Re: Porting and Relieving

[jlandry@domain.elided (John A. Landry)]

In a brilliant stroke of genius, Jim Shepherd <beltguy@domain.elided> blurted
out:

>Someone asked what relieving was.  I have held off replying to this thread
>for fear of showing my age.  Relieving was a term used for flathead
>engines.  It referred to removing metal from the block between the valve
>and cylinder to allow more air/fuel mixture into the engine.

To expand a little further on Jim's comments, porting usually refers to
cutting metal, smoothing and shaping of the *port* areas.  A popular
approach is to perform port "matching."  This is where the openings between
the intake manifold, exhaust manifold and heads are measured and modified
to increase flow.


If you look at a stock IH head, you'll see sharp edges in the casting under
the valves (in the valve pocket).  This is where the machining tools
stopped as the valve seat or insert pocket was being cut.  Any sharp edges
and ridges act to restrict the smooth flow of intake or exhaust gases.  As
Jim said, relieving refers to smoothing and shaping away these
irregularities.

Lastly, polishing is sometimes done (mostly on the exhaust side) to smooth
away cutting marks from the porting and relieving operation.  The idea is
to reduce friction to the airflow caused the surface of the passageways.

So that's what the "old timers" mean by porting, relieving and polishing.

Regards,

John L.
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