On Sun, 14 Dec 2008 10:33:49 -0500 (Eastern Standard Time) "Dan Heath" <da...@alltel.net> writes: > I noticed on Jeff's web site that the Cont cannot take advantage of having a > crossover. They claim that the VW can, I wonder why? Also, I have noticed > that the Vairs don't use crossover. Is this just for convenience, or does > the same apply?
On a 4 cylinder opposed engine like a VW or Continental, plumbing a pipe down each side is about the worst (and easiest to build) exhaust design you can make. That's mainly because of the firing order. You'll find that the two cylinders on one side fire in order, then the two cylinders on the other side fire in order. That leaves one cylinder opening it's exhaust valve right into the exhaust from the other cylinder on that side. Not a very good way for the engine to breath. A crossover exhaust pairs the front two cylinders together, and the back two cylinders together. This really is a good exhaust as you will have one cylinder firing into the crossover pipe once every revolution, but can also be difficult to build. On a Lycoming engine, this is a good exhaust as you'll only have a cylinder firing into that half of the exhaust once per revolution so you'll get some scavenging effect. If you can fit in tuned lengths into a collector like a powerflow exhaust, that's even better. On the smaller Continental engines, there just isn't enough valve overlap to get much of a scavenge effect from using a crossover exhaust. The cam is just too mild. Consequently, 4 tuned length pipes will breath as well as a crossover exhaust or headers into a collector, and is significantly easier to build. I think you'll find that most engine builders use a much more aggressive cam with significant valve overlap in the VW aircraft engines, so using either a crossover type exhaust or a 4 into 1 tuned header would work very well. A little reading at http://www.aircraftexhaust.net/crossovers.html will explain their opinion on this. There is lots of good reading about various exhaust types for aircraft at http://www.aircraftexhaust.net. And more good reading about how a tuned exhaust works at http://www.powerflowsystems.com/exhaust_process.php. On a 6 cylinder engine like the Corvairs, a pair of tuned length 3 into 1 headers are the ideal. However, guys that have done a lot of this tell me that the 6 cylinder engines don't seem to suffer as badly from using a simple log down either side as the 4 cylinder engines do. The only proof I could offer to support that would be the fact that Powerflow doesn't sell any tuned exhaust systems for the 6 cylinder Continental or Lycoming engines, but are continuously adding more STCs for less numerous aircraft using 4 cylinder Lycoming engines. -Jeff Scott Los Alamos, NM ____________________________________________________________ Click here to find the right business program for you and take your career to the next level. http://thirdpartyoffers.juno.com/TGL2141/fc/PnY6rw1UFtVVki9D87qItRZKxTQwy7qACaX27CMKS1DwxBf3J2qII/
