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Exhaust system clearances, open question

Freemasm

Well Known Member
Is there a source, rule-of-thumb, anything written about clearances between exhaust systems and, cowling, engine, components, etc?

Having a beyatch of a time and some serious decisions need to be made soon. Sorry for open question. I know that shields, insulation, etc. are part of eth game but more info is better right now. Thx.
 
I've never seen anything written, my rule of thumb is 1/2" clearance minimum from the engine as it moves around more than you would think but Ihave no data on which to base that number.
 
I’ve seen a lot of bottom cowls that show signs of heat damage. You are wise to move this up on your “things to consider” list.

If you use the Van’s or Vetterman exhaust and Van’s cowl, you will have adequate engine motion clearance on the exhaust, but not heat. But, there are proven ways to mitigate the heat issue:
- Radiant heat shields that attach to the exhaust pipe. I end up using 2 or 3 of these where the pipes are closest to the cowl.
- Insulation the the inside of the cowl. I paint the interior of the cowl using white epoxy primer. On the sides near the exhaust run I add 1/8” thick FiberFrax covered by the self adhesive aluminum foil sold by Van’s. This is done in sections to allow for aluminum tape to hold it to the cowl ever few inches.

Other issues:
- Any control cable running under the engine is covered in Firesleeve. This is an easy barrier to radiant heat. Do the same for any cables running under the engine like for the alternator and starter. For the big cables I use US made welding wire as I find it more heat tolerant than Tefzle.
- All fuel lines FWF are in firesleeve.
- Be wary of engine baffles hitting the cowl. Look for witness marks after the first flight and whenever the cowl is off. Ease the baffle corners to gain clearance.
- Look for witness marks on the baffle material to identify any gross cooling air bypass issues.
- Some people add cooling shrouds on the mechanical fuel pump. I never found a need for that.

I’m sure others will have more recommendations.
Carl
 
When I first installed the exhaust, it has 3/8" clearance to the cowl. I sent it back to vetterman and he reworked some of the bends and got it up to 1/2-3/4". I used heat barrier as well and some spots near the exhaust still show discoloration, but no heat damage is showing on the cowl itself. Any fitting, cable, rod end, etc. that is closer than about 2" got a heat shield between it and the exhaust as well.
 
If you ever started or shut down a Lyc with cowl off you will see it shakes like a wet dog after a bath trying to get you wet.

So that movement or deflection varies based on distance from the crankshaft. Since most of the shaking is torque. However there is side to side and up and down, as well as all around. Here is the deal, any part of the engine or accessory, airbox, exhaust for example needs to have room from fixed parts like cowl for example.

HOW MUCH Clearance? Depends on in which direction and geometry. 1/2" sounds fine. However 1/10th an inch may be OK. You may need a full inch or more. Some engine have out of balanced props some engines shake less. It also depends on your vibration isolation mounts. The LORD brand with the gel internal filler controls deflection while providing superior vibration isolation (IMHO).

When you get it flying YOU WILL find chafing, interference after an hour or 10 hours or 100 hours. It takes time to sort this out. But with careful thought you can minimize the interference, chaffing. However even then after 300 hours things will wear, chafe and crack if there is interference or excessive vibration. A balanced prop goes a long way in reducing wear and cracks. . Anything moving connected to or near something fixed will possibly interfere and cause wear.
 
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