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Originally Posted by 06-z0sick
Wondering if i should have the block decked to 0 deck height. Block is a late model truck block ('89) with eagle 383 rotating assembly -18cc dish 64 cc. Cam is a 235/249 .522/.509 107LSA @ .050. According to static compression calculator I'm right around 9.5 running a think head gasket .028. Car is intended for 91 pump gas street warrior. Dynamic compression says 7.4
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An 0.028" gasket with zero deck will yield a squish figure of 0.028", a little tight and asking for trouble. Most fellows will shoot for 0.035" to 0.045" squish on a SBC, so if you're gonna cut the decks to zero, use a gasket that compresses to somewhere between 0.035" and 0.045".
Now, you can cut the block decks to 0.010" piston deck height and use the 0.028" gasket, for a total squish of 0.038" or you can cut the block decks to 0.015" piston deck height and use the 0.028" gasket for a total squish of 0.043". Or make up your own combination of piston deck height and gasket thickness, as long as they total 0.035" to 0.045", you will be doing all you can to eliminate detonation and have the motor operate well on pump gas.
For you fellows and gals who don't understand squish, it is the jetting across the chamber of the fuel/air mixture as it is squeezed from between the piston crown and the underside of the cylinder head as the piston comes to TDC. This "jetting" of the mixture across the chamber eliminates lean or rich spots and generally homogenizes the entire charge to that you get a full burn when the plug fires. Of course, you need a piston with a flat area on the crown in order to mate up with the underside of the head. Many pistons do not have enough flat area on the crown to work well. The best piston to use, in my opinion, is a Keith Black hyper with a D-shaped cup. Take a look at the generous flat area on the left side of the piston crown in this illustration......
http://www.kb-silvolite.com/kb_car/p...etails&P_id=93
Also, pay attention to the piston compression height when you are planning your build. You need to add up the piston compression height, the rod length and the crank radius (half the stroke) when you figure your static compression ratio and squish. You'll note that the compression height on the piston I linked is 1.433". If you add that to the rod length of 5.700" and the crank radius of 1.875", you arrive at a stack height of 9.008". If we measure the stock block deck height at 9.025" (the published deck height of the SBC block), then we could cut the block decks to 9.008" and the head gasket thickness would be the squish. You can count on the decks to be off by as much as 0.020" from the front of the block to the rear of the block or to the opposite cylinder on the other side of the block. Suffice to say that it will not be square unless it has been decked before by another owner.
Cutting the block decks while the block is registered on the main bearing saddle will also square the block up, so that the heads sit on the decks properly (not jacked up on one end because the block was improperly cut at the factory) and the intake manifold will come a lot closer to lining up and sealing properly. You will also be moving in a direction that will equalize the cylinder volumes and giving you the same static compression ratio across all 8 cylinders, assuming of course, that you cc the chambers and equalize them.
If all cylinders are not equalized, then you will find yourself tuning the motor for the worst cylinder, the one that's detonating.
02-13-2012, 10:25 AM
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