I know it has been discussed/referenced in past, but just looking for an idea of the temperature of the exhaust as it leaves the head, and at the opposite end of the header pipe where it enters muffler.

Ceramic packing for exhaust is rated to about 2500 degrees, which is the main reason for the question.

Depending on the mixture (air:fuel ratio) it will be between 1,600 (very lean) and 1,100 (very rich). A well jetted engine running at a stoichiometric AFR of 14.7:1 will run 1,250 to 1,300 EGT. An AFR of 12.5 to 14.7 is good for an air cooled engine.

Dave,

The data comes from 40 years and thousands of hours observing and setting EGT in air cooled aircraft engines. The data above is kind of “dumbed down”.  When setting EGT, peak EGT occurs at an AFR of 14.7:1. Whether that is at 1,300 degrees F or 1,500 F doesn’t matter. What is important is you observe the cylinder that reaches peek EGT first and then set the AFR mixture based off that cylinder. Once reaching peek EGT, any change in mixture will result in a reduction of temperature. A rich mixture causes the EGT to reduce due to the cooling effect of excess fuel, while a lean mixture causes an EGT reduction due to a power reduction. What is important to know is that a reduction of 100F due to a lean mixture is different than a 100F reduction due to a rich mixture. When setting a lean cruise power setting (lean of peek) one has to carefully observe CHT to ensure that the temperature stays in the normal zone (below 470F) and certainly below the 500F redline. I typically set power at 50-100F rich of peek. This results in a CHT in the 380-420 CHT. It’s also important to know that a high EGT (above 1600) combined with a high CHT (above 460F) can cause damage to the piston.

Sorry for the long winded answer. Both Lycoming and Continental have Advisory Circulars that discuss setting power via EGT and CHT for normally aspirated, turbocharged and supercharged engines.

I’m familiar with that chart. The thought is that as you move away from the ideal AFR (lean or rich), you are making less power and therefor CHT goes down. As you know, this is a subject where there is much debate. I have observed that CHT does in fact go up with a mixture that is LOP. However, with that said I typically don’t “wait to see what happens” and immediately enrich the mixture. In reality there may be a momentary increase in CHT...I don’t know.

My experimental aircraft (RV6) with high compression pistons and a ram air induction tends to run on the high side of CHT. I also run an electronic ignition and if my memory is correct I have it set to provide as much as 38 degrees BTDC at cruise power settings, much further advanced than what a mag provides at 25 BTDC fixed. There are many factors that drive high CHT, and high cylinder pressure and ignition timing are some of those, so my “normal” reading of 380 degrees on cylinder number 3 is explainable.  This engine now has 1,500 hours on it, so I have confidence in its ability to survive.

Here is a great article from GAMI (they make fuel injector nozzles) that discusses the disconnection between CHT and EGT. I have a 4 probe EGT and I have to agree, once power is set EGT isn’t a useful tool. There isn’t any data provided by EGT that CHT won’t tell you (other than mixture).

https://gami.com/articles/egt_myths.pdf

Dave,

It isn't labeled "richest", it's labeled "richest cylinder peak EGT". What that graph is depicting is the lack of relationship between EGT and fuel flow (shown in GPH). Cylinder #3 has the highest EGT, yet the lowest FF at peak EGT.

On a multi-cylinder carburated engine it is virtually impossible to get even fuel flows across all cylinders. FI is able to meter an even distribution of fuel, EFI is able to compensate for other variations within each cylinder. Even so, all cylinders may have a different EGT.

So there's no way to know which cylinder is leanest. We know the fuel flow....but we don't know the volume of air. Without both figures it's impossible to tell. If we knew the CHT that might give insite. Might.

So we have been discussing engines that run a few clicks either side of  stoichiometric. But if you look at a lean-burn engine, engines that operate at an AFR up to 32:1 they achieve this with VVT that reduces compression and by retarding the timing. Even Homogeneous Charge Compression Ignition (HCCI), engines that rely on compression for ignition, they too run at a very lean AFR. These engines are able to run at lean AFR's without destruction of pistons and valves. The HCCI has a piston with a ridge that fits into an area on the cylinder head where all ignition occurs. Expansion happens in this ridge area first and by the time the piston moves down to reveal the rest of the piston, head and valves, the charge has cooled to the point that it isn't catastrophic, and expansion can continue.

So getting back to our dumb air-cooled and carburated engines, they can only handle so much cylinder pressure and ignition advance. They rely on excess fuel for cooling, and running lean deprives them of the cooling, thusly causing damage at the hottest points of the combustion chamber. The lean mixture isn't too hot (it's cooler than stoichiometric)...it just doesn't provide adequate cooling.

I ordered a CHT gage.  It won’t fit the spark plug configuration on the LS.  I was thinking that the 8mm stud just below the exhaust port might be a good place to install the sensor.  I have lapped three LS heads and each one had a definite depression in the sealing surface located on the underside of the exhaust port.  One actually had a significant divot where the fire ring on the head gasket had sunk into the aluminum.  I suspect it got so hot that the aluminum started going plastic.  On my recent meltdown, I found the nut on the 8mm stud finger tight.  I had checked that fastener for tightness just shortly before I had the meltdown.  To me, this area seems to be one of the hottest locations on the upper regions of the engine.  I know its not exactly on the cylinder head, but its really close.  It must be very close to the same temp as the head in that area.
 
Any of you have any thoughts on that sensor placement?

Here’s a look at the area of the sealing surface after quite a bit of lapping.

The one I ordered is a TrailTech.  It uses the washer under the spark plug sensor.  Unfortunately, the LS650 head has this tight counterbore in way of the spark plug.  It looks to me like it would be next to impossible to install the special sensing washer without doing some major surgery around the plug counterbore in the head.  The TrailTech guidance says it can also be installed under a fastener (nut or bolt).

If I use that 8mm stud, I can get a feel for the temperature extremes and how it reacts to various tuning changes in that region of the cylinder/head, then move the sensor to the rear 8mm stud, or possibly under a head nut.  Seems like it would be pretty flexible.  Right now, I'm not too keen on disturbing any of the 9mm head nuts.

I believe Dave has one of these installed under his right-rear head nut.  I think he said that the highest temp he has observed is around 300F.