| and perfectly flat rings.
Tight side clearance helps to stabilize the ring in
its groove, preventing ring flutter and reducing blowby.
Recently an A-B dyno test with a high-horsepower
big-block dramatically illustrated the benefits of gas
ports. Although we customarily use gas ports in our
565ci Super Series engines, we built an engine without
them at the customer's request. When we dyno tested the
engine, it was off more than 50 horsepower from similar
engines we'd built. The engine didn't hold pan vacuum
over 7,000 rpm, and it wouldn't pull to peak rpm. We
pulled out the pistons, drilled gas ports, and
reassembled the engine. We didn't change the rings and
didn't hone the bores, so the only difference was the
gas ports. When we put the engine back on the dyno, the
power was right where we expected it to be, the
crankcase had good vacuum, and the engine pulled cleanly
to its redline. That back-to-back test convinced me that
gas ports really work - in fact, I was quite surprised
by just how much difference they made!
Why do gas ports make power? Gas ports bleed off
pressure behind the rings quickly, so there is less
friction to overcome. They also allow an engine builder
to use thinner rings with less radial tension, which
frees up additional power. The only time you want a
compression ring to be forced tightly against the
cylinder wall is on the first third of the combustion
stroke when cylinder pressure is highest. For the
remainder of the ring's travels up and down its
cylinder, you would like to have it loaded as lightly as
possible while still maintaining an adequate cylinder
seal. Do gas ports wear out rings prematurely? I don't
see any discernible difference in ring wear with and
without gas ports. After all, it's cylinder pressure
that forces the ring face against the wall; gas ports
are just a more efficient way to apply this
pressure.
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It would be foolish to build
a racing engine with an inferior method of sealing the
cylinders. It may surprise you to learn that gas ports
are now used routinely in NASCAR Winston Cup engines. If
the rings will last for 500 miles in a Cup engine with
gas ports, do you think they'll wear out in a big-block
that makes 250 quarter-mile passes between overhauls?
Another knock on gas ports is that they will
become clogged or "coked up" in a bracket
racing engine. That may have been true in the past, but
with the sophisticated oil control techniques, pan
vacuum systems, dry sumps, and lightweight synthetic
oils that are now used on high-horsepower engines, the
combustion chambers and piston tops stay clean and dry.
If the engine is properly tuned and you keep oil out of
the chambers, gas ports will function perfectly for
hundreds of runs. Some racers mistakenly believe that
some leakage past the top ring is desirable because it
pressurizes the second ring. The fact is that the second
ring is not a compression ring - its purpose is strictly
to control oil. Any pressurized gas that leaks past the
top ring accumulates in the volume between the rings
where it unseats the top ring and diminishes its sealing
ability. That is why many Pro-style pistons now have an
accumulator groove machined between the top and second
ring grooves. The theory is that leakage past the top
ring will have a larger volume to fill before it
disturbs the ring seal. So does every sportsman engine
need gas ports? Absolutely not. If you're running weekly
bracket races with off-the-shelf pistons, gas ports
aren't necessary. But as the competition to make Quick
16 and Top Sportsman shows becomes more intense, racers
are turning to bigger, more powerful motors. When you
cross the threshold of 1,000 horsepower, gas ports
become essential for maximum performance. Ring seal is
everything in a racing engine. You can have killer
cylinder heads, an ultimate induction system, and a
top-secret camshaft, but if you don't seal the
cylinders, you're giving away power.
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