Zero deck the block or not?
04-29-2008, 11:39 AM
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Zero deck the block or not? I am in the midst of building a 355. My question to y'all is should I have my block zero decked? I have a set af AFR195 Eliminators going on as soon as the block is done. This is going to be a street, very seldom track car when done.
I just got a set of GM Impala head gaskets. I do not want to have to use race fuel so I don't want the CR too high.
Thanks
Mark
I just got a set of GM Impala head gaskets. I do not want to have to use race fuel so I don't want the CR too high.
Thanks
Mark
04-29-2008, 12:38 PM
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Deck Height or Deck Clearance:
This most important step consists of measuring the distance from the assembled piston tops to the surface of the block deck (deck clearance), and milling as necessary. The general feeling is that the total quench or squish distance should be about .040". The quench distance is the compressed thickness of the head gasket plus the deck clearance. The quench area is the flat part of the piston that would contact a similar part of the head if you had .000 assembled quench height. In a running engine, the .040 quench height decreases to a close collision between the piston and the cylinder head. The shock wave from the near collision drives air at high velocity through the combustion chamber. This movement tends to cool hot spots, averages the chamber temperature, reduces detonation and increases power. The shock wave also provides better fuel/air mixing, and this allows the fuel to ignite better and burn faster. A faster burning fuel charge means less timing is required for optimum power output. An example of this--a running 462 had a factory deck height of about .020, the deck was set to 0. There were no other significant changes to the engine (new rings and bearings, but same cam, heads, intake and exhaust systems). The optimum timing setting prior to the change was 34 degrees - that provided the fastest MPH and quickest ET. After the change to 0 deck, the optimum timing using the same Amoco gas changed to only 30 degrees total mechanical. Not only did the lowering the deck raise the CR by several tenths of a point, but by retarding the timing 4 degrees, it was possible to increase the CR even higher due to the optimum lower timing setting.
Note: Since it is the close spacing between the piston and cylinder that reduces the prospect of detonation, never add a shim/head gasket, or flat cut the pistons tops to reduce CR. If you have proper quench with 10 to 1 CR, and then reduce the CR to 9.5 by one of these two methods, you will create more ping with the 9.5 CR then you had with the 10 CR. By all means, deck the block first and under all circumstances when building an engine for optimum power output, and then determine what chamber volume will be needed in the heads to arrive at the final CR.
This most important step consists of measuring the distance from the assembled piston tops to the surface of the block deck (deck clearance), and milling as necessary. The general feeling is that the total quench or squish distance should be about .040". The quench distance is the compressed thickness of the head gasket plus the deck clearance. The quench area is the flat part of the piston that would contact a similar part of the head if you had .000 assembled quench height. In a running engine, the .040 quench height decreases to a close collision between the piston and the cylinder head. The shock wave from the near collision drives air at high velocity through the combustion chamber. This movement tends to cool hot spots, averages the chamber temperature, reduces detonation and increases power. The shock wave also provides better fuel/air mixing, and this allows the fuel to ignite better and burn faster. A faster burning fuel charge means less timing is required for optimum power output. An example of this--a running 462 had a factory deck height of about .020, the deck was set to 0. There were no other significant changes to the engine (new rings and bearings, but same cam, heads, intake and exhaust systems). The optimum timing setting prior to the change was 34 degrees - that provided the fastest MPH and quickest ET. After the change to 0 deck, the optimum timing using the same Amoco gas changed to only 30 degrees total mechanical. Not only did the lowering the deck raise the CR by several tenths of a point, but by retarding the timing 4 degrees, it was possible to increase the CR even higher due to the optimum lower timing setting.
Note: Since it is the close spacing between the piston and cylinder that reduces the prospect of detonation, never add a shim/head gasket, or flat cut the pistons tops to reduce CR. If you have proper quench with 10 to 1 CR, and then reduce the CR to 9.5 by one of these two methods, you will create more ping with the 9.5 CR then you had with the 10 CR. By all means, deck the block first and under all circumstances when building an engine for optimum power output, and then determine what chamber volume will be needed in the heads to arrive at the final CR.
