Looking radical and yet somehow normal at the same time, the headers are conversation starters. This dual three-into-one layout is optimal for flat-six engines, yet six-into-one exhausts predominate with Red Bull and airshow performers likely because the single exhaust is better for burning smoke oil. The nearly flat angle of the exhaust exit looks sharp and minimizes drag (as if it mattered on a dirty old Starduster, says the author) but is awfully shallow. The transponder antenna melted in about 5 hours and exhaust residue lays thick against the belly.
It all started when I could no longer deny that the 540 on the front of my tired old Starduster Too was begging for a rebuild—or else.
Once steeled to the reality of a complete engine overhaul, the inevitable process of accumulation began. Higher compression pistons made sense, and why not port the cylinders as well? To an old hot rodder such queries are the stuff life is made of.
A vexing question was what to do with the exhaust system. After years of hammerheads and cross-countries, the existing homemade exhaust was warped and cracked scrap. A completely new system was needed, but which system? Six short stacks? Simple manifolds? A nice custom exhaust from an aircraft exhaust specialist? What about those six-into-one headers the Red Bull crowd favors?
To find answers I read the old texts and CAFE reports, and pestered the exhaust-system specialists. The consensus was that more sophisticated exhaust systems help, but no one could say how much. My curiosity was aroused. I got power in a radical set of pipes, but there was probably some. More importantly, there was power to be lost in an indifferent design.
With the two collector and megaphone assemblies temporarily blocked, clamped and safety-wired into their final position in the lower cowl, developing the mockup headers at Aberle Custom Aircraft became a three-dimensional puzzle of connecting the exhaust flanges on the cylinders to the collectors in the lower cowl using exactly 34 inches of pipe and bends from the local muffler shop. This is an excellent challenge for the dedicated builder looking to expand fabrication skills. Aberle and Andy Paterson at ACA get credit for the header layout.
Old enough to know better, but enthusiastic enough to keep trying, I couldn’t let go the idea of trying an optimized, tuned exhaust system. The main goal was promoting equal exhaust action among the six cylinders while avoiding “bad” pipe lengths. Sound was another consideration. Loud pipes hurt aviation, and yet the weight, complexity and, above all, backpressure, of a muffler are undesirable, so some squelching or harmonizing of the exhaust note was desired. And yeah, if they made more power, I was definitely ready for that.
Step one was to decide on the basic design. Six individual pipes were quickly discarded because they did not fit the existing cowling, they offer no cabin heat, plus I feared their loudness and lack of tuning. A “manifold system” in which the primary pipes on each side of the engine all branch into each other is the most common solution for many good reasons and is commonly available, but like six individual stacks, it doesn’t offer any tuning efficiencies or sound reduction. That led to headers, where the primary pipes are joined at a collector. Cost and complexity soar out of sight, but some promise of improved engine efficiency, sound and power was tempting.
Welding the headers at your home shop means your airplane provides the necessary buck. But the author was so tickled by the pro welding on the Burns Stainless collectors that he elected to have his headers made by one of its contractors, Chris Parker at CPR Fabrication. That meant a dummy engine supplied by Ly-Con played a major role in providing the welder with a fixture to work against. Looking down on the dummy engine shows how cylinder offset, along with the reversal of the intake-to-exhaust port relationship from left to right bank, means the right-side header barely reaches with 34 inches of pipe, while the left bank snakes tightly to end up at the same spot. Burns calls for all primary pipes to be within +/- a quarter-inch length of each other, and that goal was met.
Once I decided on a tuned, equal-length header, I turned to Jack Burns at Burns Stainless, a racing car exhaust designer with credentials extending to the highest levels of motorsport, including the four-into-one header on Jon Sharp’s original Nemesis race plane. Burns doesn’t build headers per se, but does fabricate collectors and supplies stainless steel and Inconel pipe and tubing. Best of all, though, is Burns’ X-Design header design software.
It’s far beyond the scope of this article to explain header theory, so let’s just say the pressure waves racing back and forth inside an exhaust system are mind-numbingly complex. But the main idea of a header is to join each cylinder’s primary exhaust pipe to the other cylinder’s pipes so as to promote more evacuation of exhaust gas from the cylinder than is possible with a simple manifold. There are a jillion variables, and if you guess incorrectly at two of the major factors—pipe diameter and length—those pulses and waves will work against you.
The finished product is a work of 321 stainless-steel art, especially before the inevitable discoloration begins. Much consideration was given to providing adequate compliance among the pipes to avoid cracking. Ultimately the slip joints at the collector were deemed sufficient, as each pipe is separate, has at least one bend and is nearly a yard long. These factors allow the pipes to expand and squirm with heat cycling. Further, each pipe is 16 gauge from the flange to the first weld, then 20 gauge the rest of the way to the collectors. The thicker material at the flanges aids mechanical strength and crack resistance.
After plugging in a veritable truckload of data—rpm, compression ratio, valve port dimensions and flow, camshaft valve openings, closings and lifts and so on—X-Design calculates the optimal header primary diameter, length and collector design. Burns Stainless offers the X-Design service for $80 via www.burnsstainless.com.
In this case X-Design called for a dual three-into-one layout using 1.75-inch diameter primaries that are 34 inches in length. A simple, straight collector would work well, but 14-inch tapered megaphones promised to broaden the power peak, so Burns built the megaphones while I set about figuring how 34 inches of primary pipe was going to coil inside the cowling.
The Burns Stainless collector-megaphone assemblies are built from 16 gauge 321 stainless in the collector and 20 gauge 304 stainless in the megaphone. The primary pipe inputs are 1.75 inches, the narrow choke is 2.00 inches, and the collector length is 14 inches. These megaphones look terribly loud, but in reality they quiet the exhaust note compared to conventional systems. At idle the system gives a slightly hollow “bloop.” In flight the tone is mellow and pleasantly horn-like.
Making It Fit
Step one in building a header is to mock it up. After considering PVC pipe and flexible tubing as mockup material, I took Jack Burns’ strong suggestion to tack the system together from mild steel purchased at a local muffler shop. This gives by far the most accurate idea of what fits and what doesn’t. This work was a collaborative effort between Aberle Custom Aircraft in Fallbrook, California, and to a lesser degree, yours truly.
With the mild steel mockup complete, the final product was welded from 321 stainless steel by Chris Parker at CPR Fabrication, which is right next door to Burns in Costa Mesa, California. This ensured superb pipe fitting and welding, though an experienced hobby welder could get close as the final header is built up from straight tubing and pre-done bends, just like the mockup.
Burns Stainless likes a tapered nozzle at the end of its megaphones to enhance their scavenging effectiveness. Burns calls them reverse cones, and in this example they narrow the 3.5-inch megaphones to 3 inches at the outlet. The felt-tip numbers are the weight of header/collector/megaphone assemblies in pounds. Burns uses the weight as a double check of symmetry between the two sides. The 0.2-pound difference is chalked up to slightly different lengths of 16 gauge primary pipe side to side.
Finally, it was time to test the headers against more conventional aircraft exhaust systems. The freshly overhauled 540, which had been mounted on the airframe to build the mockup headers, was removed and trucked back to Ly-Con Aircraft Engines so it could be run on the dynamometer. There the engine was run with a set of six short “test stacks” along with a rather basic manifold exhaust system off of an S2B Pitts Special and my new set of headers, both with and without their collectors.
Ly-Con uses these approximately 15-inch long by 1.75-inch diameter stacks as specified by Lycoming for dyno duty. These proved remarkably powerful thanks to low backpressure and are obviously dirt cheap and dead simple to fabricate. They are noisier than a Who concert, however, blaring an annoying, raspy racket at high volume. Even inside the protection of the data room’s concrete walls the roar from this combination had everyone wearing hearing protection.
My worst fear was soon realized: There was a choke in the engine, and it wasn’t the exhaust system. Most likely it was the intake throat in the oil sump/intake manifold, which, even though ported by Ly-Con, tends to stop the party right around 315 horsepower. Still, significant trends were identified even if the difference among the systems was not as great as it would be with the addition of a cold air intake.
Another consideration was controlling the oil temperature. Ly-Con’s dyno is fitted with a gigantic oil cooler designed to keep the most Herculean of its engines running cool in the vicious Visalia summer heat (think of Sean Tucker’s 400+ hp 540 or twin-turbo Reno Super Sport Gold race engines pumping 1200 foot-pounds of torque). But this test was on a cold, sometimes rainy winter day. Even after blanking most of the oil cooler the oil temp would still vary wildly between wide open propeller blasts. Because big oil temperature swings can skew dyno results significantly, we were careful to use only those runs with similar oil temperatures with these results:
Clearly the short test stacks are tough to beat power wise, but as previously noted they have several practical limitations and are generally not a good solution on general aviation aircraft. Above all, they are hideously loud.
For clearance the headers were swapped side-to-side on the Ly-Con dyno. This is common dynoing practice and has no effect on efficiency or power production. Mario Valario of Ly-Con handled the majority of the dyno wrench work.
Running the headers without their collectors shows a real benefit to collecting the exhaust. Burns points out that all open, non-collected systems narrow the powerband and don’t scavenge as well.
The runt of the exhaust litter was the under-engineered Pitts manifold system. It definitely lost power, but even worse, its excessive backpressure raised the CHTs dramatically and somewhat more unevenly. (Good baffling will affect cooling, but a set of headers will help fundamentally lower CHTs.) Within seconds of applying full throttle, the CHTs were over 400° F and rocketing upward. I must emphasize that these mediocre Pitts manifolds are not a blanket condemnation of the manifold design. A well-thought-out set of manifolds from an aviation-exhaust specialist would absolutely do better than the Pitts pieces. Key elements in a good manifold would be stepped pipe diameters (the pipe diameter enlarges where each cylinder joins), along with general attention to detail as seen in tight tolerances and good pipe alignments. The Pitts pieces had neither of these.
Representing the humble sort of exhaust system found on so many Experimentals was this used but serviceable factory-built Pitts S2B manifold system. Built to a price from 1.75-inch stainless, this system was not only too small to handle the gas volume of a healthy 540 Lycoming, it was indifferently constructed as well. It cost power, raised CHTs and emitted an unappealing medium-loud blat. A quality manifold with stepped pipe diameters—1.75-inch to 2-inch to 2.25-inch joined to a 2.5-inch tailpipe—would have worked much better.
By contrast, my tuned headers, like the test stacks, stabilized and equalized the CHTs in the mid 300° F range almost immediately, so the engine can be run at takeoff power as long as desired. Another indication of their effectiveness is the headers made at least the same power as the test stacks but with 1 inch less manifold pressure.
Run without their collectors, the unsupported outer ends of the primary pipes literally disappeared in a high-speed blur of vibration when the power was brought up. The other result was a slight drop in torque and power—and more noise. Uncollected pipes tend to have narrower, peakier power bands, too.
The bottom line is that a properly sized manifold system is where the cost, performance, weight, inside-the-cowling-complexity and cabin heat cuff considerations best intersect on a typical aircraft engine. For all-out power on hot rod engines the headers do help. The evidence strongly suggests that development money is best spent on a cold air intake, and then a set of equal-length headers. I’m confident that my engine and header combination, so equipped, would show clear-cut power advantages to the headers. Fiddling with the camshaft (later exhaust valve closing) might also pull more power and efficiency with the header’s powerful scavenging. But even if my headers didn’t prove major power builders on the engine, their help in providing extremely even cylinder-to-cylinder CHTs and EGTs, and their surprisingly quiet operation are great. This old hot rodder doesn’t mind their wow factor, either.
A real rat rod, N771TW gets by on brute power, so the tuned headers fit its persona. Weighing 1480 pounds empty, the author’s Starduster Too rolls a little over 200 feet during takeoff and has an initial rate of climb close to 3000 fpm. At 24 square it tears an indicated 170 mph hole in the atmosphere, which feels like driving around at Vne because it’s close to it! Its optimized exhaust is one reason both CHTs and EGTs are even, and there are no temperature restrictions when running at full power. Pulled back for cruising, it loafs along at 150 mph at 2300 rpm.