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Hier habe ich ein paar Sätze dazu gefunden: www.spdusa.com/turbos.htm

The early turbo motors have a closed deck block. Up to 1995 they were build for the World Rally Champioship and were expected to see 400+hp fro just two liters. Pure racing engines. The open deck N/A motors have open spaces between the cylinders and the edge of the block that comprises the mating surface between the block and the cylinder head. The early turbo motors have a flat-machined mating surface with the cylinders and several water and oil passage holes visible on a deck of solid aluminum. The early turbo block is much stronger and more stable for this reason. It is designed to withstand the thermal stresses of turbocharging and stay together atmuch higher specific outputs than the N/A motors. The coolant circulation in the upper block and head region is particularly improved.

In 1996 the WRX turbo motors were revised to use an open deck configuration. There were two reasons for this change. The rally car rules were changed to restrict the intake to 32mm, limiting horsepower to about 300 at 6000 rpm. Second, there were several advances in coatings and thermak control of the upper cylinder area, giving much better stability of the gasket surface. The cost of sand casting thousands of closed deck blocks is much more than their die cast open deck cousins the change was made.


MfG
2-bias