To elaborate a little bit on Mark Jones: The plenum style can take longer to fabricate due to setting up the foam molds, where if you reference Dan Heaths site for how he set big pieces in place and then cut card board for the shape of the flexible pieces, makes short work of the time needed. In most cases you will need guides and hold downs like Mark Langford used, not just the glass. Also, the isolated head temps can be eliminated with a cross over that is at least 3 inches in diameter, according to testing performed by Ed James of James Aircraft, of Fort Myers, FL. He sells plenums for certified engines which eliminate the need for cowl baffling, use the round port inlets, and are made for high temp. If you call first and give him a heads up, he will make you a finished plenum from a basic one that you glass up. He also does the cowlings for RVs and their spinners that Vans sells.
The most important thing is that both sides of the engine share the air coming in, especially in climbing, so that the engine cooling is effectively from both sides, even though in a climb there is a bias to the down stroke blade side. Although Mark L has not reported any bad problems with isolated engine sides, WW recommends against it, and especially for new engines with no history, I would keep the air circulating to both sides, baffled or plenum style. According to Ed James, you can use add a cardboard tube as the mold for making the cross over tubing to what you see on Mark Ls site and do just fine, as long as the diameter is at least 3 inches. Otherwise, not enough airflow will flow to the other side for balance, and you are wasting your time. A lot depends on what you are best at making. The plenum turns the cowling into just an aerodynamic cover, while the baffled method makes it have two jobs, your choice. Good luck Colin Rainey Ormond Beach, FL N96TA
