Losing your Head

by Daymond Mayhall

My first article is going to be on cylinder heads. There are a lot of issues I want to touch on and I figured this one is as good as an argument as any of them!

Honda has been producing the XR mini since 1973 and things have stayed relatively the same (as far as motors go) since their introduction. There have been developments good and bad, displacement increases, larger head studs, 5 speed trannies, bigger cranks. They alleviated the sling catch off the right side of the crank, which saved weight, but took away the only means of an oil filtering system, and they alleviated the all cast-iron, all reliable camtower (bad deal)....and though they have pointless ignitions now, progress has definitely been at a snail's pace. Horsepower increases have also been minimal. The claimed output of the early 75 was 4.5 h.p., the current 80 is 5.5h.p., and the 100 puts out around 6.5 h.p. I realize these numbers might not be accurate, but they are close enough for me to get my point across. Our common interest lies in tweaking these pups to maximum horsepower. The early 75 motor had a displacement capability of ll6cc and Powroll claimed this motor produced ll.5 h.p. on the dyno. The modern day 100 is capable of l50cc (and slightly larger) and sources are claiming anywhere from 12.5 h.p. to the neighborhood of l5 h.p., unfortunately this number has seemed to be the horsepower limit, and along with the 150cc motor comes crankshaft reliability problems.
In my quest to build the ultimate motor I visited with a tuner in Houston named Clouse. He works with factory Suzuki on their road race program and was hired because of his cylinder head port design. I brought one of my disassembled 100 heads and we sat down and talked about what he could do for me. Clouse immediately measured my intake valve and wrote on a piece of paper, length of intake port + size of intake port + size of carburetor, he stressed the importance of getting the cylinder flowing, but said, based on the size of the intake valve, not to stray from the numbers he had given me. This shattered a few of my preconceived ideas on how things worked, but I walked away from there convinced that the horsepower restrictor on the four stroke mini lies in the size of the cylinder head valves. I've recently talked with Frank at Engines Only and the guy blew me away with what he's doing with his XR motor development. We touched on the valve subject and he said I was "spot on" with the theory. I know this is old news, but I've always wondered what an XR was capable of with a redesigned head.
Stay tuned for my next article....I plan on interviewing Frank and he's going to enlighten a few of us on what he's been doing to make his motors run. (cylinder heads among other things)!