The last one was so much fun, let's try another one.
There is a site on the internet that deals with increasing motorcycle
performance by creating high velocity ports. You can read a bunch of
articles at
http://www.mototuneusa.com/homework.htm
by a guy who has great success making motorcycle engines perform much
better by making the ports much smaller at the curve. Read the back issues
or articles. Basically he fills up the short side radius with epoxy. The
reduction in port size is startling but he claims that the port velocity is
so much higher that more hp and torque are produced.
Well, I tried it on TR4 heads. I started filling the port at the short side
radius, and with each step of filling it (reducing cross section), I
measured flow on the flow bench. What I found was that I could reduce the
diameter of the TR4 port at the bend (only) from its stock 1-1/2" until it
was down to 1-1/8" before flow started to drop off!!!!!! I have a
cross-sectioned head with modeling clay duplicating what I did and will
send a digital photo to anyone interested.
What conclusions should we draw from this? My own conclusion was that there
isn't much air following the inside or short radius. It's all packed to the
outside (not surprisingly). So flow stays the same if the short side is
built up, but just the opposite happens if the short side radius is
improperly flattened out -- then flow decreases.
Just to close the loop, though, I took my car to the chassis dyno shop and
ran back to back runs with my current head and with the restricted head.
The engine had about the same hp and torque with both heads. I can't
explain this result. Perhaps something else in the system was the
resticting factor. As I've said before, port work is a very humbling
experience.
Going back to the Motoman's article where he actually shows dyno curves, I
see that the biggest gains come between 7000 and 14000 rpm or so, although
the percentage gain looks to be the same. My tests were run through the
range of 3500 to 6200 rpm. I don't have a billet crank so that's where I
run my engine. Maybe the high velocity port approach only works out with
high rpms. I don't know. I got tired of making chips. However, I did see
another approach at my local machine shop today that I'm dying to
try.............
uncle jack
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