Cessna 120-140 Association

After searching around and seeing many members aircraft I've seen 2 major exhaust routing through the engine cowling. The first is directly out under the motor perpendicular to and through the cowling. The other is inside the cowling dumping at the air outlet on the belly. The second routing seems to be superior from and aerodynamic perspective. Currently my 140 has an 0-200 with Randy Thompson STCSA547EA and I believe the 150 exhaust STCSA648NE. There is room inside the engine cowling to easily facilitate routing the exhaust to the belly air outlet. Is there any currently available STC to reroute the exhaust inside the cowling? I’ve seen many 140s out there with the second routing but not sure how to get the change authorized. Any advice would be appreciated!

I suspect the routing you're referring to as "dumping at the air outlet on the belly" is really the early ('46) straight pipes. All the later planes with "mufflers" exited through the cowl skin. From your photos, it looks like you may have the early cowl, so there is room for the pipes to exit there.

You'll probably have to contact the STC holder for the mufflers to see if there's a way to do it. I suspect you'll need some kind of support back there.

Given how many 140s I've seen have the "dumping at the air outlet on the belly" exhaust I am hoping for some guidance on the legal part. Physically there is no issue. Safety-wise I don't think there an issue given how many already dump in this fashion. Would it require Field Approval? Is that difficult to get?

I already have an exhaust pipe from a 170 and it looks like it would bolt right up. Would just need to have some length trimmed of the end given the longer motor on a 170.

Your mufflers are installed under an STC. The STC specifies the installation. You need to start there. If the STC doesn't allow for that sort of piping then you will need to get a field approval to make the change. The '46 straight pipes have a support bracket that attaches near where the pipes exit the cowl. You will need something similar to prevent the moment of the longer pipes from causing cracks farther up in the exhaust.

You could switch back to the original installation, if you could find the parts. That would not require any sort of approval other than a log book entry.

Look in the parts catalog for the '46 engine installation if you've never seen one.

Thank you very much for the information John!

I have the stock '46 exhaust on my 140. It is LOUD. And, the heat muffs are a maintenance headache.

Personally, if I had your system I'd do nothing. You may pick up 1 mph in cruise and end up with two round patches on the bottom of your cowl. Not sure how much speed the patches will cost you...

The cowling on the early aircraft....... (1946's) exended down below the engine farther than the 1947's and later years. This provided about 3 inches under the engine that the later cowlings don't have.

This additonal spaced allowed for exhaust to be routed out directly below the fire wall, rather than well in front of it. If you can find a 46 aircraft (with a true 46 cowling, the cowlings are interchangeable between years) and stand off to the side, 90 degrees to the airplane and compare the drop to a 47 and later you can see the difference.

An easy way to tell also is the 46 has a retangular cut out at the bottom and the 47 and later is more like a shallow 'V' or a crescent shape.

I have a 47 cowling with the the C-150 exhaust. It would be very difficult to route the tail pipes out the back and have enough clearance to avoid the pipes contacting any number of other components, including the cowling.