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The Truth About EGT

How Hot Is Too Hot?

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A powerful engine will create heat-there's just no way around it. In the diesel segment, we measure the heat created by combustion in the form of exhaust gas temperature, or EGT for short. Why is EGT important? Because if your in-cylinder temperatures get too hot, you can melt pistons, crack heads, destroy turbos, and create general havoc within your engine. A diesel needs a certain amount of heat to run, but high EGT should definitely be avoided.

How hot is too hot? We get this all the time, and unfortunately, there's no easy answer. When diesel fuel is sprayed into the combustion chamber, it is ignited by intake air that has been compressed and can reach more than 900 degrees Fahrenheit. When the diesel ignites, it can create a flame front of more than 4,000 degrees Fahrenheit. The superheated flame front heats the air around it and creates high cylinder pressures (almost 3,000 psi in some cases) and temperatures (more than 2,000 degrees). This pressure and temperature rise is what pushes the piston down, creating horsepower and torque. One important thing to remember is that although this process occurs very quickly in a diesel engine, the slow-burning properties of diesel fuel keep the fuel burning for up to 40 degrees of the crankshaft's rotation, also known as crank angle.

Where Does The Heat Come From?
During the engine's power stroke, energy from the fuel's combustion pushes the piston down. But not all of the energy released from the fuel is converted into work. The unused energy becomes heat, some of which is absorbed by the pistons, cylinder walls, and cylinder head(s). The rest of the exhaust heat gets forced out of the engine during the exhaust stroke-and this heat is what we read with our EGT probe mounted in the exhaust manifold. The exhaust heat temperature that we read on our EGT gauge will theoretically always be less than the peak in-cylinder temperature. However, it is also possible that the exhaust gas temperature at the EGT probe will be more than the sustained in-cylinder temperature, especially if the engine has radical fuel tuning. This false reading can be caused by excess fuel igniting in the exhaust manifold, which creates a secondary combustion event. Either way, high exhaust gas temperatures put the engine, exhaust valves, and turbochargers under a tremendous amount of strain and can lead to component failure. This is why we monitor EGT-to keep things from melting, cracking, and breaking.

How Hot Is Too Hot?
Based on our own experience in the sled pulling and drag racing world, we can definitely say that any engine running at 1,800 to 2,000 degrees EGT for more than a second or two should be rebuilt every season and may incur turbocharger, cylinder head, or piston damage during the season. It's just too much to ask a diesel to run at those types of temperatures. For towing, we'd say 1,200 degrees is a safe number, with occasional spikes of up to 1,400 degrees when climbing grades being an acceptable risk. The concept of EGT is analogous to putting something in the oven. You can put your hand in a hot oven for a second or two, and nothing will happen. Leave it in there, and you'll get burned. The same thing happens in a diesel engine. The longer the engine spends at a high EGT, the more likely it is to become damaged.

How Do I Control EGT?
So you may be asking yourself, if high EGT is such a harmful thing, why doesn't everybody make their engine run between 1,200 and 1,400 degrees? The simplest explanation is that it starts to get expensive to try and keep a powerful engine cooled down. Adding fuel is the most common way of achieving more power in a diesel engine, and this excess fuel creates heat. It can be controlled through a combination of larger turbochargers, aftermarket intercoolers, and water injection, but those parts can cost thousands of dollars. For most people, it's easier to just buy a programmer or injectors and live with the high EGT levels. However, living with superheated EGT levels isn't a solution-it's just putting off the problem. Sooner or later the stress on the engine will start to show, so it's best to keep your EGT low, reduce exhaust drive pressure as much as possible, and go easy on the injection timing. We have a self-imposed limit of 1,600 degrees while racing, and 1,200 degrees while towing. Any higher than that, and we'll look to water injection, intercooling, nitrous, or using less throttle to cool things down.

Competition EGT: What the experts say
Exhaust gas temperature is a funny thing. We've seen engines hurt parts at 1,400 degrees, while others will live at 2,000 degrees. In common-rail engines especially, EGT will only tell you part of the story. Since it's very expensive to measure peak cylinder temperatures and pressures, we don't know exactly what's happening during the injection event. A lot of timing and injector pulse width could create a very hot condition for the piston, but since most of the energy will be used up, the EGT reading may not be that high-even though we may be doing possible engine damage. A stand-alone computer system in which all the injection parameters can be changed and modified would be a big help in making tremendous amounts of safe power with the newer, high-pressure injection common-rail engines.
Erick Brown
- Brown's Diesel
1,138hp no-nitrous common-rail Cummins

Competition EGT: What the experts say
We have a lot of 2.8 and 3.0 Class sled pullers that can't use water injection and run in the 1,000hp range. With a good intercooler, we're still able to keep the EGT less than 1,600 degrees during the pull. There is really no reason to run any hotter than that-you'll just be harder on parts. With high-pressure water injection, we can put our EGT level wherever we want. We can take an 1,800hp Modified Class pulling engine that would see 3,000 degrees without water and reduce the EGT to 1,400 degrees.
Dan Scheid
- Scheid Diesel
Diesel competition engine builder

While large turbochargers will flow more air and help reduce EGT, one can only go so large. Here is a comparison between the 60mm compressor upgrade used on Project Rust Bucket, and the 95mm secondary turbo on Erick Brown's 4,000-rpm common-rail. Most trucks would be hard-pressed to even spool a turbocharger this big.

We've spent a lot of time talking about high EGT, but what about low EGT? Shutting a hot engine off can lead to oil burning, which leaves carbon deposits (called coking) behind. Make sure your EGT gauge reads less than 400 degrees Fahrenheit before you click it off.

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