This piece originally ran in Vic’s Checkpoints column in the January 2024 issue of EAA Sport Aviation magazine.

In the November 2021 issue of EAA Sport Aviation I wrote a column entitled “Cooling Things Down.” It was meant to help builders solve some of their cooling problems by providing some insight into the causes. From several discussions with pilots and owners of aircraft since that column, it’s become clear that not everyone understands the differences between EGTs (exhaust gas temperatures) and CHTs (cylinder head temperatures), and whether they are or are not a problem.

This misunderstanding was driven home to me when one owner remarked that his EGTs were high, and he had been working on the baffling to lower them, all to no avail. I politely told him that no amount of work on the baffling was going to affect his EGTs. That made me realize perhaps some explanations are in order.

While it is true that CHTs and EGTs can be affected by each other, it is important to understand how and why that could happen. Let’s start with them individually to understand what we are measuring.

CHT is measured either by a sensor that is placed underneath a spark plug that looks a lot like a spark plug washer or by a probe that is inserted into a threaded hole in the bottom of the cylinder. When using the spark plug sensor, be sure to remove the normal copper spark plug washer.

Maximum CHTs are recommended by the engine manufacturer. Since Lycoming and Continental are the most popular air-cooled engines, I will focus on them.

The maximum allowed CHT for Continental engines is 460 degrees Fahrenheit, and for Lycoming engines it is 500 degrees Fahrenheit. There are some exceptions. The Lycoming O-235, for example, specifies less than 400 degrees for continuous operation. The max CHT for the Lycoming IO-390 and -580 is 460 degrees Fahrenheit.

Always refer to the owner’s guide for the specific model. Keep in mind those are absolute maximum temperatures. While they should never be exceeded, they are not the ideal temps for constant operation either.

Most cylinders have aluminum heads and steel barrels, and aluminum starts to reach half of its strength around 400 degrees. The closer one gets to the upper limit, there is less cushion to prevent damage.

While many older aircraft have a single CHT probe installation, with the manufacturer usually telling you which cylinder should be monitored, the best bet is to have a multi-cylinder probe installation so all the cylinders can be monitored. Troubleshooting is much easier to perform with a multi-probe system.

For certification purposes, the engines are run for 150 hours with all temps and pressures at the highest limits. An engine continuously run at those levels most likely will not reach TBO (time between overhauls). At the price of engines today, most owners want to ensure their engine will not only reach TBO but also go beyond it.

The ideal temps for Continentals are around 360-380 degrees. For Lycomings, it is less than 400 degrees for economical operation, and 435 degrees for high-power operations, such as in helicopters.

There’s a lot of misinformation out there that has many new builders and owners trying to achieve much lower temps for their engines, and it is not ideal. I see higher oil burn in engines being run at lower temps.

EGT is measured by a probe that is placed directly into the exhaust system, usually within 3 inches of the exhaust port on the cylinder. It is important on multi-cylinder installations that each EGT probe is placed into the exhaust pipe at the exact same distance from the cylinder head, or the readings will be useless.

For normally aspirated engines, EGTs will be at their lowest point during idle operations. Hint: Keeping the EGTs above 900-1,000 degrees Fahrenheit during ground idle operations will help keep the spark plugs from fouling, as at those temps the lead in the fuel will burn off. Higher EGTs will be achieved at higher power settings, and peak EGTs will be achieved by leaning with the mixture.

For normally aspirated engines, peak EGTs will be in the mid-1,500s, but with turbocharging, EGTs can be much higher into the 1,600-1,700 range, and one needs to be careful.

Overall, the EGT gauge tells us the temperature of the exhaust gas, which is really an indication of the fuel mixture coming out of the cylinder after the combustion process occurs. A rich mixture, such as during takeoff, will have a much lower EGT than a properly leaned cylinder during cruise.

Basically, it is all about the fuel mixture, and no amount of messing with the engine baffling will affect the EGTs.

One distinction between the EGTs and the CHTs is that the EGT indication is instantaneous, while the CHT is usually a little slower to react. I say usually slower, as there are exceptions.

During detonation, which is a violent and uncontrolled combustion process, the CHTs can rise so fast that you may not be able to stop them before there is damage to the cylinder. Detonation can occur when the wrong grade of fuel is used, such as fueling a piston aircraft engine with jet fuel instead of avgas.

With multi-probe installations you can begin to check EGTs as soon as the engine starts, as they will almost immediately climb to 700-900 degrees if everything is working properly. A quick glance at the EGT gauge after startup can help you verify that all the cylinders are firing.

If the engine is not running smoothly after startup, the EGT gauge can help you to quickly identify the problem cylinder. On the other hand, sometimes the CHTs can take five minutes or more to reach an operating temperature of 200 F.

In simplified terms, and under normal circumstances, the CHTs are usually kept in check by proper airflow cooling. Some aircraft have cowl flaps that, when opened, allow for an increased airflow in the engine compartment during high-power/low-airspeed operations such as in a climb. Other than cowl flaps, there’s not much we can affect from the cockpit with regard to airflow cooling.

While there are lots of things that can affect the cylinder head temps during normal operations, the one thing we can control from the cockpit is the fuel mixture. That is done with the mixture control. For the purposes of this discussion, I will leave out the potential failure items that were presented in the column “Cooling Things Down.”

For Lycoming engines, a good EGT around 1,170-1,275 degrees Fahrenheit during the climb will usually allow for proper cooling of the cylinder, assuming the baffling and other parameters are all correct. Unless departing from high elevation airports, the mixture should be in the full rich position for takeoff and climb.

Some aircraft allow for leaning in the climb above 5,000 feet, but be sure to monitor the CHTs when doing so. You should only lean in the climb for smooth operation, not for economy. Once the aircraft is leveled off, if the mixture is left in the full rich position, the CHTs should drop a substantial amount within a couple of minutes. Once the CHTs have stabilized, then you can lean for best power or for best economy.

Proper cruise leaning should be done at or less than 75 percent power. Again, you should refer to your specific owner’s manual for the proper settings. Pulling the mixture back to peak EGT will show a slow rise in the CHT as well. You will want to keep an eye on the CHTs so you don’t exceed the maximum recommended continuous operation temperature.

Remember, the CHTs are slow to react, so give them a few minutes to stabilize. Running lean of peak should see a commensurate drop in cylinder head temps, as well as a drop in airspeed, as the engine is producing less power. Lean of peak (LOP) operations is a whole different discussion, and I will leave that for a future column.

One last tidbit: When doing your run-up, take a glance at the EGT gauge. You should see a rise in all EGTs when switching to each ignition. If there is any roughness in the engine, you can quickly determine which cylinder it is, as you will normally see a drop in the EGT. If you leave it there long enough, the EGT will drop rapidly and end up being close to the CHT on the offending cylinder.

It will give you a quick clue as to which spark plug is fouled or which ignition lead is the problem. With an electronic ignition, a failed coil will usually show you two dead cylinders, so don’t bother chasing the spark plugs.

You may have to lean the mixture to get a good magneto check. Follow the advice in the owner’s manual. It will help to keep the fun factor alive.

Vic Syracuse, EAA Lifetime 180848, is a commercial pilot, A&P/IA mechanic, designated airworthiness representative, and EAA flight advisor and technical counselor. He has built 11 aircraft and has logged more than 10,000 hours in 74 different types. Vic founded Base Leg Aviation, has authored books on maintenance and prebuy inspections, and posts videos weekly on his YouTube channel. He also volunteers as a Young Eagles pilot.