EXHAUST INSULATING WRAP

Rotax Wizard wrote:

Hi All

Apparently some are not keeping up with the latest documents from Rotax on this issue.  Please see the SL, service letter, on this subject...attached.

Cheers

No malice here or nit picking just like to discuss issues to learn and educate. 

Nothing like a good educational debate to find answers and or dispel myths..

I have two questions on the pictures in the SI. The first shows over wrap which was poor installation and yes it can lead to over heating, but mounting the VR too close to a pipe can cause it to fail too and it wasn't the VR's fault. Just poor installation like over wrapping. Can Rotax say this pipe cracked because of the wrap or cracked like hundreds of others around the world without wrap? Second the picture shows the deformed pipe. Does Rotax know what the EGT's were when this happened? Were the pipes under positional stress? I think failures like any other part on the engine are caused by human installation issues which we have all seen and a failure to observe and maintain the engine properly which leads to failures like exhaust pipes. What does Rotax say about the cause of all the broken non wrapped pipes which number into the hundreds every year? These very factors alone can cause a pipe to fail and if header wrap is on them then some points the finger that the wrap caused it.

My questions are did the wrap actually cause it or did other factors play a part that aren't known or revealed?

I would bet worldwide there are thousands of wrapped exhaust. I know of well over a hundred by myself over the last 10 years with no issues.

This is similar to Rotax not wanting ethanol in the old days and now they do.

"I might even find some old notes about how much higher is the EGT in flight when measured 10 cm away from the head exhaust ports, with the wrap on. Many consider it a flashy addition to their engines, but it is not. Sometimes I am left speechless when I see those beautiful custom built café racer motorcycles of which nearly all have their manifolds wrapped all the way from the ports to the mufflers."

Multiple misconceptions here. *Of course* EGT within the pipe is higher with wrap than without, because that is exactly the most usual reason for using it. The mass of gas moving through the engine at a given rpm is essentially constant. If the exhaust gas is hotter, it's moving faster and hence has lower (back) pressure. Taking away extra heat in the form of exhaust gas is waste heat not going into valves and cylinder heads. Peak EGT does *not* coincide with peak CHT. Of course Rotax doesn't supply a mixture control for the 912/914, so one can't really explore that. This is a most excellent series which goes into details.

https://resources.savvyaviation.com/eaa-airventure-2021-slides/

Wrapping has been a standard technique with cars and motorcycles for decades. Even my RX-8 rotary exhaust is wrapped (mostly to lower heat inside the car) with no problems in spite of the rotary EGT running a good 200C hotter than any piston engine. My Ducati exhaust is also wrapped without problems. WHY? Ever ridden a bike while straddling exhaust pipes? It tends to cook body parts which I prefer to remain uncooked.

If pipes are cracking from wrapping, they aren't made from the correct material or are too thin.

Hi all

OK, wrap away, but realize that the issue is serious enough to Rotax that they say not to.  I am pretty sure if you fail an exhaust and it is wrapped that they will deny any claim given they issued a Service Letter and it was not in compliance.  A Service Letter is information of note and not mandatory, hence the statement under 1.3 in the SL

"1.3) Reason

Insulating the exhaust pipes (e.g. with exhaust wraps or ceramic coatings) leads to a significant

increase in the component temperatures of all exhaust-relevant components. The significant additional

thermal load generated inevitably reduces proven durability of exhaust components.

Exhaust wrap may also hold moisture, promoting corrosion.

The use of any insulation material is not approved by ROTAX  and may result in exhaust component

fatigue and damage."

Cheers

Roger Lee wrote:

I have heard that some have done ceramic coating, but I personally don't know of any.  I haven't read on any forum about issues, but who knows? Maybe someone here has more real time knowledge and experience with ceramic? 

Thanks Roger; I hope there will be more responses to my ceramic coating enquiry.

Additional information on my installation;

The original builder was particularly interested in keeping heat out of the engine compartment. This is mainly due to having an exit air coolant radiator (the opposite of most front mounted radiators).

Not only did he opt for ,what I think must be an expensive  competition grade ceramic coating of all four exhaust pipes, he has completely encased the muffler/silencer (standard Rotax) & tail pipe in a dedicated shielding (similar to a cabin heat set up).

One end of the muffler shielding has about 1/4 -1/3 opening to allow air to be drawn in, the tail pipe ends short of the surrounding shielding, with the intention/hope of creating a venturi effect, drawing the hot muffler air out of the engine compartment. A lot of work. In about 2 months I should find out how effective it is.

Much of this discussion has been centered around the potential benefits and adverse effects of wrapping the pipes on components under the cowl, in particular their longevity. I am aware of the adverse effect of warm intake air to engine performance but also wonder about the benefit of heat under the cowl to the carb throats. My 912UL has the ring mount, so no air box, and no heat for the carburetors to prevent carb icing other than what the tight cowling provides to them as they sit atop the engine. I don’t know, but suspect that like me, the vast majority of other owners do not have the Rotax manifold carb heat system.  I wonder if the risk of carb ice accumulation could be increased by wrapping the pipes.  In other words, is the performance trade off of using exposed K&N filters instead of the cold air intake (factory air box) year-around carb ice protection?  Does the factory air box kit include some a carb heat capability or is the manifold heat system still required?