“A little oil consumption is a good thing because it indicates that you’re right on the edge of the minimum required ring tension.”
I’m told there is no free lunch, but I am certain there is free horsepower to be found in many racing engines. What I define as free horsepower is increasing an engine’s output by reducing parasitic losses such as oil windage and internal friction. Piston rings and oil systems are two interrelated areas where you can often find horsepower without changing the basic engine combination.
How important are frictional losses? Consider that an engine with 85 percent mechanical efficiency loses 15 percent of the power produced in its cylinders to friction. In a 1,000 horsepower engine, that’s 150 horsepower that never reaches the flywheel.
If you can recover even a small percentage of these parasitic losses by minimizing friction and windage, then you will have more net power to accelerate your race car. You don’t need to buy a new camshaft or a set of trick cylinder heads to realize these gains – you simply have to liberate more of the power that the engine already produces by improving its mechanical efficiency.
The major sources of friction in an engine are piston skirts and piston rings. You can’t do much to affect the skirts, but you do have choices when selecting piston rings. When you rotate the crankshaft assembly in a short-block, you can feel just how much drag the piston rings produce. Now imagine the resistance that must be overcome when the crankshaft assembly is spinning at 7,500 rpm in a thick slurry of oil droplets. It’s easy to see where the power goes.
A well designed oil system with a proper pan, scrapers and windage tray can produce a 25 to 30-horsepower increase over a worst-case oil system. But the gain doesn’t stop there. If you can control the oil in the crankcase, then you can significantly reduce ring tension to unlock even more power by minimizing friction. That’s why I say that the oil system and piston rings are interrelated.
Some racers are concerned when they see a puff of blue smoke from an engine. A little oil smoke isn’t anything to worry about (assuming there aren’t any mechanical problems such as worn valve guides or scuffed piston skirts). All racing engines consume oil – Indy cars and NASCAR stock cars can burn several quarts of oil in a race. Drag racing engines don’t operate for extended periods, so we don’t see comparable oil consumption, but it still happens.
In fact, a little oil consumption is a good thing because it indicates that you’re right on the edge of the minimum required ring tension. What’s important is that you control the amount of oil that reaches the cylinders to prevent contaminating the air/fuel mixture to the point that the engine loses power.
You don’t need anything more elaborate than a fish scale to measure piston ring drag. Assemble the rings on a piston, insert the piston and pin into the cylinder upside down, and push the piston to the bottom of the bore. Now note the resistance on the fish scale as you pull the piston smoothly through the bore; it’s the steady pulling resistance that’s important, not the breakaway force. If you repeat this experiment with various oil ring expanders and compression rings you will see considerable differences in ring drag.
Racers tend to think that the oil ring controls the oil – that’s why it’s called the oil ring, right? But in fact, you can put together a ring package with less total friction if you recognize that the second ring plays a significant role in oil control. Think of the second ring as a squeegee: It’s a tapered face ring with the leading edge down, and it’s very effective in pulling oil off the cylinder walls.
At Reher-Morrison Racing Engines, we use that second ring to fine tune the ring package. For example, if a motor needs just a little more oil control, we might install second rings that have been back-cut to a radial thickness of .175-inch instead of rings with .160-inch radial thickness. Often a small increase in second ring thickness (and a resulting increase in static tension) will dry up the engine with only a pound or two of additional drag. To get a comparable gain in oil control by increasing the oil ring tension could add five or more pounds of drag.
Reducing the tension of the top ring and drilling gas ports is a win-win situation. A standard D-wall .043-inch top compression ring has a .210-inch radial thickness; you can certainly run this ring without gas ports because it has more than enough static radial tension to push the ring face firmly against the cylinder wall. But that radial tension adds to the engine’s internal friction because the ring drags against the cylinder wall every time the piston rises and falls. In fact, we really only want the top ring to seal against the cylinder wall primarily on the power stroke; on the other three strokes, it’s just along for the ride. So in this example, it’s an excellent trade-off to use low-tension top rings with a .160-inch or .170 inch radial thickness, and then use gas ports to apply cylinder pressure directly to the back of the rings for sealing only when it’s needed.
Among the misconceptions about gas ports is the mistaken belief that gas ports increase ring wear. That’s just wrong. In fact, gas ports allow you to reduce ring drag while sealing the cylinder more effectively. All piston rings rely on cylinder pressure for sealing; gas ports just apply the pressure more efficiently. We rebuild hundreds of racing engines, and typically see less piston ring wear in engines with gas ports than in engines with conventional rings. That’s because the engines with low-tension rings and gas ports have less drag on the three strokes when absolute sealing isn’t needed. The gas ports also allow the top rings to depressurize quickly; after the exhaust valve opens, cylinder pressure falls dramatically and the pressure behind the ring dissipates.
It’s said the best things in life are free. For a drag racer, there’s nothing better than free horsepower.