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HPBG: Racing Piston Technology

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Lightening Things Up
In case you are just now joining us, lightweight is the name of the game in the racing piston game. One manufacturer has taken it a step further by producing its own software program that can “lighten” a shelf or custom piston for a customer who may be looking for an extra edge. And who isn’t?

Internal lightening, according to this manufacturer, is one of the more effective ways to reduce weight without changing the performance of the piston. In some forgings there may be areas that are thicker than an engine builder’s application may need so the lightening program is able to remove any unnecessary material. This is like getting free horsepower. It lightens the load on the other components in the engine as well.

Excess material along the inside of a piston is cut away to match the contour of the external features. Experts use their knowledge of piston design, and computer modeling software such as FEA and 3D, and after evaluating the needs of each individual customer they decide precisely how much inner material can be removed without decreasing reliability. Each internal profile design is saved to a computer then machined on a CNC mill.

Rings and Things
The accumulator groove is the groove between the first and second compression ring. It does make the piston lighter, but the real purpose is more abstract, explains one manufacturer. Pressure spikes that get trapped between the first and second compression rings tend to unseat to top ring. This action encourages ring flutter and loss of piston ring seal.

The void created by this groove between the rings tends to average the spike pressure of combustion, keeping the pressure low enough to prevent lifting the top ring while maintaining some pre-load on the second (oil scraping) ring.

The top ring end gap is often a significant part of the problem whenever there are piston issues. Most top land damage appears to lift the land into the combustion chamber. The reason is that the top ring ends butt and lock the piston at TDC. Crank rotation pulls the piston down the cylinder while leaving at least part of the ring and top land at TDC.

Actual running end gap will vary depending on the engine heat load. Piston alloy, fuel mixture, spark advance, compression, cooling system capacity, duty cycle and horsepower per cubic inch all combine to determine an engine’s heat load.

Most new generation pistons incorporate the top compression ring high on the piston. The high ring location cools the piston top more effectively, reduces detonation, smog, and increases horsepower. If detonation or other excess heat situations develop, a top ring end gap set toward the tight side will butt, with piston and cylinder damage soon to follow.

High location rings require extra end gap because they stop at a higher temperature portion of the cylinder at TDC and they have less shielding from the heat of combustion. At TDC the ring is usually above the cylinder water jacket. However, many of today’s current designs do a better job of keeping the rings cool, according to one manufacturer.

High rings locations work well in stock and street performance applications, and it helps reduce the crevice area for lower emissions and better fuel economy.  But in really high power applications (blown or turbocharged engines, or nitrous), many piston manufacturers recommend a piston design that has the top ring somewhat lower on the piston to keep the ring away from the heat. They say this improves the durability of the piston and top ring.

If a ring end gap is measured on the high side, you improve detonation tolerance in two ways: One, the engine will run longer under detonation before ring butt. Two, some leak down appears to benefit oil control by clearing the rings from oil loading. A small amount of chamber oil will cause detonation and significant horsepower loss. The correct top ring end gap with some pistons can be 50% to 100% more than the manufacturer’s specs, says an expert.

Ring options of 1/16˝ or stock 5/64˝ are offered in many applications. The 1/16˝ option reduces friction slightly and seals better at high rpm but the drawback is that it’s considerably more expensive. Stock (usually 5/64˝ compression rings) work well and won’t break your customer’s budget. Metric ring options are also becoming more common.

Piston to bore clearances of .0015˝, .0020˝, .0035˝ and .0045˝ were wide-open throttle dyno tested by one manufacturer. After 8 hours of maximum torque and 7 hours at maximum horsepower, the pistons were examined and all looked new, except the tops had normal deposit color. Even with 320° F oil temperature, the inside of the piston remained shiny and completely clean.

Excess clearance has been shown to be safe with many of today’s race pistons. The added skirt stiffness reduces piston rock, even if it is set up loose. Less rock allows you to run a tighter quench.  Some hypereutectic pistons with over .002˝ clearance may make noise.

As they get up to temperature they may still make noise because they have a very low expansion rate. Some manufacturers may use different hypereutectic alloy that expands at different rates than some others. When the skirts stay cool they don’t grow. Running additional piston clearance because friction is reduced can sometimes have a short-term horsepower improvement.

Pin oiling should be done at pin installation. Either pressed or full-floating, pre-lube the piston pinhole with oil or liquid pre-lube – never use grease. (If you are using a pressed pin rod, be sure to discard the spiral pin retainers.) A smooth honed pin bore surface with a reliable oil supply is necessary to control piston expansion.

A dry pin bore will add heat to the piston rather than remove heat. All pistons are designed to run with a hot top surface, cool skirts and pin bores. High temperature at the pin bore will quickly cause a piston to grow to the point of seizure in the cylinder.

Sources:
JE Pistons (www.jepistons.com), KB Pistons (www.uempistons.com), Wiseco Pistons (www.wiseco.com), Diamond Pistons (www.diamondracing.net), Speed Pro (www.federal-mogul.com), Mahle Motor-sports (www.mahlemotorsports.com) and Ross Pistons (www.rosspistons.com). Other suppliers of pistons and piston rings can be found in our exclusive Engine Builders Buyers Guides.