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Notice on cylinder head No. 1, the combustion chamber has been modified and enlarged. We can only guess that this was performed because the previous owner was looking to produce some monster horsepower from their RB26 engine. Hiro says this type of modification is only necessary if you plan to drag race or run high boost. Modifying the combustion chambers to increase the quench zones can increase the chances of knock due to the inability to burn completely and efficiently. This modification also lowers the compression ratio, especially if both intake and exhaust sections of the chamber are cut.