I've got a 1967 172-H with the Av-Con STC for a lyc O-360 and CS prop. We are doing some upgrades to the plane as part of overhaul, one of which is adding the PowerFlow exhaust. PowerFlow sells the Maple Leaf exhaust fairing which apparently does a lot to keep CHT's down. The fairing is not STC'd for the H model. I reached out to Maple Leaf to see why and if the had any advice for getting it approved for my bird.

The STC only applies to I model forward because it was equipped with a Lycoming from the factory. The fairing only fits Lyc exhaust and the H models had a Conti O-300 from the factory, so STC isn't applicable to the stock H model.

The person I emailed at Maple Leaf said she was pretty sure they had sold a few to be used on earlier models that had been converted to Lyc O-360's that were granted a 337 field approval using the STC data, but did not have a copy of the approval.

Any chance one of those birds is owned by somebody here that can give me some tips for getting this approved? Seems like it should be a shoe-in approval in my mind, but I'm not the FAA.

Any advice is appreciated, as I have 0 experience in dealing with FAA for something like this.

Could this be considered a minor alteration and a simple log book entry?

bush master wrote:Could this be considered a minor alteration and a simple log book entry?

Yep. It’s just a piece of fiberglass screwed to your cowl. And it probably won’t reduce CHTs much if any.

MTV

The mapleleaf exhaust fairinf dropped my climb CHTs by about 20 dF. The firewall on my E model is right sft of a gaping hole in the bottom of the cowl. I tried to reduce CHT by flatter climb profiles. Didn't do much. fter installing the exhaaust fairing climbouts after takeoff at the same deck angle were much cooler. The hotest cylinder dropped to about 400 in the summer, and others hover around 375-395. Logbook entry because it was installed on other C172 aircraft.

PapernScissors wrote:The mapleleaf exhaust fairinf dropped my climb CHTs by about 20 dF. The firewall on my E model is right sft of a gaping hole in the bottom of the cowl. I tried to reduce CHT by flatter climb profiles. Didn't do much. fter installing the exhaaust fairing climbouts after takeoff at the same deck angle were much cooler. The hotest cylinder dropped to about 400 in the summer, and others hover around 375-395. Logbook entry because it was installed on other C172 aircraft.

Wow, that’s impressive. I can’t tell that it did anything on my 175. The lady who sold it to me at the company said that results can be unpredictable on different cowlings.

MTV

I made an aluminum shroud around my Lyc. powered 175 and dropped the Cht's about 20 degrees. I think I was getting ram air into the lower cowling reducing the Delta. The way I installed my shroud I think it is drawing some air out around the exhaust and dropping the pressure in the lower cowling.

bush master wrote:I made an aluminum shroud around my Lyc. powered 175 and dropped the Cht's about 20 degrees. I think I was getting ram air into the lower cowling reducing the Delta. The way I installed my shroud I think it is drawing some air out around the exhaust and dropping the pressure in the lower cowling.

Yes, that’s the theory with the exhaust fairing……to lower pressure in the bottom cowl.

I assume that what you’re describing is to block off any air flow from around the air inlets from going into the lower cowl? I know of at least one person who modified the oil cooler mount on rear baffle to increase air flow past #4 cylinder. A lot of work, and pretty “noticeable”.

MTV

Yes, that’s the theory with the exhaust fairing……to lower pressure in the bottom cowl.

I assume that what you’re describing is to block off any air flow from around the air inlets from going into the lower cowl? I know of at least one person who modified the oil cooler mount on rear baffle to increase air flow past #4 cylinder. A lot of work, and pretty “noticeable”.

MTV

Yes, I believe ram air was going in around the exhaust cut out, the fairing stops this and I think created a reduced pressure area. I normally see about 350 degrees and 380 or 390 with 100 degree air temps. I have no cooling baffel leaks. I also closed off about half 30% of the lower cowling inlet area, it was too large for the outlet and I think there was considerable turbulence because of this. Presently experimenting with a ground adjustable cowl flap like on the later scouts, which is dropping temps more and maybe a speed gain.

bush master wrote:Yes, that’s the theory with the exhaust fairing……to lower pressure in the bottom cowl.

I assume that what you’re describing is to block off any air flow from around the air inlets from going into the lower cowl? I know of at least one person who modified the oil cooler mount on rear baffle to increase air flow past #4 cylinder. A lot of work, and pretty “noticeable”.

MTV

Yes, I believe ram air was going in around the exhaust cut out, the fairing stops this and I think created a reduced pressure area. I normally see about 350 degrees and 380 or 390 with 100 degree air temps. I have no cooling baffel leaks. I also closed off about half 30% of the lower cowling inlet area, it was too large for the outlet and I think there was considerable turbulence because of this. Presently experimenting with a ground adjustable cowl flap like on the later scouts, which is dropping temps more and maybe a speed gain.

I think a cowl flap on these things would be great. You dancing with a field approval?

MTV

Some people have had good results increasing airflow to improve engine cooling by adding a "seaplane lip" to the lower cowl outlet.

hotrod180 wrote:Some people have had good results increasing airflow to improve engine cooling by adding a "seaplane lip" to the lower cowl outlet.

Yes, that definitely helps in Huskys, to the point where the factory created a lip, and put it on their equipment list. My 175 already has a lip, but perhaps extending it a bit would help....

Good point.

MTV

mtv wrote:

hotrod180 wrote:Some people have had good results increasing airflow to improve engine cooling by adding a "seaplane lip" to the lower cowl outlet.

Yes, that definitely helps in Huskys, to the point where the factory created a lip, and put it on their equipment list. My 175 already has a lip, but perhaps extending it a bit would help....

Good point.

MTV

If the Maple Leaf fairing hadn't worked, i had a seaplane lip from a c170 ready to go. The hole in the bottom cowl of my C172E already has a vestigle However, it's ineffective at high AOA on climbout near Vx and even Vy. The Maple Leaf fairing plugs a big hole in the cowl that leaked a huge amount of air past the stack even near Vy.