What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Exhaust Wrap

RV6_flyer

Well Known Member
Benefactor
This was the top story on VAF for the day 30 November 2016.

2016112913513431-FullSizeRender-XL.jpg


Just want to add the above is an OPINION that I do not share.

I have used exhaust wrap on my RV-6 for 19-years and over 3,000 without the issues in the letter.

I started using exhaust wrap at the recommendation of an RV-3 owner / A&P / IA that had a 360 under the cowl. He had one of the old Tolle exhaust systems that was a rigid exhaust unlike the "Vetterman" exhaust that most of us use. The Tolle exhaust systems were known to crack and break welds. Once he wrapped the exhaust, his cracks and breaks happened a lot less often.

There are as many opinions on the subject of exhaust wrap just like there are on primer, nose wheels, tailwheels, ignition system, engine manufacturers, spark plugs, paint schemes, etc...

Everything we do in aviation has risks. Exhaust wrap also has risks. IMHO, the risks are worth it to me for the lower under cowl temperatures that I have on my aircraft by using it. YMMV. IF the exhaust wrap use required that i replace the exhaust system every 500 hours it would be worth it to me.

Now others can comment with their opinions. I am sure that there are as many opinions on the subject as there are readers to this forum. Maybe I am the only successful, happy user of exhaust wrap.
 
SS

Gary,
Much respect to your opinion!
Was the old Tolle exhaust systems SS?
Larry or Clint told me the wrap cause the molecular level of the SS to move around after my wrapped exhaust slip joints "ovaled" after wrapping.
Discussion stated after he sad he could tell I had wrapped it!
FYI...Larry did fix it....and IIRC for free! I have not wrapped since.
 
Exhaust wrap

I had an experience similar to what the writer head with two major differences number one it was an automobile not an aircraft number two it was mild steel tubing and not stainless. just saying
 
I also had to wrap the exhaust in my long EZ to minimize the hot air being used for cooling on that up draft cooling situation. I did experience a couple of exhaust pipe cracks on the SS material and had to keep a close check on that issue. I believe there were two sets of manifolds installed over the 8 years of flying. On my 9 I have an area of about 8 inches wrapped where closee to other stuff and cowl that I need to keep the heat off of. Overall I agree that the complete wrap will shorten the life of an exhaust system but in some situations it can be used for advantage.
 
I also respect your opinion very much Gary.
My own experience was from a crack under the wrap. It is difficult to inspect for cracks without removing the wrap. When I did discover the crack it was not repairable due to "changes in the properties of the material based on elevated internal exhaust tubing temperatures". We could not weld the crack and had to replace the exhaust system.
Although I can weld, anything on my airplane goes to a very experienced, and well know Van's supplier of weldments. If he can't weld it, nobody can.

I chose to protect the cowling in other ways, with shields and foil. This was on the Bucker, not the RV.

My Champ, which I "borrowed" for a year while the Bucker was being restored had wrapped exhaust and had been flying for hundreds of hours with no known issues.
 
FWIW

Well well. This thing is just a matter of degree, not an absolute black and white opinion. Pun intended.

When wrapped, the external heat transfer from the tube to the air outside it will be reduced. This results in a higher in a higher metal temperature. This elevated temperature can several things, depending on the absolute level. One is oxidation at the external surface where oxygen content is ~20% not the 3-5% in the exhaust. This is a fact, oxidation rate increases. Even 316, 321 stainless will oxidize. Another effect of the higher temperatures is a reduction in fatigue strength of the material. It is a natural, normal effect of elevated temperatures.

So, does wrap cause cracks? Not directly. If oxidation progresses and the material is thinned, or if the inherent alternating stress is high on a particular hanger or pipe design, then the elevated temperature might lower the fatigue strength enough to result in a fracture.

Metallurgy effects notwithstanding.

Not surprisingly, based on these factors (and base material and thickness) your results will vary. Maybe we can identify what material, design, thickness tolerate the wrap. I would like to wrap, but have not because of the unknown.
 
Last edited:
All exhausts are not the same in terms of how they load their component parts. Applications that cracked simply exceeded available strength at that particular point in the structure, and the others didn't.

There is a loss of strength as temperatures are elevated. Impact strength of 321 stainless after 1000 hours at...

900F, 95% of the unheated condition.

1050F, 85%

1200F, 65%

How much does wrapping raise tube temperature? I don't know, but I can see it would not take much to lose a lot of toughness. There is only a 300 degree spread between those data points.
 
A buddy built a Murphy Rebel------then put it on floats a year or so later.

Due to the drag and weight of the floats, he put a turbo on the engine. Turbo required entire new exhaust system. To get better perf from the turbo, he wrapped the exhaust pipes.

Exhaust failed in less than 10 hours with wrapping and weight of the turbo.

Like said above, different planes will act differently.
 
One is oxidation at the external surface where oxygen content is ~20% not the 3-5% in the exhaust. This is a fact, oxidation rate increases. Even 316, 321 stainless will oxidize. .

I believe this was why, or at least one factor, in my exhaust being un-repairable. It just would not take to welding. I was watching under the hood and it was interesting to see how the material behaved.
 
I would like to SEE WHERE his exhaust failed. We all know that a weld on an exhaust stack can fail even if it is not wrapped. I am wondering if it failed at an area that was already heat affected by the welding process or if it was in the middle of the pipe, etc...

On a different note, I wrap my heat exchanger (not the pipes) each winter in order to get a warm cabin. BTW, it works GREAT! I have done this for two winters. Each spring, I remove the heat exchanger entirely. I inspect it and replace it in the fall. In the two years that I have been doing this, I haven't experienced any ill effects. I have 500+ hours on the ship now.

I will keep you posted if there is a failure that I attribute to this method.

:). CJ
 
All exhausts are not the same in terms of how they load their component parts. Applications that cracked simply exceeded available strength at that particular point in the structure, and the others didn't.

There is a loss of strength as temperatures are elevated. Impact strength of 321 stainless after 1000 hours at...

900F, 95% of the unheated condition.

1050F, 85%

1200F, 65%

How much does wrapping raise tube temperature? I don't know, but I can see it would not take much to lose a lot of toughness. There is only a 300 degree spread between those data points.

Good question. I believe that the difference in temperature with and without wrap is negligible. Our exhaust pipes get almost as hot as the exhaust gas with or without wrap and is weaker after many hours of operation. Temperature delta between inside and outside will be slight. The bad thing about the wrap is that it will keep the pipe hotter for a little longer than it would without.

IMHO, an exhaust with wrap on it should NEVER have wrap over a weld or expansion joint. At least that is the way I have done if for 19-years of use.
 
Interesting that Bruce Bohannon's Flying Tiger Rocket/RV hybrid and Dave Anders RV-4 (both record holders) have wrapped exhaust.
 
Interesting that Bruce Bohannon's Flying Tiger Rocket/RV hybrid and Dave Anders RV-4 (both record holders) have wrapped exhaust.

And they both have a higher maintenance hours to flying hours ratio than the standard fleet, which allows them the luxury of doing some things that require a little more "hand-holding" and keeping an eye on. I don't know that those examples would be valid to use as comparisons for the rest of us.
 
And they both have a higher maintenance hours to flying hours ratio than the standard fleet, which allows them the luxury of doing some things that require a little more "hand-holding" and keeping an eye on. I don't know that those examples would be valid to use as comparisons for the rest of us.

You hit the nail on the head here. There is a difference between building for reliability and building for performance. If you want high reliability along with the ability to go on long trips, night flights, instrument flights, or desire or fly alot between maintenance checks, then build stock and take adavantage of the best practices.

Nothing wrong with building for performance, but sometimes it comes with higher maintenance costs due to repairs, shortened component/engine life, and the need for more frequent checks. It may limit your type of flying depending upon your risk levels.

Vic
 
OK, that's a valid point. I am under the hood so often on my airplane changing stuff and experimenting that I forget some people go oil change to oil change without taking the cowl off.

However, the point remains that there is plenty of evidence to support the assertion that a wrapped exhaust provides performance benefits, and more so if the engine is tuned to take advantage of it. And secondly, there are some who wrap (or in my case ceramic coat) their exhaust and suffer no maintenance penalties.

It's a lot to ask of this forum, but it would be nice to figure out why some have success and some have failure - rather than just the blanket advice that "...wrapping is bad..."
 
Last edited:
I don't wrap. I like Clint's work and wouldn't dream of covering it. As for inspections, I guess you could just give a wrapped pipe some "inspectiness".
 
I believe that the difference in temperature with and without wrap is negligible.

You are hearby challenged with assigning hard numbers to your belief ;)

Seriously, temperature-related loss of strength is the accepted issue, and without actual temperatures, we have nothing.
 
You hit the nail on the head here. There is a difference between building for reliability and building for performance. If you want high reliability along with the ability to go on long trips, night flights, instrument flights, or desire or fly alot between maintenance checks, then build stock and take adavantage of the best practices.

Nothing wrong with building for performance, but sometimes it comes with higher maintenance costs due to repairs, shortened component/engine life, and the need for more frequent checks. It may limit your type of flying depending upon your risk levels.

Vic

Higher maintenance cost with exhaust wrap? Then why is with 19-years and over 3,000 flight hours I do not have issues? Am I the exception to having exhaust wrap without issue?
 
And this is going to vary widely with operations type - someone who always run 50F ROP is going to see a large difference in "typical" exhaust temps from someone who runs deep LOP. Lifetime exposure is probably what matters here, which would weight cruise operations heaviest.

I'm wondering if the oxygen content in the exhaust plays a considerable factor in the equation. ROP operation has very low oxygen content while LOP has quite a bit.
 
And this is going to vary widely with operations type - someone who always run 50F ROP is going to see a large difference in "typical" exhaust temps from someone who runs deep LOP. Lifetime exposure is probably what matters here, which would weight cruise operations heaviest.

I'm wondering if the oxygen content in the exhaust plays a considerable factor in the equation. ROP operation has very low oxygen content while LOP has quite a bit.

The exhaust gas temperature is exactly the same 50 ROP as it is at 50 LOP, no?

Exhaust oxygen content is about 1% at best power, 2% at stoichiometric (counter intuitive) and around 3.5% in in the 70 LOP range.

Skylor
 
The exhaust gas temperature is exactly the same 50 ROP as it is at 50 LOP, no?

Yes, but it's not the same for all throttle settings. My airplane will spend the majority of it's life in cruise at 18"-20" MAP in the midteens - some others might spend the majority of their life in the low altitudes running full-out for racing or aerobatics, while others like low-and-slow. It stands to reason that these conditions will have an effect on the lifetime of wrapped (or unwrapped) pipes, but we just don't know enough right now to say what effect or how much.

We don't know what we don't know, but we know there's a lot of it.
 
We don't know what we don't know, but we know there's a lot of it.

Well stated. Gary knows what he knows from his personal experience on his airplane. In fairness, we assumed my exhaust was not repairable due to the change in the material because of the wrap. We have repaired other exhaust systems of similar type and age with no issues, but it was still an assumption.

Lots of variables and I appreciate Gary's post and the chance to have the discussion again.

So, after all of this, I am now in the "I just don't know" camp. Still, if I can find another way to deal with the heat, like shields, foil, etc... I probably would lean that direction. Those are tangible and I don't have to wonder about them.

Why is it the more I learn about something the dummer I feel? At this rate, I will be a complete idiot in a few more years. (Don't comment if you feel I might already be there. I am happy and it's working for me ;)
Jethro Tull " The wise man never knows how it feels to be thick as a brick"
 
Higher maintenance cost with exhaust wrap? Then why is with 19-years and over 3,000 flight hours I do not have issues? Am I the exception to having exhaust wrap without issue?

Good question - Gary, do you know the original material and thickness for your exhaust pipes? That might be a place to start with understanding your exhausts' longevity. And - do you know the specs for the materials used in the wrap?
 
Good question - Gary, do you know the original material and thickness for your exhaust pipes? That might be a place to start with understanding your exhausts' longevity. And - do you know the specs for the materials used in the wrap?

Vetterman exhaust. Not sure if it is T304, T409, or 321. 304 costs less but do not know what Vetterman exhaust uses. I do not believe in wrapping welds or any part that was inserted into the system for movement.

Wrap was purchased from Aircraft Spruce.

Just because it works for me does not mean that I recommend it for you.

I like the way Ceramic coating on exhaust works BUT I am afraid to use it as a break in the exhaust system after ceramic coating cannot be repaired.
 
I used the high temp "2000" product from Jet Hot and was able to grind off and re weld the collector after coating with no ill effects. This exhaust was mild steel, BTW.
 
For me, I defer to the experience of others when they clearly have been exposed to a lot more data and real world experience than I.

Larry Vetterman has been involved with exhaust systems on RV's for as long as I have been working with them (28 years).
He has probably seen just about everything that can happen to and/or be done to an exhaust system.

He will flat out tell you not to use thermal wrap.... that best case it will guarantee a reduced lifespan of your exhaust system to some degree, and worst case it will cause a potentially catastrophic failure at the most inopportune time .

Maybe that is only the case for systems made by his company. Maybe other "coatings" would be ok. I don't know. All I do know is his many years of experience is enough to convince me that my exhaust system is one critical component that I will not experiment with, and I always recommend that others don't either.

Why isn't this the universal experience of everyone that has installed thermal wrap (or other thermal barriers)? I don't know, and I don't know anyone else that does either. Since we don't know, it is not a gamble I am willing to make and I recommend that others don't either.
 
Exhaust wrap

I will give my opinion on this ongoing subject. Yes, I have seen about everything when it comes to exhaust systems, and regarding wrapping it with any product. I have seen many systems returned for repair after it was wrapped and most or all of the things that make stainless tubing, i.e. 321 which is all I ever used a short time after we made some systems in mild steel, were cooked out of the material. A simple crush test using a pair of channel lock pliers and just a small amount of pressure would break right thru the tubing. In most cases the tubing would be a non typical black color. Not only that, many systems had large bulges or were no longer round, but more of an oblong shape.
I tested it once to see the supposed power gains etc, claimed by some folks, but I didn't see any difference there. What I did see was higher egt's. No I didn't leave it the very long.
So my opinion remains the same: I cannot recommend using it. Like I used to tell builders that called and ask me about, run your system as it came out of the box and it will give you many hours of trouble free flying. Larry V
 
Larry, I can't find fault with your vast experience nor am I surprised by your finding no performance gain with wrapping. Your conclusion seems to be missing a subtle, but important element however. The theory holds that gains in power from thermal wrap come from a change in exhaust tuning due to elevated tube tempreature. Since the speed of sound is directly related to the temperature of the gas, wrapping changes column temperature and therfore the resonance. That said, if the engine is not tuned to take advantage of exhaust resonance to begin with, then simply wrapping an exhaust is unlikely to provide any gain beyond reduced radiant heat transfer to the induction system.

I would readily concede that there is probably a statistically insignificant number of owners who have taken the time to tune their exhaust, induction and ignition to match their mission profile exactly, however. So to your point, for the overwhelming majority of the fleet, the disadvantages of wrap largely outweigh the advantages.

And if I was expected to stand by my exhaust products with your stellar reputation, I wouldn't enourage wrapping either.
 
Last edited:
Back
Top