Second to oil pressure, exhaust gas temperature (EGT) may be the most critical operating parameter on your diesel engine, because excessive EGT can bring a host of problems that fall under the meltdown category, both figuratively and literally. Every material has a melting point, some lower than others, and when things get too hot, expensive parts within or attached to your engine start welding themselves together or disintegrating into the exhaust pipe.
Many people know that excessive EGT is not a good thing, but there's an equal amount slightly confused by all the numbers being tossed about, concerning what actually constitutes high EGT and where to put the EGT gauge probe, since their exhaust brake or big downpipe has a fitting but the manifold doesn't. We'll give you a few clues here, with the caveat that every engine is different, and, like EPA mileage estimates, your EGT will vary. Also, note that like any temperature, EGT can be measured on many scales, so for the sake of consistency, every temperature value mentioned in this article is given in degrees Fahrenheit.
One element in the confusion is an apparent lack of standard in measuring and setting EGT limits. While one engine manufacturer might say the EGT maximum is measured no more than 6 inches from the cylinder head, between cylinders No. 3 and 4, in the center of the pipe, another builder will use a different formula. The good news is that most engineering bases EGT on the "turbine inlet temperature," meaning in the exhaust stream just before it enters the turbocharger.
You could measure EGT at each exhaust port as many engine developers do, but since that gas flow is an individual pulse, the average is not as high as it is at turbine in. Furthermore, you need a rather expensive thermocouple and data acquisition to measure such rapid fluctuations with accuracy.
With enough perseverance, you can often find a maximum EGT for either turbine in or turbine out; the latter is often the results of measuring the same engine at two points, simultaneously. However, the delta cannot be considered a constant among different engines or states of tune on the same engine.
Another element involves conditions in which maximum EGT is measured. Obviously, engine dynos and test stands are quite consistent, but if you're dragging that overweight toybox up a 7 percent grade with the A/C on because it's 110 degrees outside, your engine might reach maximum EGT faster than during the builders' tests, and it may not drop as much between turbine in and turbine out because of underhood airflow.
Increasingly, common variable geometry turbochargers also deliver different deltas across the turbocharger because as the vanes move, temperature and pressure in the exhaust changes. At maximum load WOT and high boost, the variance between turbine in and turbine out tends to parallel a conventional turbo, but part-throttle mid-rpm loads may not.
If you remember high school chemistry, that pot of pasta you boiled last night, or the last time you accidentally leaned on your air compressor, you'll recall that temperature increases with pressure. In fact, this is the principle that drives your diesel engine by using compression to generate enough heat to start the combustion process.
As the exhaust gases come out of your engine and across the turbocharger, the heat energy and pressure in them are used to drive the compressor wheel, thereby dissipating some heat energy in the exhaust gases. As a result, peak EGT typically drops 300-400 degrees between turbine in (TI) and turbine out (TO).
Stress the word peak in the above generalization because without some boost pressure, little heat energy will be expended as exhaust gases cross the turbine. At idle, the turbocharger is doing virtually no work, and the measurable difference in EGT ahead or behind it won't be nearly as significant. One must also remember that turbocharger pressure ratios (pressure into compressor to pressure out of compressor) do not always correlate with boost levels.
To better find out how the numbers usually play out, we checked with Banks Engineering, Edge Products, and TTS Power Systems. We also fitted out a test mule with a dual-reading Westach pyrometer and identical probes, with one thermocouple at the collector in the manifold about 1 inch ahead of the turbocharger housing and the other in the exhaust brake about 1 inch behind the turbine wheel.