What is the difference between the 4032 and 2618 alloys commonly used for forged pistons? 4032 is an aluminum alloy with a high silicon and nickel content. It has excellent wear and abrasion resistance and maintains its dimensional integrity over a longer life cycle. Additionally, it has a lower coefficient of thermal expansion allowing pistons to be installed with smaller piston to cylinder clearance resulting in a tighter combustion seal with less noise. These characteristics make it most suitable in high performance street driven applications.

2618 is an aluminum alloy with moderate copper and magnesium content but no silicon. It is a denser alloy with higher thermal conductivity and expansion rate. While these properties require it to be installed with increased piston clearance, it is beneficial in terms of reducing crown temperatures. 2618 has greater strength and ductility making it more forgiving to extreme conditions and detonation. Tensile strength of 2618 is approximately 64ksi vs. around 52ksi for 4032. However, its wear resistance and ring groove dimensional integrity suffers over the long term making it more stuiable for racing applications where shorter life cycles are anticipated and expected.

Can you talk about the design features of the Cosworth pistons? The Cosworth Subaru 2.5L pistons utilize a bridged slipper piston ("boxed bridge"). This produces a lightweight piston design with excellent strength and crown stiffness. The short stiff piston pin and reinforcement struts of the bosses keeps pin flex to an absolute minimum. The short partial skirt design and its Zylan coating runs in the bore with a minimum of friction and wear. This design has evolved out of Cosworth's Formula 1 engine development programs. Cosworth's 2.0L pistons are not a bridged design but still a slipper piston with a short pin.

Through very careful piston design and the use of an offset piston pin in combination with the use of 4032 alloy, Cosworth is able to provide a piston with a very low noise level even on cold starting. The crown is highly polished which helps to reflect heat and reduce carbon build up. The ring pack used is of excellent quality from a Japanese ring supplier. These features ensure the best possible results in a high performance daily driver.

Does Cosworth use tabbed oil rails? Yes they do. A tabbed oil rail is required on a horizontally opposed engine because it prevents the possibility of oil leakage past the rings on shutdown should the ring gaps align at the bottom of the piston.

What type of piston pin clips are used? Cosworth uses a high tension wire pin retaining clip. They have no tangs and cannot be compressed with pliers. Installation is best done with Cosworth's special installation tool. Removal requires patience and a little practice and can be accomplished with a small flathead screwdriver and a 90 degree pick. However, these clips are extremely reliable even under the most extreme conditions.

Which compression ratio should I choose? Higher compression ratios improve off-boost responsiveness and efficiency. However, raising the compression ratio increases the likelihood of engine knock and therefore fuel selection and tuning becomes very critical. Longer duration cams can allow the use of higher compression ratios because of the delayed intake valve closing which reduces the dynamic compression ratio.

In terms of the Subaru 2.5L engine, we do not recommend the use of the 9.2:1 CR with pump gas. It is best suited to high octane race fuels as well as E85.