Mike,
When you polish a port, you clean up the little irregularities along the
walls. This can be done until your port shines like the chrome on the
typical Triu*ph "go fast" part. Porting involves actually reshaping the
pathway to enhance the air flow. Port matching is when someone carefully
matches the openings in various parts (head, manifold, carb) to eliminate
steps that could upset airflow. Of the three, porting is where the big
increases in airflow can come from if done correctly.
In a perfect world, the intake and exhaust ports on an engine would carry
air only and be straight, round tubes with smooth walls sized to provide a
ram effect at the appropriate rpm range. In the real world of MG's, the
ports have changes in cross section where the are siamesed, make a 90
degree bend at the valve, and have the valve/guide/stem in the way of
perfect airflow. The castings are rough, the parts don't fit well, and
there is poorly atomized gasoline to be carried along. Using a die grinder
(or a dremel if you have a few months to kill) to open up the areas of
constriction and smooth the airflow brings you closer to your ideal world.
A typical ameteur mistake, however, is to just go in and start hogging out
the material wherever you can reach it. Bigger is not always better
(watch it, Bob). What you want to do in a street engine is minimize flow
separation by smoothing everything out. The bowl area above the valve is
the place where most engines, MG's included, need the most help. You want
to open up along the "long" wall (the top of the port) as much as possible.
This usually involves working around the valve guide- removing as much as
you feel comfortable without compromising support/heat transfer or
discovering a water passage. On the short wall, you want to make the
radius of the curve leading up to the valve as gradual as possible in order
to reduce flow separation and "tumble" of the mixture as it flows around
the valve.
If you are just looking for ultimate power, then the rules change and you
worry less about finess and more about ultimate volume. This approach
usually destroys engine performance at low rpms because of the
"sticking/breaking off" of fuel along the polished walls as Bob so artfully
crayoned onto his continuous tablet.
Someone else mentioned go carts and other small engines. Single cylinder
engines and two strokes react differently than automotive engines. In a
piston ported two stroke, for instance, the carb just dumps fuel into the
crankase so mixing isn't that important. Polishing makes a lot of sense in
these. Also, if you always port and polish, how do you know if porting
only would have been better?
As far as the other question about whether rich mixtures make engines run
cooler, this post is long enough so I will stick to the simple answer.
Yes, it does. I am sure that one of our aviator friends on the list could
provide more detail if they choose.
Regards,
Bill Eastman
61 MGA and who still has (but refuses to open) his college textbooks on IC
engine theory
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