| As
Reher-Morrison Racing Engines has evolved over the
years, I have seen the differences between Pro Stock and
high-horsepower sportsman engines become less
discernible. Once upon a time, there was a common belief
that some parts were just too exotic for sportsman-type
engines. When my friends and I built small-blocks in the
back room of an auto parts store in 1972, we thought
that roller cams and custom pistons were wildly
sophisticated. Now, of course, such components are
commonplace in every eliminator.
In previous columns I've written about the benefits
that Pro-style cylinder heads and dry-sump oil systems
bring to fast bracket and heads-up engines. My topic for
this month's column is gas ports - two words that can
strike fear in the hearts of bracket racers and
sportsman engine builders.
For years we've heard that gas ports are too
effective to use in bracket racing engines. We've been
told that gas ports shorten ring life and wear out
cylinder walls. But just as my teenage aversion to
roller cams and California pistons was based on
prejudice rather than facts, I have reexamined the myths
and misconceptions about gas ports in sportsman engines.
I've come to the conclusion that gas ports are essential
for maximum performance in a high-horsepower engine. |
First a definition for any
non-gearheads who may have inadvertently stumbled onto
this column: a gas port is a hole drilled from the
piston deck (a vertical gas port) or from the top ring
land (a horizontal gas port) to the rear of the top ring
groove. The purpose of a gas port is to apply combustion
pressure directly to the top ring, forcing the ring face
firmly against the cylinder wall. The number, size, and
location of gas ports vary with bore diameter, engine
type, dome design, and the engine builder's personal
preferences. A typical big-block piston, for example,
has between 12 and 16 gas ports that range from .040 to
.060-inch in diameter.
The fact is that all top rings rely on gas pressure
to seal the cylinder on the compression, power, and
exhaust strokes; static ring tension is primarily
responsible for sealing on the intake stroke, when low
pressure exists in the cylinder. In a piston without gas
ports, the ring is pressurized by gases that work their
way to the back of the ring through the clearance
between the ring and its groove. Production pistons
customarily have .002 to .004-inch side clearance to
allow this pressurized gas to reach the cavity behind
the ring. Gas ports pressurize the back of the ring
directly, so the ring-to-groove clearance can be reduced
significantly. Some clearance, of course, is still
required to prevent micro-welding between the ring and
piston; I recommend at least .0012-inch side clearance.
Naturally, such tight clearances require precisely
machined grooves |
