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
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)!