M20J Muffler shot - AME says its because I fly LOP

[andrem]

At my 50 hour inspection the AME reported that the muffler is shot and pieces of it is actually gone. He asked if I fly LOP - I said I do on occasion. He said that's the reason. Anyone else have an opinion in this regard?

 

50 hours ago the muffler was pressure tested and found to be fine - now it's a mess.

[rbridges]

doesn't LOP generally drop CHT and EGTs if done right?  seems like that would prolong muffler life IMO.

[jetdriven]

There's rumors of an "oxidizing flame" burning through the metal. On the A&P test there is a section that talks about excess of acetylene is a hotter flame to cut through metal, but that's a torch. Seems to me the muffler wears out like most of the exhaust does, it wears thin and then cracks. I had to replace my tailpipe last year at annual. I went to polish it but first had to knock the rust off it with sandpaper. Close inspection revealed very tiny fissures, like a dry lakebed. I showed the IA. He took a bodyman's pick hammer and punched a hole in it like paper. It was only .004 thick there.

[Bob_Belville]

doesn't LOP generally drop CHT and EGTs if done right?  seems like that would prolong muffler life IMO.

LOP lowers CHT but probably not EGT. I usually run either 100 (or more) ROP or about 40 LOP so ROP is cooler EGT by 60F.

[DS1980]

It's possible you're not running lean enough. What do you run at?

Also I would ask your mechanic to explain LOP to you. If they are unable to expand on this thinking of LOP burning exhausts, you need to get another mechanic. The belief of OWTs by a mechanic is evidence of character traits of people you don't want working on your airplane.

[Cruiser]

First, I am not saying LOP is the reason your muffler is worn out, but you also don't say how much total time is on the muffler. They can last from 500 hrs (or less) to as high at 1500 hrs (or more) 

 

Second, if and I am only working a theoretical example here: 50 hrs LOP at a two gallon per hour (2 gph) fuel savings is going to be around $600.00 or about the cost of a new muffler. So, with all the other benefits of LOP operation you are getting the muffler for free.

 

Third, I don't believe LOP burned out your muffler.

[Awqward]

That reminds me...I had forgotten....at my recent annual my IA pointed out that the exhaust tubes at the cylinder heads did not appear to be discolored (a good thing)...and said that he sees many engines that run LOP with orangey, clearly overheated exhaust tubes....(a bad thing)....and implied that I should not run LOP....which I don't....but mainly because I don't have an engine monitor installed (yet)

[DS1980]

Even if running LOP wears our exhausts faster, I would consider this acceptable. Cruiser illustrated the mathematical reason and from a mechanical standpoint I had a complete exhaust failure when a mechanic did not torque the nuts properly and both headers fell off into the cowling. A little noisier but nothing more.

 

Still don't think LOP is the cause though.

[Guest]

I would think that the valves and the stacks would experience higher temps than the muffler, whether ROP or LOP. You muffler is likely due for replacement because its old, nothing more.

Clarence

[Bob_Belville]

I have a PowerFlowSystem. I dropped Jim Shafer a note earlier this evening for their comment. 

[Andy95W]

Time to buy a Power Flow, maybe?

And you don't have to tell your spouse you're getting the Power Flow. You're just replacing the muffler because it wore out.

[wiguy]

 It probably just wore out & needed to be replaced. The total hours would give an idea.

[carusoam]

From an energy/temperature point of view...

Running LOP is cooler than running at peak.

The fuel is mostly burned in the engine.

Very little is left to burn down stream.

The effect of excess air is mostly there to add cooling, similar to excess fuel without the benefit of heat of vaporization.

Technically the excess gets in the way of molecule collisions and slows the burning process a tad. (Where's Ross?)

ROP:

Where does the excess fuel go?

When does it meet more oxygen?

How hot is it when they meet?

From an oxygen point of view.

Oxygen is always in excess.

It IS available to oxidize other things.

Hot metals in contact with excess oxygen are a concern worth discussing...

What can be done?

The automotive industry has been using more stainless steal since the 90s. Pipes got replaced because they were used well, were also replaced because they sat without being used at all. The only thing worse was being used sometimes...

I would expect that a new aviation exhaust would select better materials than what was available when the plane was new.

Excess fuel or excess oxygen is a pretty nasty environment for metal objects.

Ask about the new exhaust system. Is it made of some version of high temperature stainless steel (Inconel)?

How old (age) is the exhaust system we are talking about? Prior to the 90's? Was it made with high nickel alloy stainless steal?

My 65' C had exhaust tips that were eroded at the weld lines. The weld materials were not as resistant to the environment as the pipe was.

This thought process has lead me to a similar conclusion that I arrive at when it comes to training or purchasing. Find the expert and pay him to get the right answer.

Often, our experts advertise and sell the product you are looking for. Take this question to a couple different exhaust system suppliers that advertise in Trade-a-Plane. Who else would know more?

Who would want to know this much about that?

Best regards,

-a-

[DaV8or]

As others have stated above, it would not surprise me at all if LOP caused premature exhaust system failure. Excess oxygen in a heated environment usually means increased rate of corrosion. However, nobody has studied or tested this theory so nobody really knows. It doesn't seem to be a widespread and common problem though. Many people are running LOP these days and there doesn't seem to be an epidemic of rotted mufflers. Also keep in mind that in my experience, mechanics will use just about any excuse to bad mouth LOP ops. I think nearly all I have met have said to me, "Don't do it!"

[ArtVandelay]

Don't know about LOP, but I know if you ground run the engine without letting it get to normal operating temps, you will leave moisture in the muffler, as well as the oil.

[Guest]

Maybe I'm confused, but isn't the "Peak" portion of LOP and ROP the hottest or peak temperature in the exhaust stack, hence the name? It would seem to me that operation on either side of the "peak" would be cooler and therefore less stressful to the exhaust system

[carusoam]

It would be cooler.

But is the exhaust comprised of excess gasoline or excess oxygen?

Excess gas has been known to ignite in 'pops' that lead to damaged exhaust. Very rich mixture settings.

I think the maintenance concern for LOP in this thread is sending oxygen into an already hot metal structure and possibly causing oxidation that can't be caused in an oxygen free or low oxygen environment.

In this case it is not how hot LOP or ROP the exhaust goes. But how much oxygen is available at hot temperatures. Some rust may appear in a very red color. This would lead to the discussion of surface oxidation or inter granular oxidation in the exhaust component?

So is cooler with more oxygen better than cooler with less oxygen?

Realistically, if this IS a concern, we would want to find out what stainless steals are being used in similar exhaust applications.

My R came with LOP in the POH. I don't recall any changes to the exhaust system. That would have been expensive enough to remember.

My C never ran LOP, it couldn't, and I had to replace a few internal exhaust parts.

If needed, buy the proper quality that is required by the application. It may take a call to the exhaust parts manufacturer and ask what is required for an R vs. What is required for a C? Or J?

I have used 306, 308, Inconel, and X-alloy in machine applications. They are all considered stainless steal. They have different other properties that make them the right stainless steal for the application.

I would expect that a design / approval prior to 1980 may not have the best technology that has come to the front of the technology curve.

My 1990s car exhausts are still being used with original parts (not cats). My 1980s exhaust got replaced after many years.

The automotive world isn't perfect. I have had two brake lines rust and leak in different cars in the past two years. (95 firebird, 02 Tahoe). No new cars in my near future...

There is a stainless steal for all these things. Why is it not being used in your application?

That's what I'm trying to say.

Best regards,

-a-

[jetdriven]

It probably just wore out & needed to be replaced. The total hours would give an idea.

Exhaust wears out. What was the total time on the part?

[WardHolbrook]

It's possible you're not running lean enough. What do you run at?

Also I would ask your mechanic to explain LOP to you. If they are unable to expand on this thinking of LOP burning exhausts, you need to get another mechanic. The belief of OWTs by a mechanic is evidence of character traits of people you don't want working on your airplane.

My thoughts exactly. Might be time to look for a mechanic who is a bit more up to date.

[aaronk25]

If LOP saves 2gph x $5 per gallon that's 20k over 2000 hours....sounds like a person could buy a few exhaust systems for that much!

I doubt LOP decreases exhaust life by any measurable amount.

[aaronk25]

If LOP saves 2gph x $5 per gallon that's 20k over 2000 hours....sounds like a person could buy a few exhaust systems for that much!

I doubt LOP decreases exhaust life by any measurable amount.

[philiplane]

It is simple. Peak EGT is the most stressful on the engine and exhaust. Either side of peak, the stresses go down. But on the lean side of peak, they decrease rapidly with every degree past peak. Leaner is better. 

 

That said, any muffler with more than 1000 hours on it doesn't owe you anything. Most OEM's recommend replacement, not repair, somewhere between 1000-2000 hours due to thermal fatigue.

[andrem]

Hi all, total time is 1350 since new. Maybe it was just time. It was pressure tested 38 hours ago and was fine - now it's got a big hole in it - hence the questions.

[PTK]

At my 50 hour inspection the AME reported that the muffler is shot and pieces of it is actually gone. He asked if I fly LOP - I said I do on occasion. He said that's the reason.

Did he say anything about the 1350 hrs of the muffler in service?

I'd say get a new mufler and a new mechanic while you're at it!

Anthony makes an excellent point in the need to match materials to the application.

I had a Coleman grill that lasted 15 years of heavy use. (We grill year round. Even with 2 feet of snow I shovel a path to the grill!)

I went through 3 burners during that time. They were not ss. I just got a brand new Weber Summit. SS burners and everything else! I expect it to last double that.

The Coleman was a good grill for the money. It doesn't owe me anything. The Weber is better.

The difference is selection of proper materials for the application and of course made in the good ole US of A!

[jetdriven]

Sorry, PTK, but Weber gas grills are made in china starting about 2007.

They are still far better than the others offers, though.