Polishing within the combustion chamber is legal. This design is limited to a final compression ratio of 12 to 1. 11. Cylinder heads of cast iron construction are allowed any modifications. This combination is limited to a final compression ratio of 9.5 to 1. 12.Intake manifolds must be of cast aluminum alloy construction.
Carburetor is limited to one single four barrel per engine.
A maximum 2.5-inch spacer located between the intake manifold and the carburetor is legal. Exhaust manifolds maybe cast or fabricated water jacketed construction, with no more than 15.5 inches of runner length from the cylinder head exhaust surface to the beginning of the common collector flange. Tailpipes may include the use of crossflow designs. Boats must display the shiftable capability of forward, neutral, and reverse through a transmission on both engines. Multi-speed transmissions arc not legal 17.