Stainless Steel Exhaust Systems
January 9, 2012 Leave a comment
One of the most popular products we sell are Stainless Steel Exhausts for Corvair powered planes. We have been continuously making them since 2005. In this post we will cover the different systems that we make, talk a little about the pros and cons of certain designs, and look at some applications.
Prior to stainless, we built systems out of mild steel and had them ceramic coated. They looked great, but actually had a shorter life than plain painted steel. This is a surprise to many people, but here is why: Ceramic coating really works. It is a great heat barrier. A normal mild steel exhaust lives for a while as long as it can run cool. Ceramic coating the outside of it makes it look good, but it is actually hurting the system because it is trapping the heat in the metal. All affordable ceramic coating is done on the outside of parts. Very high end shops like Jet Hott charge several hundred dollars for a system because they use special tools to apply the coating on the inside of the pipes.
In 2003, I built an exhaust for our 601XL, N1777w. It was made from mild steel, but it was ceramic coated by the Moore Brothers, a very high end shop in Florida. The coating alone cost $300. It worked, and the best evidence of this was the fact that the heat muff for the carb didn’t work because the coating prevented any useful heat transfer. The system also racked up a lot of time on our plane, and it held up well. When other Zenith builders wanted to follow our success, I began to look at stainless as a better material for production exhaust systems.
Stainless is inherently a better material for exhausts because it is stronger at elevated temperatures and it is very resistant to corroding. Both of these are a big deal in aircraft because you can’t tolerate any kind of an exhaust leak in a plane. Everyone first thinks about carbon monoxide getting in the cabin, but my real concern is the possibility of starting a fire in the engine compartment. It is remote, but it is something that experienced aviators actually consider more of an emergency than having an engine quit on you. The strength and rust resistance of stainless, combined with good materials and welding techniques, applied to a design that has been flight proven not to resonate or crack on your airframe is the answer to minimizing your risk.
The stainless we use is an alloy called 304. It is the standard alloy of certified exhaust systems. The main tubes of our systems are bent for us by a shop in Florida that specializes in robotically bent tubing. They actually make the OEM systems for Lycoming and Continental, and make STC’d systems for companies like Powerflow. The head pipes on our systems are CNC machined from solid 304 bar stock. (They are made in the same shop in Florida that produces our Gold Prop hubs.) We have a separate shop that produces the Heat Muff Box Ends and another company that makes the tight radius front pipes. All of the systems are TIG welded in our hangar with 308L rod while they are pressure back purged with argon gas. Getting very expensive American made subcomponents from four different shops together in one jig and welded is something of a logistical challenge, but the end product is well worth the effort. In the past seven years we have produced about 250 Stainless Exhaust Systems for the fleet of Corvair powered planes. Chances are, most of the Corvair powered planes you have seen in person or seen in photos have a stainless Exhaust System that came out of our shop. Virtually every Corvair powered Zenith has an Exhaust System of ours on it.
To get a look at one of our Stainless Exhausts in action, watch this video of Jeff Moore’s Corvair powered Merlin on floats:
Jeff is from Newfoundland, Canada. His aircraft previously flew with a Rotax, but he has opted to repower his plane with a Corvair that he built with our conversion parts, http://www.flycorvair.com/products.html. His engine is a 2,700 cc 100 hp engine with all of our Gold Systems and a Weseman bearing. Jeff built his own mount utilizing one of our pre-welded trays. The Exhaust seen in the video is one of our Universal #2 Systems.
Below are three of the four production Stainless Exhausts we make. Universal #1 is the Exhaust System that is used on KR-2s and Cleanex airframes. Chris Smith’s “Son of Cleanex” was the first aircraft to fly with this system. The Universal #2 is the system that we make for aircraft like Jeff Moore’s Merlin. It fits a broad variety of planes like John Pitkin’s Kitfox 5 and Russ Mintkenbaugh’s Wagabond. It combines good motor mount clearance with the ability to work with a high thrust line. It is also a good match for a Pietenpol. Universal #3 is specifically bent for aircraft with a very low thrust line, like a Tailwind. Ordering information is on our Exhaust System page, http://www.flycorvair.com/uniexhaust.html
If you have any questions about which model is correct for your plane, just send an e-mail or give me a call on the shop line, (904) 529-0006.
Our fourth production system is our Zenith 601-650-750 System. This is specifically engineered to fit in the Zenith’s engine compartment, which has plenty of room, but the Exhaust has a sophisticated shape because it passes through the mount and clears the nose gear installation. This has proven to be the most popular system we sell. The Zenith has a particular motor mount geometry that requires this Exhaust to fit the engine correctly to the airframe. While some aircraft like Pietenpols utilize stock car exhaust manifolds, this is not an option on a Zenith because the car manifolds actually hit the upper tubes of the motor mount. Thus, a stainless system is an upgrade on a Piet, but a requirement on a Zenith.
One of the first things people ask about the systems is if they would make more power if they looked like aftermarket headers for cars. The magic answer is no. I tell them that you don’t have to take my word for it, you can just ask our Dynomometer. Before we came to the design we use, we tested lots of prototypes and systems. The technical reason why these compact systems do not restrict performance has to do with the camshaft pattern and the rpm range we use. The OT-10 cam has very little overlap, which is one of the reasons why it makes good torque. Engines like this, especially ones with 3,500 rpm power peaks, don’t see the same benefit from a full tubular exhaust system that a 7,000 rpm V-8 car with a high duration cam does. There are three basic goals served by making the most compact stainless system: First, it is lighter than something elaborate. Second, it is structurally stiffer, and therefore it is not prone to vibration damage (our Exhausts are cantilever off the bottom of the engine, they do not require tail pipe brackets nor secondary mounts). And last, it has a lot less surface area to radiate heat into the engine compartment. This last one is a bigger point than many people suspect. If you operate your aircraft in a very hot climate, this makes a difference on whether it is susceptible to vapor lock. I saw an experimental that had terrible trouble with vapor lock, yet when the builder looked in the engine compartment he missed the concept that his flat black mild steel tubular exhaust pipes were radiating the vast majority of the heat that was bothering his carb and gascolator. A poor exhaust system choice can easily put as much heat into the engine compartment as the engine itself. You are far better off having this heat run out the exhaust pipe. Stainless is a poor conductor of heat, and it does not radiate heat well. Combine this with a compact design, and you have the making of a cooler engine compartment.
The second most common question is about how loud the system is without mufflers. You can watch a number of videos, but they don’t give you a good feeling about the level of the sound. In person, most people are very impressed with the sound of the engine; throaty, but it doesn’t have a harsh bark. Some of the video shorts near hangars sound harsh because any aircraft turning a prop makes metal hangars resonate like steel drums, and microphones are very good at exaggerating this frequency. Out in the open, the engine is not loud. A subjective comparison; a Cessna C-172 in the pattern of your airport is a lot louder than a Corvair powered plane. It is also an effect of the engine’s cam timing; the low overlap means the cylinder is done burning by the time the exhaust valve opens. This short duration also has a secondary effect: Only one of the exhaust valves in each head are open at the same time. Each side of the exhaust system only has to serve one cylinder at a time, contributing to low back pressure. If you would like to use a muffler, you will end up with an exceptionally quiet aircraft. We flew our Pietenpol with a muffler for many years, and many people thought it was one of the quietest aircraft they had heard. Although many people think of quiet engines as being down on power, this is only true on engines that have a long duration camshaft design. Taking the muffler on or off our Piet only reduced the static rpm by 20-30 rpm.
On the subject of custom exhausts, I have produced a number of one-off designs for builders. Most of these were for engines equipped with 140 hp heads (they have a different size exhaust stack), or for a one-of-a kind airframe. If you find yourself heading in this direction, give me a call, and we will talk it over. It is also worth mentioning that we can install oxygen sensor bushings in the exhausts for builders who want to use an air/fuel meter. This is a concept with some appeal, but 90% of our builders still opt to use EGT probes, which are placed into the exhaust system after it is installed by drilling a small hole in the tubing.