LOP & Exhaust System Life

[Jeff_S]

I'm an avowed LOP flyer, enjoying the fuel economy and lower CHTs that result. But there's no doubt that EGTs are higher, which leads to my current situation.

 

Plane is in for annual, and we knew the muffler baffles were showing signs of stress and probably needing work, plus ever since I bought the plane the muffler shroud had a little gap on one end.  We decided to send the whole exhaust stack off for inspection, and the word is it essentially needs a full overhaul, with pitting and metal loss in many of the pipes. I'll have to look at the logs to see when the last maintenance was done, but I suspect it might date to the last engine overhaul which was 13 years/1200 hours ago.

 

So my question is twofold. For those who've had their planes long enough, what is the expected life of an exhaust sytem? And secondarily, is there any evidence that the higher temps of LOP operations can shorten this life, or are the systems designed to handle these temps?

[201er]

My plane needed the muffler overhauled on purchase. And I highly doubt it was flown LOP prior. And lycoming allows economy cruise at 75% at peak which is the highest possible EGT you could get. If you fly LOP, you're already some degrees cooler than that number. That said, I'm not sure if peak +/-25 is a bad place to be for your exhaust system.

[aaronk25]

I'll start with I haven't had my plane that long but after 300 hours of LOP high power settings the exhaust is in good shape.  How many auto exhausts are still on cars after 23 years? (it looks like your bird is a 1990)  and just because its overhauled doesn't mean there still isn't metal fatigue, the only way to get new metal is to get a new exhaust.  Might be a good time to consider a high performance exhaust system such as power flow.  

 

certainly LOP operation runs 30-100 degrees warmer but thats only a increase of 14% at the most, so I'd say there is no drawback to running LOP.  Even if there was a shortened exhaust life you could by a lot of exhaust systems from the savings in fuel. 

 

Sounds like you're doing a good job of operating your aircraft.  Besides, ever notice how much cleaner your oil stays running LOP.

 

Aaron 

[Parker_Woodruff]

40 degrees LOP is the same temp as 40 degrees ROP.

The only way ROP can have cooler EGTs is if you're comparing a setting that is farther ROP than LOP. Is that the comparison you're making?

[pinerunner]

It may not be just a matter of temperature. LOP by its nature has oxygen in the exhaust and so will subject the exhaust system to oxidizing conditions. ROP uses up all the oxygen and produces reducing conditions (yes I'm a chemist) in the exhaust. I don't have particular information on how tough our exhaust systems are but I'd expect it to be tougher LOP.  I'm mostly an LOP fan though low time. I wonder if any of the long time LOPers could chime in on the longevity of their exhaust systems.

 

Dave

[fantom]

Mine lasted about 12 years from new, with maybe 40% being LOP back then (2006).

 

I, for one, don't like the higher EGT temps when 20 LOP, compared to 100 ROP. So far my rebuilt exhaust system looks OK after 7 years and maybe 50% LOP.

[gwcolwell]

Overhauled engine installed spring with GAMI'S & new Power Flow exhaust and Top Prop.  Tail pipe chrome discolored badly first LOP hour of operation.  However, fuel consumption dramatically better than before.  So much better, I'll put up with shortened exhaust life expectancy.

[201er]

If shortened exhaust life expectancy is a reality... could anyone who is mech/money savy do a cost analysis difference between the cost of exhaust replacement vs savings on fuel/cylinders?

[aaronk25]

40 degrees LOP is the same temp as 40 degrees ROP.

The only way ROP can have cooler EGTs is if you're comparing a setting that is farther ROP than LOP. Is that the comparison you're making?

Your right, what I should have said is that in many instances operators without turbos might cruise at 30 degrees LOP while it might be necessary to be 80-100 LOP, so in that aspect LOP on average is hotter.  Sorry, I see how my thought was wrong.  With that I cruise at full throttle at peak in the winter and 5-10 degrees LOP in summer at 4K (which is in the red box) but chts are 350s and I fly 250 hours a year and my exhaust is in great shape.  

 

Aaron 

[201er]

With that I cruise at full throttle at peak in the winter and 5-10 degrees LOP in summer at 4K (which is in the red box) but chts are 350s and I fly 250 hours a year and my exhaust is in great shape. 

In theory (according to red box concept), even if CHTs are in line, being over 65% power and in the red box leads to excessively high ICPs. So at that point it may not be the heat but the excessive pressure that is your enemy. That's not my opinion, just how I understand what I've read about LOP ops and red box.

[Cruiser]

my aftermarket muffler was installed with the FREM "0" time engine in 2008. I ran it consistently LOP (including break-in) for over 500 hours when the muffler was replaced last year because the fire tubes inside were almost completely gone. Externally, the muffler looked perfect and was working fine.

 

5 years and 500 hours on a $700 muffler. Was it normal? I have heard of mufflers going much, much longer but they may not have the internal fire tubes in them anymore and are still in service. My mechanic would not allow it. You be the judge.

 

LOP = about 1000 gallons of fuel savings @ $6/gal............ I will continue to replace mufflers (if necessary)

[DS1980]

certainly LOP operation runs 30-100 degrees warmer. Not at best economy.

 

The only way ROP can have cooler EGTs is if you're comparing a setting that is farther ROP than LOP. Is that the comparison you're making? Bingo!

 

40 degrees LOP is the same temp as 40 degrees ROP. True, but why would you be running at 40 degrees LOP when best economy is 50-75 LOP? On the lean side, 25 degrees is a HUGE difference. The intent of LOP operations is to 1.) Reduce engine wear (CHT) and 2.) Run at best economy.

 

LOP by its nature has oxygen in the exhaust and so will subject the exhaust system to oxidizing conditions. Only if the exhaust system uses a mild steel.  There are many systems that use stainless steel or have been treated and painted to resist corrosion.

If shortened exhaust life expectancy is a reality It's not. Did we all just create a forum blunder and agree by consensus? What evidence is there of LOP reducing exhaust system life because of higher EGTs? The motto of Wendy's is "where's the beef?" We need to be asking this question. Where's the beef (evidence) that this is true?

 

This entire thread is based on the assumption that LOP creates higher EGTs. None of this is true when operating lean enough, which is the entire idea! Great discussion though! I like it.

 

Quiz:

What's the #1 rule in engine management?

 

It's ok to run ROP, if you run rich enough. And it's ok to run LOP, if you run lean enough.

[DS1980]

In theory (according to red box concept), even if CHTs are in line, being over 65% power and in the red box leads to excessively high ICPs. So at that point it may not be the heat but the excessive pressure that is your enemy. That's not my opinion, just how I understand what I've read about LOP ops and red box.

Awesome! Add 1 to the LOP Revolution!

[Jeff_S]

Good points everyone...thanks for weighing in.  In my case, I definitely run with higher EGTs when I'm LOP because I typically cruise WOT and about 10-35 dLOP, which keeps my CHTs in the low 300s.  My plane would become a glider if I tried to run 80 dLOP.  But since I have no idea how the engine was run prior to my 350 hours, and I don't know how old this exhaust system is (just haven't scoured the logs) I'm going to chalk this up to normal maintenance.  What's two more aviation units among friends, eh?

 

(Note to the guy with the questions about Mooney ownership costs: there's my supplemental maintenance expense for this year!)

[Alan Fox]

I'll start with I haven't had my plane that long but after 300 hours of LOP high power settings the exhaust is in good shape.  How many auto exhausts are still on cars after 23 years? (it looks like your bird is a 1990)  and just because its overhauled doesn't mean there still isn't metal fatigue, the only way to get new metal is to get a new exhaust.  Might be a good time to consider a high performance exhaust system such as power flow.  

 

certainly LOP operation runs 30-100 degrees warmer but thats only a increase of 14% at the most, so I'd say there is no drawback to running LOP.  Even if there was a shortened exhaust life you could by a lot of exhaust systems from the savings in fuel. 

 

Sounds like you're doing a good job of operating your aircraft.  Besides, ever notice how much cleaner your oil stays running LOP.

 

Aaron 

Quite a few car exhausts last 20 plus years , The majority of cars now have stainless exhausts and they generally last the life of the car......That being said most of the aircraft exhaust failures I have seen are from cracks , and temperature is temperature regardless of which side of peak you are on....I think that mixture has little or no effect on the life of an exhaust....

[KSMooniac]

Vibration is the worst enemy of our exhausts IMO.  

[aviatoreb]

My exhaust needed to be replaced at ~850hrs, due to cracks.  My understanding is that this is entirely independent to this discussion of exhaust temps.  Is that right?  More so due to vibration.

I can say this, not all exhaust systems are inexpensive.  I cannot remember what it was as it was just after I got the aircraft, like within that first year but after 100hrs under my watch.  It is quite expensive for exhaust in the rocket - I think it was $3k plus a good bit of time to get it off and back on as everything is very tight firewall forward in rocket.  The replacement exhaust was much thicker in the section which cracked as I am guessing it had been a chronic problem and they had addressed the tendency to crack.

[N601RX]

Other than viberation the other thing that is rough on these exhaust is frozen joints. The aluminum case and heads expand and contract a lot and if the joints in the exhasut are frozen it will put extra stress on them. This is the sam reason the intake runners and pushrod tubes are designed to slide in and out.

[jetdriven]

[/quote

I have found After 2 years of this that 15-25 lop is the best efficiency.

[DS1980]

I have found After 2 years of this that 15-25 lop is the best efficiency.

This is entirely possible for a specific engine set up. I would have to say overall, and the LOP pioneers agree, that best economy happens around 50-75 degrees LOP. The real question is that does-for YOUR engine- 15-25 degrees LOP keep your CHTs within range? You're one of the more experienced pilots on this forum with regard to LOP operations so this question is rhetorical for you and a case in point for others new to this LOP silliness.

[201er]

I think the fundamental difference between the way Mooney pilots and Bonanza drivers fly LOP is that the Bonanza guys are really looking to save gas (hey, don't blame em when ROP is 15gph) while the Mooney guys are trying to go FAST without frying the engine. We don't need to worry much about gas. Once we're on the LOP side of things, squeezing more gas savings becomes unimportant compared to maximizing speed!

[Alan Fox]

I think the fundamental difference between the way Mooney pilots and Bonanza drivers fly LOP is that the Bonanza guys are really looking to save gas (hey, don't blame em when ROP is 15gph) while the Mooney guys are trying to go FAST without frying the engine. We don't need to worry much about gas. Once we're on the LOP side of things, squeezing more gas savings becomes unimportant compared to maximizing speed!

Mike , there you go off topic again...The topic is does LOP affect exhaust life , and the answer at least on my planes is NO They are made of Alconel and don't corrode.....They crack , from vibration mainly ....... 

[DS1980]

I like kittens.

There, now at least Mike's post was about airplanes.

[fantom]

Byron, or someone with a J, what is your speed loss at 60 to 70 degrees LOP, compared to 15 to 20 degrees LOP, so about a 50 degree delta?

[KSMooniac]

There is no need to fly 60-70 LOP in a normally aspirated plane unless one is WOT and down low, at 3,000' MSL or so.  I don't recall off the top of my head, but I think the ADDITIONAL speed loss for another 50 LOP would be pretty significant... in the 5-7 knot range.