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Can Thinner Head Gaskets Improve Outboard Performance?

Thinner cylinder head gaskets can increase compression in an engine by reducing the volume of the combustion chamber. In a 2-stroke outboard engine like the Mercury 2.0, 2.4, and 2.5 Liter V6 Outboard, the head gasket sits between the cylinder head and the engine block, sealing the combustion chamber.


When you replace the stock head gasket with a thinner one, you effectively decrease the space between the cylinder head and the engine block. This reduction in thickness reduces the volume of the combustion chamber when the piston is at Top Dead Center (TDC), which increases the compression ratio.


Increasing the compression ratio may improve engine performance by boosting power output and torque. Nevertheless, it is important to consider that adjusting the compression ratio can impact engine reliability, fuel octane demands, and the risk of detonation (pre-ignition) if not handled correctly.


It is crucial to consult with experienced mechanics or engine tuners before making any modifications to the head gasket or compression ratio of your Mercury V6 outboard. They understand the specific requirements and potential risks associated with such modifications.


But, here are a few examples of available Mercury 2.5 Liter Cylinder Head Gasket by thickness:


  • A gasket thickness of 0.75mm (0.0295" thickness) is suggested only for pistons below the deck height.


  • A gasket thickness of 1.00mm (0.0393" thickness) is suggested for pistons at/under .002" over deck height


  • A gasket thickness of 1.10mm (0.0433" thickness) is suggested for pistons at/under .006" over deck height


  • A gasket thickness of 1.20mm (0.0472" thickness) is suggested for pistons at/under .010" over deck height


Sometimes, in cases where the 2.5 block is damaged and requires deeper decking than usual to achieve a flat and even surface, a thicker 1.5mm gasket may be necessary. This additional clearance is essential to prevent the piston from protruding excessively from the deck and potentially coming into contact with the head.



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