by Mike Melvill
The 4-pipe exhaust system that I built back in 1986 has served faithfully for 1360 flight hours, with only a couple of minor cracks (after the original trial and error test program). For some time now, I have wanted to try a 4-pipe exhaust system that does not run the high pressure exhaust plume through the prop, right at the hardest working area of the prop blades. I have discussed this idea over the years with anyone who would listen, but just never got around to doing it somehow. I guess I have just been too lazy to get it done, but I also knew that my existing exhaust system had performed essentially without problems for many years. I worried about going through all the development hassles that I was sure I would have to endure if I went ahead with my plan.
Just a few weeks ago I decided to bite the bullet and try it. I have long felt that the high pressure exhaust pulse trashing what is essentially the highest activity area of each blade, was probably costing some prop efficiency, and may have been causing some of the vibration felt in our Long-EZ. I also hoped that it might be quieter if the exhaust was not being cut to pieces by the high speed prop blades.
As I saw it, I had two choices:
(1) I could route the exhaust out-board far enough to completely miss the prop (maybe the best choice?) or
(2) I could route the headers to direct the plume through the prop, close to the spinner, where the prop blades really aren't very aerodynamically useful, and just as important, where the rotational speed of the blade is a lot slower.
I decided the problem of running a hot exhaust pipe inside the wing roots was too great, so I designed and built a system that consists of four separate pipes that terminate about 1-inch inside the cowling cooling air exit. My cowling goes aft much further than the standard plans-built cowling, and I also have a 9-inch long heavy duty prop extension. The exhaust pipes end about 4 3/4 inches in front of the leading edges of the prop blades, close enough to the spinner so that most of the grayish exhaust deposits end up on the spinner! (One of the disadvantages of this system).
I did feel that in order to see any improvement in prop efficiency, I would have to run the exhaust through the prop as close-in to the spinner as possible, and I have done just that. I built the exhaust system, one header at a time, in place, on the airplane/engine. I felt this was the only way to ensure that it would fit, would not interfere with the top or bottom cowling, and would point right at the prop where I thought it would cause the least disruption to the smooth flow of air into the prop disc. I ordered all of the tubing from Ken Brock and started cutting and fitting.
I used Hot Stuff model glue to "tack' the pieces together, then drove over to Scaled and tacwelded the pieces. I had to bead-blast the Hot Stuff glue off the headers before I could finish weld, because Hot Stuff could contaminate the weld. After almost two weeks of evening and weekend work, I had the exhaust system done. I had modified the rear baffle (see photos) and patched up the areas of the cowling where the old 4-pipe exhaust system used to exist.
Left side - Two exhaust pipes supported by aft baffle extension. Note gas deposit on prop very close to spinner.
Right side - Shows routing of pipes and the baffle support
I now had a much cleaner, neater-looking installation, but how would it fly? My first impression, when I started the engine, was that it was much louder! It certainly sounded different. On the take-off roll, I noticed it was much smoother. In fact, smoothness is the most noticeable change. Up at altitude, after I had flown it around for a while at different power settings, I opened it up to full power and let it settle for several miles. I did this at an altitude and temperature where I had gathered this same data with the old exhaust system.
The final result was that at exactly the same engine RPM, manifold pressure, and fuel flow I was looking at five knots indicated faster! I was blown away. I honestly never expected that large of an increment. The prop must be that much more efficient without the high-speed exhaust plume disrupting the "sweet spot" on the prop. I have since heard from other builders and flyers who have assembled similar exhaust systems. Several achieved similar results, and others gained nothing.
I believe you must direct the exhaust plume through the prop as close to the spinner as you can in order to get this kind of improvement. If you just move the plume inboard a few inches, chances are you won't gain any performance. The disadvantage of the dirty spinner is overshadowed by an even more important concern to me - the fact that I once again have an unproven exhaust system that I can't have much confidence in until I get several hundred hours on it with zero failures. I was so sure of my last exhaust system, it is an uncomfortable feeling at this point. In fact, for now, I remove the cowl and inspect the new exhaust after every flight. Only time will tell if this was a good idea.