Of course the big difference is that for bike guys, 6000 RPM is a fast idle.
In 1972 the Honda 350 Twin STREET bike had a redline of 10,500. My current
favorite sportbike--an MV Agusta Senna--has a redline of 16,500. It
transforms from a rational (okay, semi-rational) 80 horsepower, tractable
scooter to a 142 HP monster starting about 12,000. I figured out quite a
while ago that the lesson I learned from years of tinkering with bike
engines didn't necessarily translate.
But one lesson that YOU learned is that bigger ports aren't necessarily
better. Possibly if you can change the other tuning factors to take
advantage of the increased velocity, you might have a gain.
-----Original Message-----
From: owner-fot@autox.team.net [mailto:owner-fot@autox.team.net] On Behalf
Of Jack W. Drews
Sent: Tuesday, June 14, 2005 6:32 AM
To: fot@autox.team.net
Subject: RE: 225hp
A really interesting thread. But in one respect the question of noticeable
difference in performance sounds similar to the investigation that I did a
couple of years ago on a completely different subject.
i became aware that in current motorcycle racing engines, there were some
tuners who were getting significant increases by making the intake ports
smaller, to promote high velocity. One expert claimed a 5% increase in
torque. I tried this on the flowbench and was astonished to find that I
could make a TR port 40% smaller at the bend and see very little difference
in flow (as measured on the flow bench, remember). So I reasoned that the
guy was probably right and that flow would remain at least as good,
resulting in increased velocity.
I prepared a head this way (building up the short side radius with epoxy)
and took a trip to the chassis dyno, where I tested a head using my normal
prep against this high velocity port head. Guess what? Performance was the
same, within the accuracy limits of chassis dyno testing.
I went back and found dyno charts on the 'high velocity' approach and sure
enough -- there was an increase, starting at about 6000 rpm and continuing
right on up to 14,000 rpm. Hmmm. That's just a little outside my rev range.
I concluded two things from this. One thing was that hp increase claims need
to be looked at pretty carefully. The other was that changes to the short
side of the port probably don't mean anything. Unless, of course, somebody
has found some neat trick there.
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