Reply to "373 engine build"

Tai,

In order to reap benefits from your closed chamber cylinder heads, the clearance between your piston domes & the flat surface of the cylinder head, at top dead center, MUST be no greater than 0.040". The head gasket is about that thick (0.038"). Therefore the piston dome, at top dead center, should be level with the block, or perhaps even protude a very small amount. It requires machining about 0.025" - 0.030" off the block. This process is referred to as "zero decking" the block. Not only does this process bring the pistons even with the block at TDC, it also "levels" the cylinder head mating surface (AKA the deck), because most often, from the factory, the deck is not parallel to the cranshaft, resulting in variations in compression between the cylinders front to back.

Now here's the big news. A zero decked Cleveland, with a 0.038" head gasket, flat top pistons, 4V closed chamber heads, standard stroke crankshaft, has a compression ratio of about 10.0 to 1. Although advertised as 10.7 to 1, as it came from the factory the Cleveland was actually about 9.5 to 1.

As far as the compression ratio of your motor, you didn't mention if you have a standard stroke crank, or if your block has been zero decked. If so, then you are in the ball park of 10.0 to 1.

When computing compression ratio, all the small volumes in the combustion area must be added into the calculation, not only the volume of the combustion chamber in the head, but also the volume of the hole in the head gasket, the volume of the valve reliefs notched into the pistons, the volume of the area between the cylinder wall & piston above the top ring, plus the volume above the piston at top dead center if the block has not been zero decked.

George