PISTON ALLOYS AND HIGH PERFORMANCE

Which alloy is strongest? Answer, it doesn't matter. All piston alloys used by the industry today are strong enough, including cast iron. The real story is in more technical terms like fatigue strength, thermal conductivity, wear resistance, expansion rate, coefficient of friction and specific gravity.

Thermal conductivity is probably the least understood of all terms as it applies to a piston running in an engine. The effective conductivity of a piston (not the alloy) can be altered with coatings, surface area, section design, polish, and top land design. Ideally, the combustion surface of a piston would run at a little over 500�f and not exceed 600�f. The 600�f not-exceed temperature is the most important when it comes to engine life because a 600�f piston top can ignite the fuel mix independent of the spark plug.

Our performance is that we will make higher Hp and better low RPM torque with the best economy and smog numbers. It also suggests that we will have to maximize design efforts to cool the piston to keep the piston top below 600�f. On the opposite end of the spectrum regarding thermal conductivity is our forging alloy 2618. It is the most conductive alloy used by anyone making performance pistons. When the top gets hot the whole piston gets hot and expands accordingly. Noisy when cold and just fine when warmed up. The relatively cold piston top does hurt low RPM power and economy some, but design features can offset some of the shortcomings. Some forgings have been made with a slot at the oil drain back area for the purpose of restricting heat flow to the skirt. The design works and allows forgings to run almost as tight as hypereutectic pistons. Unfortunately, the heat slot weakens the piston below what is required for modern high Hp engines.

The coefficient of friction of all materials is pretty much the same when lubricated as the oil really determines how much slip you have. The unlubricated condition is the important number, especially since it is closely tied to wear and gauling. An engine seeing detonation tends to burn the oil off the cylinder walls. The surface finish on all KB Pistons is designed to put the oil back, but severe detonation can produce a situation with a dry cylinder and a tight piston. The hypereutectic alloys, those with at least 16% silicon (4% free particulate), have a structure somewhat similar to fiberglass. This hypereutectic alloy will slide on the free silicon when oil is not present. This phenomenon is what is responsible for the almost never-gaul never-wear reputation of KB Hypereutectic Pistons.

Specific gravity of piston alloys does affect the weight. Keith Black did make some magnesium pistons that worked. For the most part, though, most lightweight materials tried as piston material fall from thermal conductivity, wear, or fabrication problems. Currently the big savings in weight comes from design changes and the use of real long connecting rods.

Expansion rate varies from aluminum alloy to aluminum alloy with about a 15% total spread. Our forged pistons expand about 13% more than our hypereutectic. Big deal! 15% of 2/1000's of an inch is only .0003". Twice, nothing is still nothing. Why can't we run .002" clearance on performance forgings? The expansion of a piston is controlled by two factors, coefficient of thermal expansion and temperature. The expansion rate is the small player, but the temperature is drastically affected by the thermal conductivity of the piston. All successful forging alloys send combustion chamber heat to the piston skirt quickly, and hot skirts require the extra skirt clearance.

Strength and ductility are often confused terms. Most all pistons are more than strong enough at room temperature, with a slight edge going to the forging alloys. At high temperature the hypereutectic alloy has the edge strength-wise. The problem is if your pistons are 800�f and strong the engine is hypereutectic alloy is a slow conductor of heat. The benefit in in detonation mode and will continue to escalate temperature to destruction. (Direct injection engines may allow higher piston top tempertures.) Ductility is the main area where forging alloys really win. Short of breaking a wrist pin, forgings usually stay attached to the connecting rod even with nuts, bolts, and valve heads sharing the same combustion chamber space. A dropped valve on a forging is more likely to stick in the piston and limit damage to the cyliner head, rod, and piston.

In summary, we make forged 2618 and 18% hypereutectic pistons to hopefully offer the best piston choice to the end user. The hyper pistons have been designed to run forever and are a little more high tech. The forgings are safe. They are excellent when used in development type engines and some very high heat engines. Top fuel and 5 Hp/cubic inch plus engines see a lot of heat, and it is a little easier to cool a forged piston top. Hypereutectic pistons are a little less likely to form cracks than forgings because the alloy structure is somewhat like fiberglass. Cracks occur from flexing. The KB Hyper Pistons are designed not to flex. If the engine builder leaves a rod bolt in the intake, this soon becomes a component flex test. There are no winners ... and piston, cylinder, and cylinder head are usually proven not very flexible.

John Erb
Chief Engineer
KB Performance Pistons

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