first of all, understand that things start getting really strange when RPM
increases substantially. Honda had a three cylinder 50cc engine way back in
the late 60s that revved to 21,500 RPM. they ran the engine with intentional
detonation because controlled burning was too slow--by the time the
wavefront reached the cylinder wall the engine was too close to BDC for it
to do any good. And this with a bore about the size of a thimble. At racing
motorcycle engine RPMs (9000 to 14000) the flow in the intake works largely
by inertia, there is a substantial amount of inertial packing of the
fuel/air mix at the intake valve. Tract resonance is also extremely
important for packing and keeping the low pressure node away from either the
jet area or the butterfly area of the throttle body. When it opens, the mix
near the intake valve comes in. at peak RPM, stuff that's way out at the
carb or throttle body doesn't get through before the valve closes, it packs
up against the valve for the next cycle. A lot of modern racing bikes have a
sort of plenum in the the intake valve area, partly because with two or
three intake valves it makes geometric sense, and partly because that's how
you pack a bunch of mixture into the combustion chamber. The amazing thing
is that these engines can idle at all (idle being 3500-4000 RPM). Little of
this has any bearing on our engines. Most bike engines--even those from 20
years ago--don't make any horsepower until they hit 7000 RPM or more.
My MV Agusta has injectors that are suspended about three inches from the
mouth of the throttle body. There's a lot of black magic going on in the
intake that has nothing to do with what a flow bench would show.
-----Original Message-----
From: owner-fot@autox.team.net
To: fot@autox.team.net
Sent: 12/9/2003 7:18 PM
Subject: cylinder head flow
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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