Exhaust Heat

[Andy Walker]

So given that all the "standard" Sonex engines (Aerovee, Jab 2200/3300) are very heat sensitive, does anybody *not* wrap their exhaust in heat wrap? It seems like people having heat issues might try that first thing, but I see posts of people who have struggled with heat issues for a long time saying they are "considering" exhaust wrap. I definitely plan to wrap mine, and take that extra heat off the table up front.

Also, does anybody have any experience with the ceramic coatings like Jet-Hot? I wonder if that would be more or less effective than the fiberglass wrap. It's certainly more expensive... :shock:

[dbdevkc]

I used exhaust wrap when building my Shovelhead Harley. It is my *understanding* that the ceramic coatings (inside the pipes) are more effective. It is worth noting however, that the main reason for doing a wrap or coating on a motorcycle or car is because they affect engine performance. I do not have a firm grasp on the physics of it, but the idea is to keep the exhaust gasses hotter as they leave the engine by stopping the release of heat from the pipes which then somehow increases the speed (or slows the natural decrease of speed) at which the exhaust is evacuated. It is generally considered a good thing for performance, but not sure how that would affect an aircraft engine, nor do I know what the higher EGT's would mean as that is something that is carefully monitored.

[radfordc]

Is it a "given" that wrapping the exhaust will make the engine run cooler? I can see that the air temp's in the cowl below the engine may be cooler, but the air that cools the engine isn't really affected by the exhaust pipe temperature. Also, if you keep the heat in the exhaust pipe, this heat may conduct up into the heads and make the heads hotter.

Charlie

[Andy Walker]

Good questions. I'm assuming that exhaust gas is a particular temperature, and it's better to dissipate that heat through the exhaust system than radiating some of it out into the engine compartment inside the cowl. Maybe this is incorrect, but heat-soaking cooling fins, the engine block, and electrical components by exhaust heat sounds like a Bad Thing [tm].

[sonex293]

I ceramic coated my exhaust and did notice a reduction in heat under the cowl. I was trying to control the heat to prevent any fuel issues with the EFI system. I did wrap the section of exhaust at the 3-into-1 junction. I was having quite a bit of blow-out at that point and the wrap seems to have provided the fix.

If you do wrap your exhaust, please add an item to your Annual Checklist to remove and check for corrosion etc. You don't want to miss a problem because you can't see it.....

[MichaelFarley56]

My first homebuilt, when first test flown, did not have any exhaust wrapping and after the first few landings we noticed the cowling itself was pretty warm. To prevent any worries we wrapped the exhaust and it made a huge difference. On my Waiex I've already wrapped the exhaust. My thought is that it certainly isn't going to hurt anything, so why not? I'm not sure how close the exhaust pipes run to the cowling but I'd rather keep everything as cool as possible.

[uteboy]

I had heat issues with a small engine bay in a car I built that had a V8 fitted. The thing that made the biggest difference was allowing the under bonnet (hood) air to escape more easily. I did coat the headers on the outside surface with ceramic which was not a cheap excercise but I cannot say that it made a huge difference to temps but it did eliminate corrosion as ten years on not a spot of rust anywhere.

[Andy Walker]

I had heat issues with a small engine bay in a car I built that had a V8 fitted. The thing that made the biggest difference was allowing the under bonnet (hood) air to escape more easily. I did coat the headers on the outside surface with ceramic which was not a cheap excercise but I cannot say that it made a huge difference to temps but it did eliminate corrosion as ten years on not a spot of rust anywhere.

Airflow is definitely a key aspect, but on the Sonex we're a bit constrained by the baffling, shape of the engine/mount/cowl, and similar issues. You might get better flow-through by putting extra/larger holes in the bottom rear of the cowl to allow air to flow out, but then you might not get enough pressure under the cowl, which is also critical to cooling. Cowl design is a kind of black art, IMO, and I'm reluctant to make major changes that might need to be undone later. At least until I've flown the thing enough to really understand what's happening. :)

[radfordc]

You might get better flow-through by putting extra/larger holes in the bottom rear of the cowl to allow air to flow out, but then you might not get enough pressure under the cowl, which is also critical to cooling.

What you don't want is higher pressure in the bottom part of the cowl (below the engine). The air flow through the cooling fins depends on the pressure difference between the air above the engine and the air below.