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)!