Rob, This is really good information for anyone wanting to design their own exhaust. I'd like to know what books you are referencing as I'd like to have a copy on my shelf.
As you point out, there are ideals and compromises. Just running 4 separate head pipes made it challenging to find sufficient room inside the cowling without getting in the way of something else or exposing other parts in the cowl to excessive heat. And then there's the bending equipment. I don't own any, so my compromise was to go to the local shop and have an bunch of 90 and 45 degree angles bend into mild steel pipes, then cut and weld to fit. You are correct about the pipe sizing. I used 1.5" OD pipe for three reasons. One was to keep the torque range where I wanted it. The second was that 1 1/2 OD pipe fits in the exhaust flanges perfectly. Just taper the tops to smooth the flow inside the flange. Additionally, using 1 1/2 OD pipe was simply lighter than building it all with larger pipe. Using Rob's number of 77" for the ideal tuned length for 2600 RPM, 77" is too long (and heavy) to be practical for this plane. So I used 38" length as a half wave length for 2800 RPM. I did not do the calculations for these pipes. A friend that has more expertise in this field than me did them for me and I had to decide on what compromises to build into them to make it fit my plane. It should be noted that the reason for using stainless exhaust in aircraft is primarily for weight savings. Mild steel works fine, but is thicker and significantly heavier than stainless. I use mild steel because I'm not set up to weld stainless. If I was, I would have purchased a host of pre-bent stainless parts and built the exhaust from stainless. Additionally, as Mark pointed out in an earlier post, the grade of stainless used is important. -Jeff Since I'm referencing many of Rob's comments below and the information in his post is very good, I left his email intact, despite the length. -- Rob Priddy <robpridd...@hotmail.com> wrote: Jeff, I've been reading alot about exhaust designs and requirements. I'd like to share what i've found if thats alright. Perhaps some others can benefit as well if you already knnow this stuff. But, I might say that the reason for your better performance is in comparison to the old system, the new system has more 'equal-length' exhaust runners. it's not about the valve overlap to take advantage of "crossover" exhaust design here. It's about equal flow from each cylinder and the scavenging of it to improve the breathing out of and into the cylinder. The old system had the front exhaust runners twice as long as the rear cylinder runners before combining into a single pipe of the same size, at somethign like a 90 degree angle. As you pointed out, this was turbulent and restrictive once they combined , and even more so once both were put into the same pipe size fo rthe remainder of hte exhaust path; however, the real effect of this was changing the mixtures in each cylinder because the breathing was not all the same. The golden point of power is to maintain 250 feet/second of exhaust gas velocity at operating RPM. that's where you find the peak of torque. So you never had a chance at a smooth running engine because the velocities were all over the place, and the rear cylinders were probably pressurizing the exhaust runner of the front cylinder as well. Now, you fly with a very much equal length system, the breathing is all very similar and so are the mixtures. Therefore a much smoother running engine with power for the top end. When pipes merge, they need to do so with a collector while running parallel to each other. because its a slow turning engine means nothing to exhaust flow. it needs a clean straight path as much as a race engine does... Now, ideally, for a torque engine like this, the perfect design would be a 4 into 2 into 1 system, with runner lengths and inner tube diameter sizes selected for the RPM range you would like to maximize power at. A 2600 rpm target would work out overall for the O-200, but on a side-note, as slow rpm engines favor longer exhaust pipes, there might not be any room for all the pipes to join into a single pipe out. The goal is to get all the pipes together to help scavenge with its exhaust velocity, but, done so with equal lengths for balanced smooth maximized power. So going to one of my books "four stroke performance tuning" yields some nice formulas to figure this all out. If we go with straight pipes like your using, a 77 inch pipe length for each cylinder with a 1.35 Inner Diameter (or whatever you cna find thats very close) would work golden. anything larger in diameter would move the power band higher outside of the red line range, and anythign shorter would rock the torque band lower before the torque peak, making it perform a bit flatter till the top end. But, back to the book formulas, In short, using the O-200 camshaft continental part # 530788 (and lifters part#530872 to supposedly prevent metal-making with that cam), the results say to have the primaries from each cylinder measure 15 inches total length with an inside diameter of 1.3476 inches, into a secondary of 1.77 inches inside diameter and a length of 62 inches before collecting into a final "split-interference" type of collector and into a single exhaust pipe out the bottom of 1.815 inside diameter and you could probably get away with an 18 inch long pipe even tho the formula says much longer. i don't think it would disrupt the power produced at that point in the system. now the problem.....how to fit all that inside the cowl! again, use the pipe sizes you can find as close to those numbers. theyre just what the calculator has spit out. now im not a fabricator, nor am i a pro, but this is what the books have told me about. hope it helps. -Rob ____________________________________________________________ Visa, MasterCard, AMEX & Discover. 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