Tech Talk #22 – The Importance of Gas Ports

DavidTechArticlesBy David Reher, Reher-Morrison Racing Engines

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 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. 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.