A point I like to think of is that 50 or 80 degrees "from" "peak" on the rich side and on the lean side is, the same temperature.  For piston engines, running 50 degrees lean of peak is the same temp as running 50 degrees rich of peak, so the valves are seeing the same temp - so why are some engine "gurus" so adamant about cruising 50 deg rich of peak?  It makes no sense to me.

 

So, I believe there is no such thing as "EGT too high".  There is only "peak EGT", whatever it is for your installation.

 

David 

 

There is indeed a "too high" EGT in both piston and rotary piston engines. And for turbocharged engines of both types this figure is even more vital to survival.

 

In many injected piston engines, take off power is adjusted using the fuel flow meter, and that number will deliver an EGT that is far below peak EGT to the rich side. In NA piston engines it is possible to detonate the engine on the ground so the high fuel flow and well rich of peak EGT gives you the protection you need by a slight bit of fuel cooling, and the slow flame front travel of the over rich mixture. The slow flame front organization acts like retarded timing and mimics a higher octane fuel and that helps stave off detonation also. The other advantage is that cylinder head temps are lower in the over rich mixture. So the while the exhaust valve sees the same gas temps at the same EGT both rich and lean of peak, the rich of peak picture has the closed valve sitting on a cooler valve seat, in a cooler head, so valve head temps will be lower.

 

I have never seen a NA rotary detonate to destruction, but you can do it if you try. In the car WOT at too low an RPM will do it. But in general terms the NA rotary will not detonate unless some other element has failed beforehand. The turbo rotary is another story. The features that protect the NA rotary from detonation assist the turbo in detonating. The long cool chamber with areas of poor combustion near the apex seal, when overpressured become the starting points for autoignition.

 

Best power in both engines (piston or rotary) is found on the rich side of peak EGT. For piston engines around 1450 to 1500 degrees but it is different for each engine. In the rotary about 1550 to 1575 degrees and again different for each engine.

 

Some builders don't like over 1600 degrees and others say 1700 max. Since is possible to incinerate the stock apex seals going very far above 1700 degrees, my thinking is that you would want to stay away from high EGTs by going well lean of peak and back some 100 degrees, or just stay away from it on the rich side. Even stainless looses strength at very high temps, so longer exhaust life and slightly lower cowl temps are available.

 

Passing into a high EGT during a shift in a race car is one thing. Sitting on that number for hours on end in an aircraft is another matter.

 

The probe needs to reach into the center of the flow to get an accurate number. Daryl Drummond likes the probe 3" from the port face. They don't last long that close to the port.

His point is that he wants the EGT not a number that has been reduced by cooling the header. 

 

 

Lynn E. Hanover