2.2E Pistons - Hypereutectic ok?

[mattC2993]

I soda blasted and measured up my old stock 2.2E pistons and 5 are on size and one is 0.1mm undersize. Strange. The cylinders are scrap so I am back to looking a new P&Cs. $4k for Mahles is a no go. Can anybody offer an opinion on the AA hypereutectic high compression (w/Biral Cylinders) for $900? Its just a stock rebuild. Thanks.
84mm Porsche 911 Hi Comp, Piston & Cylinder Kit | AA Performance Products

[chris_seven]

Quite a difficult question to answer.

The Eutectic Composition of Silicon in binary alloy with Aluminium is 12.2%. (You can still find reference to this value being 11.7% as the initial measurements made in 1926 were later proven to be incorrect).

This means that 'hyperecutectic' pistons will have greater levels of silicon than either eutectic or hypoeutectic pistons.

Alloys with between 16% and 19% Silicon are commonly used for the manufacture of cast pistons and as they tend to expand less than the alloys with lower levels of Silicon they are routinely used in production road cars.

They tend to need lower cold clearances which is a benefit for normal day top engines run in a variety of conditions.

They do have weaknesses.

Hypereutectic Alloys tend to be more brittle than alloys with lower levels of Silicon and are more likely to suffer damage if they experience detonation than pistons with lower silicon content. They are also more likely to suffer from fatigue failures in hard used engines.

Hypereutectic Alloys are also more difficult to forge than alloys with lower levels of silicon.

In general I would think that a 4032-T6 alloy which has 11% Silicon and can be readily forged is a very good choice and very similar to the composition of the original Mahle 911S piston material.

For a road going car in a moderate state of tune such as a 911T I can imagine these parts would be OK but I wouldn't use them in a 911S motor.

I would almost certainly buy the cylinders but would use Woessner Forged Pistons which are made from 4032-T6. They also have coated skirts and offer good value for money.

JEs are typically made from 2618-T6 which has very little silicon expands more and is more susceptible to ring groove wear than the alloys with higher levels of Silicon. Clearly the JE is a good piston but we have used Woessner for 7 years now and have had great success.

For our 'competition' engines we use Omega Pistons and again specify 4032=T6.

[mattC2993]

Hi. Yes I am familiar with hypereutectic. I am specifically interested in the use of the AA pistons in the short stroke 2.2 application. For another $500 I can get the JE forged pistons but I'm not sure I need them. I've heard of Woessner but not sure where they are sold or if they would offer any savings. I might try and find one good replacement piston and use my stock set with the AA Biral cylinders. Thanks.
ps Woessner doesn't have a listing for the 2.2E piston w/9.1 compression in their catalogue