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Written by Vic R   

Exhaust Gas Temperature (EGT) and Cylinder Head Temperature gauge (CHT) can be used to help adjust air mixture. CHT and EGT each tell you slightly different things, and using them together tells you some things that neither one could tell you alone. EGT has some advantages because of its basic construction and its mounting location. A thermocouple responds very quickly. Because the CHT sensor has to respond to the temperature on the outside of the head, it cannot respond to changes in combustion temperature as fast as the EGT probe that is directly in the exhaust gas stream. Secondly, the EGT probe is not exposed to the outside air; it is not affected by changes in outside temperature. By comparison, since the CHT is measuring the temperature of the cylinder head casting itself, and since the cylinder head is one of the engine's primary means of shedding heat to the air, the cooler the air, the cooler the CHT reading and vice versa. For quick, consistent temp readings, EGT is a better option if you had to choose.

But what exactly are we trying to determine with these temp sensors, anyway? EGT and CHT are simply ways of trying to judge the relative fuel/air ratio. We all know how critical it is to have the carb mixture correct, by adjusting the carb needles on a 2 cycle. And it's generally agreed that the leaner the mixture, the hotter the engine will run. But what is really happening inside there? Does hotter always mean better, or just sometimes? Well, the truth is, it's mainly a matter of air. Many of you have had the experience of hitting the set-up just right in practice and then waiting excitedly for the race, certain you're going to blow 'em all away this time. But when the time comes for your race to start, suddenly you've lost that wonderful top-end RPM you had in practice, or the clutch just won't pull like it did in practice, or some other problem pops up to spoil your day. You havent changed a thing, but the air may have changed things for you! As the air temperature goes up, or the humidity goes down, or a storm front blows in, the density of the air changes, and that changes the fuel/air ratio that your carb delivers. If you don't recognize what's happening and adjust accordingly, youre going to suffer.

So how can you stay on top of the effect that changing air conditions is having without bringing your own weatherman with you to the track? With an EGT gauge you can take a lot of the guesswork out of carb tuning. 'Remember we said that it was generally agreed that a leaner fuel/air ratio was always hotter. And when we asked if hotter was always better? Well, you guessed it, neither one is true. If you get the fuel/air ratio too lean, the combustion temperature will actually go down! "Wait a minute," you say. "I know that when I lean the engine out it just keeps getting hotter until it sticks!" If all you have to go by is CHT you're absolutely right. When your engine gets too lean, the skyrocketing temperature you see on the CHT is probably not really an indication of hotter combustion. Most likely it's a warning sign of DETONATION. Detonation is the collision of two flame fronts inside the combustion chamber, where there should be just one, and it's the single biggest cause of heat related engine failures. Savvy drivers can often sense that an engine is slowing down and richen up the mixture to control the detonation. But you don't need decades of experience to spot detonation before it puts you on the trailer for the day. Detonation floods the combustion chamber with heat, so the CHT goes up, but with CHT and EGT readings, if you see CHT rising and EGT going down, it's a sure sign of detonation. As a guide - a Yamaha S max EGT temperature is 650 degree C .

A quick adjustment will restore the power and save that expensive rebuild. Even with just EGT, it's a lot easier to get the most out of your engine without burning it down. EGT should climb as the RPMs come up on the straight, and then drop when you lift for the corner. If it drops when you're pulling off a hard corner, or under acceleration, you're on the detonation expressway back to the shop for a rebuild.

So to summarize, we know we want to run the fuel/air ratio as close to ideal as possible. And we know that the ideal fuel/air ratio should produce the hottest combustion flame. While the cylinder head temperature gives us some indication of the combustion temperature, it can be slightly misleading because of air temperature or other weather conditions. Because of the mass of the cylinder head, CHT can take a few seconds to register a change in internal temperature.

At a new track, most of us aim for that magical temperature reading that has always told us we're on target - at home. To get that number back when we're at a new track we'll play with carb settings, pipe length and gearing. And yet, that strange reading will probably be telling you that the gear you've got is wrong. Try leaving your engine/pipe adjustments where you know they work and see if just a few teeth more or less won't pull your engine back into its ideal operating temperature range.

Home Setting :- 80 Teeth

In this example, you have found your engine runs its best at your home track with 80 teeth, a 5/8 turn high speed jet setting and a temp gauge reading of 160 degrees C. The stop watch and reading of the spark plug and piston crown have consistently confirmed this. Jet Setting 5/8 turn = 160 degrees C. 

Now you move to a new track

Geared too low :- 84 Teeth  (engine will run cooler)

Too many teeth on the back when you don't need them can keep the engine from working as hard as it should, thus causing it to run too cool. Without considering the gearing, you might find yourself leaning the carb down dangerously low to get the temp reading back up to 'normal'. this risks a stuck piston. Jet Setting 3/4 turn = 120 degrees C, Jet Setting 1/2 turn = 150 degrees C,  Jet Setting 5/8 turn = 135 degrees C.

Geared too high :- 76 Teeth (engine will run hotter)

Too few teeth on the back when you actually need more can overheat the engine and cause it to run hot. You might find yourself richening the carb way past normal in a vain attempt to bring down the temp gauge reading, but the engine will feel lazy and loose acceleration. Jet Setting 7/8 turn = 160 degrees Jet Setting 3/4 turn = 175 degrees C,  Jet Setting 5/8 turn = 190 degrees