EGT: What’s normal

I own a 1974 Grumman AA-5, SN550. Last winter I had the engine (O-320) rebuilt. I have a question about EGTs.

I have new probes in all four exhaust pipes. What I notice is cylinders one and two run colder than three and four. I assume that this is expected since one and two are in front. Number three runs hottest. I find that four is about 75° colder and two is about 200° colder while one is the coldest — about 250° to 300° colder than number three. The above conditions were in cruise flight, 2,450 rpm at 3,000 feet msl.

Is the temperature spread between numbers three and one OK?

I think a lot of factors affect EGT. It would be helpful if you could review what’s normal.

MICHAEL MILLER

Richland, Wash.

Michael, thanks for your question regarding Exhaust Gas Temperatures (EGTs). This is a subject that raises all kinds of questions. I’ll try to simplify it as best I can because I’ve gotten the feeling over the years that folks make it more complicated than need be.

First of all, if you have a normally aspirated carbureted engine like your O-320 in your Grumman AA-5, the EGT is less critical than, for example, if you had a turbocharged engine where temperatures can and do run hotter.

On these less complicated engines, the EGT offers us a simple operating reference as to what’s taking place in the cylinder being fired. You’ve pointed out that you experience different temperatures in cylinders one and two versus three and four. This is typical for a carbureted engine because, as you might have guessed, the fuel/air mixture on these engines is poor, at best, simply because of the way the fuel and air are mixed. The fuel and air are mixed in bulk, if you will, unlike in a fuel-injected engine where the fuel is introduced directly into the individual cylinder on most injected engines.

When you couple this basic and rather crude way of introducing the fuel/air mixture into the engine’s induction system we get somewhat less than perfect results with regard to distribution. In case you’ve never taken notice of the intake pipes on your engine, check them out the next time the cowling is off and you can see them well. You’ll notice that they are not of equal length and this also contributes to the less than perfect mixture by the time the fuel and air get to the cylinder combustion chamber.

Now let’s see if I can explain the difference in the readings you see in the cockpit. My prelude about poor distribution should have given you a hint that from this we’d expect that not all temperatures will be the same. There is no rule of thumb as to what is or is not acceptable when comparing differences of EGTs on a carbureted engine or any other engine. You will see much less of a differential on a fuel-injected engine, again because of the fuel being injected directly into each individual cylinder, which gives us much better distribution among the cylinders.

The main thing to keep in mind is that the only temperature that must not be exceeded is the one set for basically turbocharged engines. The maximum temperature for most, but not all, is 1,650° F. On a normally aspirated engine I don’t believe you can get there regardless of what you do. Anything less than that is fine and is of no concern.

The readings you mentioned seem “”normal”" and may change sometimes as power is applied or reduced. It also may change when you lean the engine at or below 75% power in cruise. Again, this is a result of the entire induction system and is nothing to be concerned about as long as it remains pretty much the same.

If you notice a sudden change compared to what you’ve experienced, then a chat with your maintenance technician might be in order. It may indicate a leaking intake gasket if, for example, the EGT goes hotter than you’ve seen and the CHT follows along with it and it’s constantly that way.

There is another factor that also can influence the EGT readings. Proper installation of the probes is vital, especially as to the installed position in relation to the exhaust pipe flange and the installed depth in the pipe. I’ll assume in your case that this installation was done in accordance with the probe manufacturer’s recommendations.

I hope this has been of some help and convinced you that you aren’t seeing anything out of the ordinary for your type of engine.

Paul McBride, recognized worldwide as an expert on engines, retired after almost 40 years with Lycoming. Send your questions to: AskPaul@GeneralAviationNews.com.

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