Exhaust Systems

When I first started thinking about an exhaust system for my tri-gear RV-6A, I assumed I would buy a ready-made stainless steel crossover system similar to the one I had installed in my first RV-6 (the taildragger, that is).

That assumption, however, became a quandry after I learned that I could get a complete mild steel exhaust system for about $250 . . . this was considerably less than half the price of the stainless steel exhaust unit I originally had in mind.

Are the stainless steel stacks really that much better? Are they worth the extra cost? Maybe so. But that is not to say that a mild steel exhaust system has a short life or is inferior.

For my part, the cost of either ready-made exhaust system was still more than I had ever expended on any previous project.

Except for one donated ready-made system, all of my earlier projects were fitted with "do it yourself" exhaust stacks of one type or another.

Somewhere in my distant past, someone convinced me that if an exhaust system lasted 100 hours without failing it would go 1000 hours. I assume that profound utterance applies to the common automotive type mild steel stacks as well.

At any rate, my own experience seems to bear this out because none of my homebuilt engine installations has ever suffered an exhaust system failure. And yet, each exhaust installation was as different from the other as night is from day.

My first airplane, an Emeraude, was fitted with a brand new $28 J-3 exhaust system . . . guess how long ago that was? It never failed.

Another had a liberated Cessna 150 system complete with two used stainless steel mufflers straddling a set of used stainless stacks. It never failed.

A third was fitted with a scratch built mild steel installation using VW pipes (with lousy welds, yet). Years later, another VW installation did have better welds - but they didn't fail either.

More recently, each follow-on project was economically equipped with homemade stainless steel stacks assembled from used units purchased at fly-ins, or built up of new stainless steel exhaust flanges, straight pipe lengths and preformed elbows. No, I didn't do the welding, a friend Heliarc welded them for me (and, boy, does a builder ever need a skilled friend or two). Again, no failures.

For the life of me I don't know why some of my earlier installations remained trouble-free for the several hundred hours each of them was flown. A little luck, perhaps, and plenty of well located expansion (slip) joints, and properly supported tail pipes, I guess.

Since I have never experienced a failure in either the ready-made or homemade exhaust systems (mild steel or stainless) I have use, I have no preconceived preference for one type of metal over the other.

I do consider myself fortunate that I no longer have to build my own exhaust system and can, if I prefer, purchase a ready-made set from any one of several reliable sources.

A number of these ready-made exhaust systems are designed and custom-fabricated to fit specific aircraft designs - designs like the RV's, T-1 8's and Mustangs to name a few.

Incidentally, the newsletters, subscribed to by builders for their particular aircraft, are often good information sources for various component parts - including complete exhaust systems.

Naturally, if you prefer, you can still fabricate your own exhaust system using parts available from one of the homebuilder suppliers, or from some local muffler shop.

But, before you can buy a complete system or build your own, you must first decide whether you want to have those expensive stainless steel stacks - or the less expensive mild steel variety.

Mild Steel vs Stainless Steel Exhaust Systems
Anyway, what difference does it make if you install a mild steel exhaust system instead of a stainless steel system? Your engine won't know the difference. Your pocketbook will though.

I am sure you are aware that the mild steel exhaust pipe installations in automobiles are reliable and hold up very well.

Of course, the prevailing belief among many homebuilders is that mild steel pipes, especially in infrequently flown homebuilt aircraft, are bound to suffer more rust than even those seen in the most abused and neglected cars.

Although the mild steel exhaust pipes will pick up a film of surface rust almost immediately, the assumption that destructive corrosion is quick to follow is not a valid conclusion.

A local VariEze has been flying for years with a mild steel exhaust system that was originally installed eleven years ago. How about that?

I know of stainless steel installations that didn't last half as long . . . they often burned out in and around the mufflers, or suffered cracked exhaust flanges.

Well, now, if rusty pipes offend your senses, you could:

1. Paint the pipes with a high temperature (2,000 degree F., preferably) automotive header paint. Properly done, this high temperature paint is quite effective and will enhance the appearance and prolong the useful life of your automotive pipe exhaust system for many years. Follow the painting instructions carefully. This is the least expensive and most commonly used treatment for protecting mild steel stacks against rust.

2. Have the mild steel pipes coated with Jet-Hot, a relatively new high-temperature coating for exhaust systems.

3. Have the stacks aluminized.

4. Have the exhaust system chromed.

Sometimes there can be an unexpected advantage to using a mild steel exhaust pipe installation.

For example, in the event you do develop a broken pipe on a cross-country flight, the chances of getting the mild steel pipe welded most anywhere are a lot better than trying to find someone who can repair and reweld stainless exhaust pipes.

A word of caution. Some coated mild steel exhaust pipes may be difficult to repair. Aluminized pipes are probably the worst in this respect. That is, they cannot be reliably re-welded.

It is true. A mild steel exhaust system will be heavier than a similar stainless steel system because the automotive pipes have a heavier (.050") wall thickness. In contrast, stainless steel pipes have a thinner .035" wall thickness. Even so, my ready-made (Vetterman) mild steel crossover exhaust system in the RV-6A weighs only 12 pounds.

The price difference between a mild steel exhaust system and that of a stainless steel system, on the other hand, is considerably greater than the modest weight difference.

That dollar difference alone can make the automotive pipe exhaust system quite irresistible to a budget minded builder - whether he buys or builds his own.

If You Have To Build Your Own

If you can buy a ready made exhaust system designed for your aircraft type, I would recommend you get it rather than to try to build your own . . . it might be cheaper in the long run.

There is more to fitting and developing an exhaust system than merely routing the exhaust gases from each cylinder overboard.

John Thorp, designer of the fabulous T-18, used to say that a well designed crossover exhaust system could net a 10% increase in power. However, that may not be true compared to the semi-tuned four pipe exhaust system I bought from Larry Vetterman. It could be true, though, when you compare a crossover system efficiency with that of a common two pipe "Y" branch installation.

When building your own exhaust system, you should realize that if it is improperly done, your exhaust system could suffer from unnecessary power losses due to abnormal back pressures in the system. The installation might even fail after a few hours due to the lack of properly located expansion joints.

What kind of exhaust system would you build? A crossover system? A semi-tuned four pipe exhaust system? Maybe a simple exhaust installation with four short straight stacks? They all work.

However, unless you have the technical background for that sort of thing I would suggest doing it the easy way - duplicate one of the exhaust installations in use in some recent vintage store-bought aircraft.

Making Your Own Exhaust System

If you are a careful buyer, or a good scrounger, you can effect significant savings by building your own mild steel exhaust system . . . especially if you can do a creditable job of gas welding the mild steel pipes.

Attempting to weld your own stainless steel exhaust system, on the other hand, may not be as rewarding as it requires special welding techniques, more know-how, and perhaps, some expensive welding equipment.

Although most commercially produced aircraft use expensive mufflers, you will probably fabricate your exhaust system without them to simplify the installation and reduce cost and weight.

The argument made for eliminating mufflers is based on the premise that they are heavy and that homebuilts have very tight fitting cowlings with almost no extra space for mufflers. (Those excuses, incidentally, don't hold water in Switzerland and in other environmentally sensitive countries.)

1. Begin fabricating your exhaust system by making your own mild steel exhaust flanges - or you can simplify the job by purchasing a set to fit your engine (Lycoming or Continental).

2. Start to assemble the component parts by welding the exhaust flanges to short pipe sections about 4"-6" long. Use 1-3/4" dia. pipes for a Lycoming, and a 1-1/2" dia. for most Continentals.

3. Next, temporarily bolt the pre-welded exhaust stack stubs to each cylinder. This will establish the correct exhaust flange alignment for each cylinder and will accurately position the stub stacks. The other exhaust system pipes can then be fitted and clamped to these stubs.

4. Decide where to locate the slip joints and ball joints for a trouble-free exhaust system.

5. Cut, fit and clamp each pipe section together.
  
NOTE - If you think that figuring out how to fit and hold the various pipe sections in place while attempting to assemble and tack weld your exhaust can be difficult, you're right. It is no simple undertaking. The easiest way to do this is illustrated in one of the photos. Notice how the separate pipe sections are clamped together with small metal strips and steel hose clamps to form the assembly. This steel clamp jigging technique is especially effective when you have to work alone.

6. After all of the pipe joints have been immobilized with the steel clamps, remove the assembly from the engine and tack weld the joints.

7. After tack welding, reinstall the pipe assemblies on the engine to double check the routing and clearances. Then you can take the tack welded exhaust stacks to your favorite welder and have the welds completed . . . either that or do it yourself.
  
NOTE - Do not buy or use curved automotive pipe sections that have a rippled or buckled inside radius. You should be able to purchase smoothly bent exhaust pipe sections at most any muffler shop.

You might, on the other hand, find a local shop equipped with mandrels that does custom bending of pipes for hot rodders and the automotive set. This would permit you greater flexibility in obtaining the exact bend radiuses you need in your pipes.

Such a shop can also expand the diameter of one end of a pipe so that it will form a slip joint when assembled with another section of pipe.

Avoiding Exhaust System Failures
Exhaust systems deteriorate and fail because of high engine operating temperatures, vibration that causes metal fatigue in stress concentrated areas, and wear at joints and in clamped connections.

A number of our local area homebuilts have suffered exhaust system problems over the past few years. These problems are quite typical and included cracked exhaust flanges, burned and cracked stacks, and broken tail pipes as well.

Most of the cracks were detected in the welded areas of the exhaust flanges. However, a couple of the aircraft did have exhaust pipes break off completely in flight. Fortunately, no engine compartment fires occurred.

Most of the affected homebuilt owners attributed the failures to the relatively thin walled stainless steel pipe used, and to improper welding technique. Not all the problems were limited to stainless steel exhaust systems.

For my part, however, I believe almost all of the failures in homebuilt exhaust systems can be traced to improperly supported tail pipes.

Sometimes, inexplicably, what appears to be an adequately braced installation does develop a crack, or breaks off completely.

In those instances one can only assume that the design and fabrication of the exhaust system didn't provide for properly located expansion joints, and flexible joints that would have allowed the exhaust pipes to expand and contract during normal engine operations.

It is obvious that exhaust system problems are aggravated because they operate under some pretty severe conditions.

For example, the exhaust gases exiting the engine are so hot they heat the pipes red hot . . . about 1400 to 1600 degrees F.

Further downstream, the temperatures are significantly lower causing an uneven heating and expansion along the length of the pipes.

To make matters worse, a sudden cooling of the exhaust pipes caused by prolonged power off letdowns can add still more abuse to the welded joints, and to the entire exhaust system.

I don't recommend wrapping exhaust pipes with one of those automotive "Exhaust Insulating Wrap" kits regardless of the claims made for them. A local Mustang II equipped with a mild steel exhaust system suffered complete failure of the pipes in all of the bend areas. His thermal wrapped pipes had completely burned through.

Do remove your cowling at frequent intervals and carefully inspect the entire exhaust system - inch by inch.

Here are a few things to look for:

1. Exhaust leaks and cracks. These areas usually show up as gray-white streaks.

2. Use new copper/asbestos exhaust gaskets when installing or replacing your exhaust system. Do not reuse the old gaskets.

3. After the first 10 hours of flying time, re-torque the exhaust flange nuts. Be sure you have a plain steel washer and a new star lock washer under each nut.

4. Look for loose or broken clamps and connectors.

5. Check to see that stacks are not dented.

6. Look for cracked or broken stacks and tail pipes.

7. Check heat muffs (and mufflers, if installed) for condition and broken connectors.

8. Be sure the SCAT ducting is in good condition.

9. Finally, and this is most important, check to see that the exhaust pipes are braced directly to the engine and not to the engine mount or to the fuselage structure.

Remember this . . . the best place to cope with exhaust system problems is on the ground, not in the air!