Low-tension rings can free up some horsepower but are not recommended for street applicati
When the piston is at BDC and moving up, the rod is being compressed, and when the piston is at TDC and moving down, it is experiencing tension. The most violent force for the rod to endure is from the tension created at TDC during overlap where the piston is not cushioned by compressed air. A piston assembly (piston, rings, pin, and pin clip) that weighs 600 grams, for example, will weigh 11,250 pounds at 6,000 rpm during overlap. The rod must hold on to the piston as it reverses direction without distortion, which can cause breakage or bearing failure. Heavier pistons make it worse.
Using Scat as an example, its Street I-beam rod will withstand a 6,000-rpm pounding with a piston assembly in the 600-gram range. Its Pro Comp I-beam will survive a 600-gram piston at 7,500 rpm or a heavier piston at a lower speed. The next step up would be the H-beam, which can take whatever you can afford to throw at it. Chances are, though, if you are building an engine that requires an H-beam rod, the pistons will be lighter, allowing more rpm and more rod longevity.
Pistons
Much ado is made about forged pistons, which are stronger than cast ones, but unless you're running a power-adder of some sort, your stock pistons will probably survive life inside a street engine. If you're looking for more compression, there are good, inexpensive cast and hypereutectic pistons available in the aftermarket. Go with your machinist's recommendation.
Coatings on pistons can be a good idea for a street engine. Coating on other parts is just
Most factory replacement pistons come with 5/64, 5/64, and 3/16 rings. Aftermarket pistons are made for 1/16, 1/16, and 3/16 rings. They are thinner and offer less friction than the 5/64 rings but are slightly less durable. The piston manufacturer will usually determine the type of ring you should use.
Coatings
As technologies advance, there are a lot of new high-temperature and low-friction coatings available for engine components, the benefits of which are mostly realized in race applications. Johnson says pistons with friction coating in their skirts are useful and can offset some wear in the cylinders over the life of the engine. But coated bearings can probably be passed over by the budget builder. "They're a safety factor, not a power gain," Johnson says. "A bearing is a spacer and should never make contact with the parts it fits between. If you do get contact, the coating will give you a little extra margin of safety before the part fails." The point is, you shouldn't be making contact in the first place, and you have bigger problems than coated bearings will solve.
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JMS Racing Engines
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