Wben it comes to lowering the compression ratio, dished pistons are an alternative option versus using big chambered heads. With that being said, there are several options on the dish design. Here are some pics of various dished pistons for the Y. For an all out blown application, I lean heavily toward the reverse hemispherical design.

The idea behind the reverse hemispherical dome is to concentrate the combustion pressure on the center of the piston. This reduces the piston skirt friction that is normally induced when the combustion pressure is non-centered on the piston top with a combustion chamber specific or ‘D’ shaped dish.

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I did use the reverse hemi dish on the naturally aspirated EMC Y-Block engine which made 528 HP on the UNOH dyno while burning 86 octane fuel (MON method). Here at the shop that same engine made 540 HP when tuning for a peak horsepower number and not worrying as much about the low end torque values which were otherwise needed to help the EMC score. The EMC engine had a very long connecting rod (6.750”) which maximized dwell time at TDC and helped to defray any detonation issues that may have been present if using a 6.300” or shorter rod with the lower octane fuel. Because of the rod length and paying particular attention to the dynamic compression ratio numbers, I didn’t worry about maximizing the quench on that engine.

Most custom piston manufacturers can put the desired type of dish in a piston if given enough information. If attempting to machine a dish into an existing piston, it’s important to first check the piston deck thickness to insure that there is enough material remaining after the machining operation in which to support the combustion pressure. On normally aspirated engines, I prefer a minimum of 0.200” of material and on boosted applications a minimum of 0.300” of material depending upon the amount of boost.

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The top ring land is also a consideration in that you want to make sure any machining to the piston top clears that area adequately.

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It’s still a good idea to deck the block even for a supercharged application. Besides insuring that the decks are parallel or flat, decking will help to have all the pistons at the same depth assuming the decks are cut parallel with the main bores or the engine was mocked up with its respective pistons in the bores and cut accordingly. I prefer the installing of the respective pistons and rods in the corner bores and then cutting the block accordingly. This can be performed without having the rings on the pistons and in many cases without actually installing the caps on the rods in which to do this.

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Zero deck or up to 0.010” in the hole with a 0.045” thick gasket will still insure a given amount of quench even on a piston that has a semi-hemispherical reverse dish. Making the dish smaller in diameter and then deeper would also help to increase the quench versus the larger dish shown in the pictures posted earlier. The piston blank being used may dictate to a large degree what you can do with the dish.