Chunks fell out of muffler inlet

[Helirich]

2. If you run with no headers (or manifolds), when the exhaust exits into the much cooler ambient area, the exhaust valves will be heated and cooled (by a waive which I'm not going to get into) at such extremes that they suffer from fatigue. Pronto.

Not sure about this statement. I've seen aircraft exhaust stacks so short that I could see the exhaust valve. This was on WWII aircraft that fly at high altitude. It can be -50 at altitude. I had a discussion on this very subject with an old coot at an air show. He said there is so much hot air coming out that no cold air gets in. He did say it was a worry when they shut down an aircraft in freezing weather. (Cold breeze)

[David T]

Not sure about this statement. I've seen aircraft exhaust stacks so short that I could see the exhaust valve. This was on WWII aircraft that fly at high altitude. It can be -50 at altitude. I had a discussion on this very subject with an old coot at an air show. He said there is so much hot air coming out that no cold air gets in. He did say it was a worry when they shut down an aircraft in freezing weather. (Cold breeze)

You make a lot of references between cars and aircraft. While they do have things in common, there are too many differences to compare them directly. For example cars get to coast most of the time and rarely run at full power. Most internal combustion engines will run better with a tuned exhaust. BUT you can only tune it for a very narrow range of HP or RPM or torque.

[Ron]

Not sure about this statement. I've seen aircraft exhaust stacks so short that I could see the exhaust valve. This was on WWII aircraft that fly at high altitude. It can be -50 at altitude. I had a discussion on this very subject with an old coot at an air show. He said there is so much hot air coming out that no cold air gets in. He did say it was a worry when they shut down an aircraft in freezing weather. (Cold breeze)

First I shouldn't have said "pronto", especially on a non-performance engine.

Anyway, it will be significantly much hotter right behind the valve when there is a manifold. Heat is still being produced (and held) until combustion completes there. I'd bet if you ask the old coot, he will tell you that they used to cut the pipes (or open collectors) off right where the discoloration ends to tune them...time to turn it loose.

Never, say never.

[Helirich]

I think when you guys say "tuned", you are perpetuating the myth that "some back pressure" is good. When they tune an exhaust, they are trying to have negative back pressure (vacuum) at a certain RPM. They actually would like this negative back pressure at all RPMs, but that is impossible. So they pick a useful RPM for what they are trying to do. Ron mentioned that headers help in low end torque. This is often true, but it is not the only "tune" there is.

David T is correct in some things on aircraft are unique. Like their engines are tuned to run in a very tight RPM range at the expense of all the rest. That's why they idle so bad. The differnce between cruise power and max power might be less than 400 RPMs. In the case of helicopters and constant speed props, there is no differnce.

Ron, I will say, I agree with you on the short stacks have to be bad for temp fatigue and I was arguing that to the old mechanic. One thing he told me was the TBO (time before overhaul) was as low as 250 hours on those aircraft. He said that most got shot down before reaching that. (Not a perticular comforting thought) So maybe breaking valve heads is not a concern.

I'm not trying to start an argument here. I'm just saying that I've not heard of any "good back pressure" unless you are defining "negative back pressure" as back pressure.

[Stainless]

Well, after pulling the motor, I removed the crossover pipe and cat and was surprised at how good the cat looked on the inlet side. It was hard to get a good picture of the outlet side due to the curving of the pipe, but I could see that place where some of the internals were missing. I'm currently considering going aftermarket with my exhaust, so I'll noodle on it for a bit to see what route I decide to go.
PXL_20210817_025442289.jpg PXL_20210817_025506253.jpg

[TTait]

Might be time to buy a boroscope/endoscope inspection camera for your phone or laptop...