

Other experts point out that installing the thermocouple in the exhaust manifold, ahead of the turbo, raises the possibility of the thermocouple breaking loose in the heated air stream and damaging the turbo. With greater accuracy comes greater risk.

One expert gauge manufacturer suggests that mounting the thermocouple in the down pipe and watching for a sustained temperature of 1050 F, is just as good as mounting in the pre-turbo position. Logic would suggest that if the temperature can vary as much as 400 degrees from the input to output side, then the sustained TOT temperature to be monitored under extreme load should be more like 870 F (1270F "straightening" point minus the possible 400 F variation.)

The most convenient installation of the thermocouple for a TIT application is in the lowest point of the driver's side exhaust manifold. The manifold itself is made of a high silicon cast iron and is very easy to drill and tap. There is a place on the manifold that makes for an easy installation and since it's the lowest point of the exhaust manifold, the metal chips are easy to clean out. Greasing the drill bit to capture and contain any loose metal chips does not seem to help much. Some owners drill with the motor running to blow the metal chips out in real time.

The best location for installation of the thermocouple for a TOT application is in the down pipe as close to the turbo as possible.

This article is based on our own research through trade literature, and discussion with gauge manufacturers. You will generally find that neither the manufacturer of the truck, the engine, or the various manufacturers of pyrometers take a public position on where to put the thermocouple. Neither do we. In short, watching your EGT's is highly recommended if you operate under heavy load. And just as in deciding how fast to drive, there is risk and reward in deciding where to place the thermocouple.

References: Port a Diesel - STS Turbo

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