AFR vs. EGT Gauges: An Inside Look At How they Function And Tuning Benefits of Each

Air fuel ratio and exhaust gas temperature gauges are both useful tuning tools with unique pros and cons. We take an inside look at how they function and how each can best benefit specific applications.

We often take for granted that we live in an era of technological wonder. Instead of paging through the Sears catalog and waiting weeks for a mail order to arrive, practically anything you want can be delivered to your door overnight. In our pockets, we carry devices that can tell us where we’re going with a precision measured in inches, provide access to the entire sum of human knowledge, and condense dank memes and funny cat videos out of thin air. And most significantly (at least for gearheads), you can stick an affordable device in your car’s tailpipe and know exactly how rich or lean your engine is running at any given fraction of a second.

It wasn’t always that way, though. Before the introduction of the wideband oxygen sensor back in 1992, options were limited, and it took years for the technology to drop in price enough to become truly widespread among both professional tuners and serious do-it-yourselfers. In those days, if you wanted to have any idea of what your air/fuel ratio was doing moment to moment, you relied on an exhaust gas temperature gauge. While an EGT reading wouldn’t tell you your mixture directly, it gave crucial insight into whether you were running lean or rich if you knew how to interpret what it was telling you.

So in the age of Amazon.com, smartphones, and inexpensive and accurate wideband oxygen sensors, is there still a place for the EGT gauge in a tuner’s arsenal of tools? We say yes, for a number of reasons. To understand why, we’ll take a closer look at how a wideband O2 sensor works, what an EGT gauge does, and how the combination of the two can expand your understanding of what’s going on inside your engine.

Necessity is the Mother of Invention

Oxygen sensors were a crucial technological advance necessary to make electronic fuel injection truly practical. Ever-stricter emissions regulations and fuel economy standards in the early 1980s meant that time was running out for the venerable carburetor, but without a way to “close the loop,” early fuel injection systems were no better at controlling the fuel-to-air mixture than the hardware they were replacing. The answer was the oxygen sensor – technically, an electrochemical fuel cell that, when exposed to exhaust gas and an outside source of reference air, could determine in real-time when an engine was running at the chemically “correct” stoichiometric 14.7/1 ratio, where all the available fuel was being mixed with the right amount of oxygen.

While knowing when the exhaust flow crossed that perfect ratio was good enough to give first-gen electronic fuel injection systems the ability to self-correct (and improve both emissions and fuel economy), narrow-band oxygen sensors were a blunt instrument at best, only accurate at the stoichiometric point and reading simply “rich” or “lean” on either side. In order to be really useful as a tuning tool, the oxygen sensor had to evolve into a “wideband” device capable of accurately measuring lean and rich mixtures.

Oxygen sensors are mounted with the tip flush with the exhaust pipe (right), while EGT probes need to be directly in the exhaust stream to produce an accurate reading.

At the risk of oversimplifying how a wideband oxygen sensor works, the breakthrough happened when the narrowband sensing technology was combined with a solid-state “pumping” cell – the wideband controller still tries to keep the narrow-band “reference” cell at a perfect stoichiometric ratio, but it does this by using the pumping cell to add more or less exhaust gas to the mix to achieve it. By measuring the amount of work that the pumping cell has to do to keep the reference cell consistent, the wideband controller can precisely determine how rich or lean the exhaust mixture is.

So if we can directly measure the air/fuel ratio in the exhaust stream, why bother with an EGT gauge? To answer that question, we need to look at exactly what information the EGT reading is giving us, how that information relates to the engine’s operation, and how to use that information effectively.

Tracking Temperature

At its most basic, an EGT setup consists of a thermocouple (an electronic component that produces a variable signal depending on temperature) and some way to display or record the signal it produces. “In the automotive space, all EGT products are using a ‘type K’ thermocouple probe,” explains Innovate Motorsports’ Felipe Saez. These types of thermocouples are sensitive across a temperature range between -330 and +2,460 degrees F, and because they’re inexpensive and decently accurate, they can be found in everything from furnace thermostats to remote meat thermometers for your Thanksgiving turkey.

The sensor itself can take several forms. “The faster response probes have an exposed bare wire at the tip, but are more susceptible to wear,” Saez explains. For EGT applications, the thermocouple is usually metal-sheathed, which greatly improves probe longevity in the real world, but means that the response to temperature changes in the exhaust stream will be delayed by the amount of time it takes for the sheath to heat up or cool down.

Even if the thermocouple was magically able to provide instant temperature readings, though, that information wouldn’t be as meaningful as you might suppose. Per Bruce Tucker of JBA Speed Shop, “Indicated EGT is somewhat misleading, because the sensor is typically located three or four inches downstream from the combustion chamber, and most of the time – during the intake, compression and power strokes – the exhaust valve is closed. So, no exhaust gas is flowing out. When the exhaust valve first opens, if you could measure it accurately, the exhaust temperature would initially be very hot but then cooling rapidly as it exits and expands. Then, if there is valve overlap, it would be scavenged by cold induction air. The result is that the actual exhaust gas temperature is changing very quickly and the sensor only provides the actual temperature of the probe.”

Because it’s not an instantaneous readout, the EGT reading is actually delivering insight into a number of different things going on inside the combustion chamber, instead of a simple glimpse at the mixture. All other factors being equal, exhaust gas temperatures will peak at a stoichiometric 14.7/1 air to fuel ratio (for gasoline) and fall off on either side of that, whether rich or lean.

However, in addition to running rich or lean, “Cam design and timing (duration, lobe centerline, lobe separation, and installed position), spark timing (advance / retard),  compression ratio, and fuel burn rate (octane) will all affect EGTs,” explains long-time drag and land speed racer Russ Eyres. “On a well-tuned, properly configured performance engine, the most significant factor that will change the EGT reading is spark timing,” Eyres continues. “An engine running retarded spark timing will read with very high EGT’s.  Cam timing, especially early exhaust opening events, will also impact EGT readings without noticeable results on engine torque or horsepower.”

Per Eyres, “Many ‘program engines’ such as 9 to 1 compression ratio engines and restricted intake engines (such as classes limited to 390 CFM 2 barrel carbs) will also read with higher EGTs than that same engine without those limitations. And finally, the fuel being used will impact the EGT.  Fuels that are higher octane or with lots of anti-detonation additives usually burn slower and have the same EGT behaviors as do early exhaust opening cams.”

It’s also important to recognize that peak EGT readings don’t necessarily coincide with maximum stress on the engine. More mechanical energy being extracted from the combustion process for a certain volume of fuel being burned means less energy headed out the exhaust, and it’s well known that peak power is almost always found on the rich side of stoichiometry, which will read cooler than peak EGT. “Conventional wisdom says that peak EGT typically happens about 75-degrees F lean of best power, and temperature falls off from there in both the lean and rich directions,” Tucker explains.

How you use an EGT gauge will depend on whether you’re using a single probe mounted in the collector, like you would with Innovate’s MTX-A Analog EGT Gauge or MTX-D Digital EGT Gauge, or a multi-channel datalog setup with one probe per cylinder, like the combination of Innovate’s TC-4 PLUS four channel thermocouple amplifier and Modular Tuning System PL-1 Pocket Logger. You may be thinking that this is a pretty obvious statement, since it’s intuitively clear that a probe located a few inches from the exhaust valve will read temperature differently from one that might be two feet or so downstream, but like so many things in life, it’s not as simple as it seems.

Location, Location, Location

For optimum responsiveness, multi-channel EGT systems should have their probes placed high in the exhaust runner in a consistent position to ensure cylinder-to-cylinder readings are meaningful. Per Eyres, “Normal EGT [thermocouples] are placed from 1 to 3 inches from the header mounting flange, which puts them about 6 to 9 inches from the actual in-cylinder burn (adding the exhaust port length to the EGT position). Normally [they] are placed on the top or upper run of the exhaust header/manifold to see the greatest flow density, and they are placed at a common depth into the flow path to avoid different measurements due to flow lamination and pressure variances.”

You’ll find examples of this in practically every photo of a set of dyno room headers you’ll ever see, but Eyres prefers an even closer location to the exhaust valve, adding, “I place my thermocouples about a half-inch into the header.”

With a single-channel EGT gauge, there are two primary options for probe placement – in the collector, where temperature data from the entire bank of cylinders is averaged out, or in a single exhaust runner, positioned in the same way you’d place it for a multi-channel setup. The trio of 1990’s DSM cars – the Mitsubishi Eclipse, Eagle Talon, and Plymouth Laser – were a perfect example of the latter strategy, with many tuners advocating EGT probe placement in the #1 cylinder manifold runner. In instances where one particular cylinder is known to run consistently leaner than the rest due to air or fuel distribution issues, this approach provides the greatest degree of safety.

Regardless of whether you’re using a multi-channel EGT system or a single analog gauge, and irrespective of probe placement, the important thing to remember when tracking exhaust temperature readings is that the number on the gauge is only useful in context, and unlike wideband AFR readings, there is no single meaningful answer to the question, “What should my exhaust gas temperature be?”

It’s All Relative

The first widespread commercial application for EGT gauges came in the 1960’s, when Alcor introduced the original analog gauge for piston-powered light aircraft. It found its way into the instrument panel of countless Cessnas (and Pipers, and Beechcraft, and every other brand during the golden age of general aviation) and changed the way private pilots flew.

Aircraft piston engine technology tends to be very conservative, biased strongly in the direction of safety and reliability at the expense of power or efficiency – after all, if your car engine dies, you pull over to the side of the road, but if your airplane engine does the same thing, you’re in serious trouble. As a result, before the introduction of FADEC (the aviation equivalent of EFI), pilots were not only responsible for managing engine power via the throttle setting, but also directly controlled the fuel-to-air ratio via a mixture knob, because at the time manual controls were more reliable and safer.

The first commercially available EGT gauges appeared in the general aviation market, and they graced the instrument panel of many a Cessna. Note that there are no temperature markings on the gauge face – just hash marks every 25 degrees, and a yellow pointer the pilot could move to mark peak EGT. This reflected the fact that adjusting the mixture was done by leaning the engine out until peak temperature was reached, then adding fuel until a ‘safe rich’ reading of 75 degrees below peak showed on the gauge.

The Alcor EGT gauge finally gave pilots a way to monitor and control their AFR, beyond the arbitrary “rich for takeoff, lean out for cruise” settings for the mixture. What it didn’t do was give an exact temperature reading – the Alcor gauges (and their subsequent multi-channel versions) simply had hash marks every 25 degrees F. Pilots would set their cruise mixture by adjusting for peak EGT, then richen the carb until the reading fell by the desired 75 degrees or so – this was the sweet spot for fuel efficiency versus stress on the engine.

Of course, this simple approach couldn’t last, and when competitors introduced gauges that displayed actual probe temperature down to single-degree accuracy, Alcor followed suit. Regardless of whether that was actually useful information, it became the de-facto standard. Rather than using exhaust gas temperature readings in a practical way, pilots became fixated on what the “right” temperature readings were, and as the technology made its way from aviation to high performance automotive applications, that same flawed philosophy followed.

Practical Magic

“As far as I know there is no uniquely correct EGT number,” states Eyres. “The ultimate goal is to not damage the engine; so as long as performance is not degraded the objective is to keep EGTs as low as possible (under 1250 to 1300 F for a probe placed in the runner).” Remember, of course, that far more than just the rich/lean status of the mixture has an effect on EGT – per JBA’s Tucker, “A rapidly rising EGT reading may be a sign of bad things happening (in other words, detonation).” The ability to effectively utilize an EGT gauge depends on learning what your baseline temperatures should be, and interpreting changes from that baseline.

Multi-channel EGT systems add another dimension. “Generally speaking, one looks for consistency between cylinders, because any noticeable variation means that the cylinder that is out of step with the rest has some sort of problem like a flow blockage, an inner chamber variation, or a cam lobe shift that is causing the odd EGT,” Eyres explains. “On an EFI engine, a simple fuel rate change to the ‘out of step’ cylinder will aid in detecting the cause of the variation by either confirming or rejecting fuel (and air) flow variation as the cause.”

“Cylinder to cylinder variations that are major (over 100 degrees) are clear indications that that cylinder is not working efficiently,” he continues. “Especially if the variation is intermittent or a new variation not seen on prior sampling. Reading the EGT will not always tell you what is going wrong, but it will tell you that something is awry, and for max power you need to find the cause. I have frequently seen individual cylinders ‘drop out’ sporadically during a test pull (at high RPM or boost), then rejoin the firing group as the engine slows down. This phenomenon is common in supercharged and turbocharged engines.” Under high boost at high RPM, the ignition system may no longer be capable of delivering enough energy across the spark gap to fire the dense mixture in the cylinder consistently, causing misleading oxygen sensor readings.

When partnered with a wideband oxygen sensor, an exhaust gas temperature gauge, especially a multi-channel system, provides an extra window into the engine’s combustion process that can save time when chasing the initial tuneup or adapting to changing weather and track conditions, and will definitely help save parts when things start to go wrong. “An oxygen sensor in a bank of cylinders will tell you the average of how well those cylinders are consuming the available free oxygen,” says Eyres. “If one of those four cylinders sporadically misfires, the O2 sensor will read that there is more free oxygen available and indicate that more fuel is needed.

Addition of that added fuel might cause the offending cylinder to mis-fire more often and the O2 sensor would then ask for even more fuel. This would continue (with most systems) until the offending cylinder (and possibly neighboring cylinders) was mis-firing so frequently that it would result in loss of power and become detectable with human ears or physical vibrations. With a [multi-channel} EGT the offending cylinder would be detected instantly by a drop of the EGT for that cylinder and the remainder of the cylinders would not have been subjected to an over-rich condition.”

While the humble exhaust gas temperature gauge may have been made obsolete for specialized task of tracking air/fuel ratio by the wideband oxygen sensor, it still has an important place in the tuner’s and racer’s toolbox once you understand what it’s telling you. No serious engine dyno test facility will do without EGT monitoring, and thanks to affordable and accurate single and multi-channel systems from companies like Innovate, it’s a smart choice for serious street-driven and race builds too, especially when partnered with a wideband sensor.

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