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Old 01-15-2011, 01:00 PM   #52
RalliSpec
Former Vendor
 
Member#: 39
Join Date: Jun 1999
Location: Hainesport, NJ, USA
Vehicle:
98 Prodrive WRC

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A few comments....

1) Piston clearance is measured at the skirt. So thermal expansion data does not tell the whole story with regards to why you need to run certain piston clearances. You have to take into account thermal conductivity between the crown and the skirt as well as piston design to determine how much expansion will occur at the skirt relative to the thermal load the piston is experiencing.

2) Cosworth's alloy is essentially 4032. Alloy 4032 is more desireable for an engine being built as a daily driver or for an endurance application because of its increased wear resistance and other properties. It will maintain a better ring seal and reduced oil consumption over a longer period of time under normal driving conditions. I would only use 2618 in a racing application where high detonation resistance and thermal conductivity are necessitated but where regular overhauls are to be expected.

3) It is only assumed that USDM STI pistons are hypereutectic although their problems seem to bear this out. Older JDM STI models with forged pistons (Ver.7 and older) used 4032 alloy pistons. There are some newer cast alloys that have been developed with higher tensile strengths and improved ductiliy...its possible that they are being employed on JDM STI models rather than hypereutectic alloys. Who knows. But the change from forged to cast pistons was probably necessary to meet stricter emissions standards.

4) I would be a little wary of using piston dome coatings where the piston was not designed with it in mind. Anything that interferes with the thermal conductivity through the piston is going to alter the expansion characteristics of the piston. Some of the better pistons out there are machined such that they have their best fit when up to temperature. The design process involves analyzing how the piston shape changes as heat is conducted through the piston. The question would be whether the advantages at the crown offset the potential disruption of the expansion process.
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