Exhaust wrap.

[wockda]

Anyone have advice on how to keep my exhaust from burning the cowling (see pics) . Also, should I change the trailing edge of the cowling ? I am planning on installing a ram air intake (Vans) and making a boat tail style lower cowling and also replacing the prop with a Silver bullet. Currently I get 2350 rpm on the ground and 2900 rpm @4°C , 6500 msl . Any ideas/ criticism welcomed. Thanks 

[Marc Zeitlin]

2 hours ago, wockda said:

Anyone have advice on how to keep my exhaust from burning the cowling (see pics) .

Clean the inside surface of the cowl near the exhausts - completely free of grease and oil. Then sand it to scuff a bit. Then use Red High Temperature RTV as an adhesive, put a layer of 1/16" Fiberfrax over the area to be protected. Then cover the whole area with AL tape:

https://www.aircraftspruce.com/catalog/cspages/3mproptape.php

Then seal the edges of the tape with a bead of Red RTV.

2 hours ago, wockda said:

Also, should I change the trailing edge of the cowling ?

For what reason? What do you propose changing it in to?

[wockda]

Not sure, I was just asking. Thanks for the info on the fiberfrax

[slk23]

You can reshape your cowling to reduce airflow separation and reduce drag but it's a big job.  I'm in round 2 of that and it's taking months to get the shape to my satisfaction.

If your exhaust system will be needing replacement anytime soon you could go for in-cowl design.

[wockda]

I will be changing the lower cowling to fit the ram airbox in and getting a new prop to hopefully get an increase in speed. The Hendrickson prop has worked ok for years but I’m sure I can get some gains with a little work. This is what I like about this forum with the exchange of ideas and knowledge. 

[eric r]

Jet Hot coating will keep the exhaust pipes from burning the cowling. They coat inside and out of pipes. The downside (if any) is that exhaust gasses exit the end of the pipes at or very nearly the same temperature they exit the exhaust valve! Be careful where they point!

I have a set of Hal Hunt pipes for O-200 that I had coated by them, and also a 6" Sabre prop extension not yet installed. My concern is the heat of the exhaust plume aimed at prop disc. If the prop is "clocked" correctly the hot exhaust exits between blades, but I still plan some very short runs followed by shutdown to see where prop temps are going..

A set of those pipes are up for sale on the forum right now.

BTW, I have used Jet Hot on race cars and motorcycles for years. my experimenting with other less expensive coatings did not turn out as well.

To put the snake oil guys at bay, I assure you that when I coated race car headers, a carburetor jet change was required to take advantage of new found power. when I treated IO-470C exhaust manifolds and down pipes on a Bonanza, there was an immediate gain in static RPM and takeoff RPM on takeoff. That in itself is an obvious indication of power increase. I got the same boost that seaplane guys have been doing for years when they "adjust" the prop governor for more takeoff RPM.  Also, after race car shut down, and immediately after exiting the Bonanza, I was able to place my hand on the exhaust header and the Bonanza down pipe without leaving skin on the metal ...palm, not finger.

When I did the Bonanza, I conferred with a DAR and an FAA rep who both said that is was akin to painting exhaust and not considered a major change. I'm sure some would disagree, but common sense prevailed then.

[eric r]

Oh yeah, and "exhaust wrap" will work, but it absorbs oil, absorbs moisture, and bakes the pipes continuously, so I'm not a fan. Works on race cars, but eventually destroys the metal headers. Not a scenario you want on the airplane. You want to inspect for cracks, etc. to avoid avoid catastrophic failure of metal.

The fiber fax on close surfaces will work well if also topped off with reflective material. My concern with that is keeping it outside the prop disc area in case something decides to depart.

[slk23]

@wockda : I think the improvements you're planning are all worthwhile.  Before first flight in '03 I changed my cowling to use a NACA scoop (from Lightspeed Engineering) and a boat tail.  I used a Van's airbox turned toward the rear and later replaced it with a custom version that provides a little more clearance.  Around '06 I switched out my Hendrickson prop for one from Gary Hertzler.  It performs better and just as importantly for me, is safer since it's unlikely to catastrophically fail if a fuel cap went through it.  Now I'm working on extending the trailing edge of the cowling with the hope that I will get better ground/taxi cooling by reducing the gap between the cowl opening and the prop disk.  There's also a side benefit of smoothing out the curves leading to the trailing edge and keeping the airflow attached longer during flight.