Another interesting piece from Hondaman:
The whole situation is actually pretty subtle and a little complicated,
but goes like this:
The original cam profile produced a torque peak at 5100 RPM and the HP
peak at 8200 RPM. The 4.00x18 rear tire with the 2.81 final drive put
the speeds of an upright rider at 132 MPH in top gear at 8500 RPM.
By the K1, the cam lost almost 3 degrees of intake dwell because Honda
was suffering terrific warranty costs from abusive American riders:
their response was to lower the torque peak a bit, and the RPM of that
peak dropped to 4800 RPM, while the HP peak dropped to 7800 RPM. The
larger sprockets, at nearly the same ratio, were introduced to reduce
chain wear and breakage. The result was a top speed of 128 MPH, because
the engine would not reach 8200 RPM in high gear anymore: it could
typically just touch 8000 on a fresh tuneup. These engines lost about 2
HP (65) to the K0's 67.
When the K2 came out, the warranty problems had been reduced to just
broken chains and bad frames (due to their haste at building them, they
sometimes skipped a few steps here and there on some items) for the most
part. The countersprocket was incremented another tooth to reduce chain
wear even further, and the exhaust pipes were changed to the more
restrictive HM341 types. The cam lost about 1 more degree of dwell and
about .010" of intake lift. This moved the torque peak down to 4500 RPM
and the HP peak to 7800 RPM, and it lost almost 5 HP from the K0 model's
engine (about 60 HP). At the time, the vast majority of the 750s were
found on the nation's highways at 55 MPH (the 'new' speed limit), so the
increased low-end torque helped with the highway RPM of 3950 at 60 MPH
with a 4.00x18 tire. Honda didn't miss this trick, as it lowered the
engine speed just enough to quiet the intake roar about 6 dB. This was
very welcome behind the fairings we all had, which reflected the engine
noises right into your face. The noise difference between 60 MPH and
4100 RPM 66 MPH is dramatic.
In the mid-year K3, the HP dropped a lot, due mostly to the rocker
shafts being locked in place and the final drive bearing being increased
to 2 rows wide. This lost almost 3 HP all by itself: the pipes received
even more restrictive baffling for a while. By the K4 the engines were
only producing about 45 HP. This was not enough torque to pull the
typical bike above 100 MPH, and we had lots of complaints about it. I
"fixed" it by gearing the bikes back down with a 17T countersprocket to
get the engine RPM up a little higher toward the HP curve above 4500 RPM
when in top gear. This would let most of them break 100 MPH (to about
110 or 115), but the reduced HP forced the riders to have to lay down on
the tanks to achieve it. (This was how Kawasaki got so many Honda riders
to switch to their new 900-4: it could break 125 sitting up again).
So, the gearing itself does not tell the whole story....
On the Old Factory K1, the ideal setup is the 17/48. It follows the
cam's power profile almost exactly, and can let the engine hit 8200 RPM
in top gear if it is in good shape, reaching 128 or a little more. My
Old Factory K1 had this setup, and the only bikes that could beat me
were the K0 sandcast ones. ;-)
HM
