Head gasket
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Although static compression ratio should be determined by the piston configuration and the volume of the combustion chamber, small compression ratio adjustments are possible by altering gasket thickness as long as they don't throw you out of spec on the squish dimension (piston crown to under side of cylinder head with the piston at TDC). Generally speaking, this figure is 0.035" to 0.045". There will be some flex in the crankshaft, rods and pistons as they whip around at speed and this clearance will be diminished as a result, with the piston coming in close proximity of the underside of the cylinder head. This will "squish" the otherwise dead mixture out of the area and jet it towards the spark plug, thus fully mixing the mixture, contributing to more complete combustion and the elimination of detonation. The best piston to use for this is one which has a dead flat area where it will meet the cylinder head. Stock Chevy pistons, for instance, have only a thin ring around the perimeter of the piston to accomplish squish. Absolutely flat-top pistons will work best, such as the ones offered by Keith Black. Even the dished pistons offered by them have the flat area opposite the dish. | Although static compression ratio should be determined by the piston configuration and the volume of the combustion chamber, small compression ratio adjustments are possible by altering gasket thickness as long as they don't throw you out of spec on the squish dimension (piston crown to under side of cylinder head with the piston at TDC). Generally speaking, this figure is 0.035" to 0.045". There will be some flex in the crankshaft, rods and pistons as they whip around at speed and this clearance will be diminished as a result, with the piston coming in close proximity of the underside of the cylinder head. This will "squish" the otherwise dead mixture out of the area and jet it towards the spark plug, thus fully mixing the mixture, contributing to more complete combustion and the elimination of detonation. The best piston to use for this is one which has a dead flat area where it will meet the cylinder head. Stock Chevy pistons, for instance, have only a thin ring around the perimeter of the piston to accomplish squish. Absolutely flat-top pistons will work best, such as the ones offered by Keith Black. Even the dished pistons offered by them have the flat area opposite the dish. | ||
| − | This is probably one of the most important areas of engine building and should be considered carefully before ever buying any parts. You must know what the piston deck height (distance from the crown of the piston to the block deck surface with the piston at top dead center) is before continuing. I'll use a 350 Chevy for this explanation. If the piston deck height is, for instance, 0.025", then a gasket with a thickness of 0.015" (Fel-Pro FPP-1094) would put the squish at 0.040". If the piston deck height is, for instance, 0.012", then a gasket with a thickness of 0.028" (GM 10105117) would put the squish at 0.040". Mix and match the piston deck height with the gasket thickness in your particular application to achieve this ideal 0.035" to 0.045" squish. | + | This is probably one of the most important areas of engine building and should be considered carefully before ever buying any parts. You must know what the piston deck height (distance from the crown of the piston to the block deck surface with the piston at top dead center) is before continuing. I'll use a 350 Chevy for this explanation. If the piston deck height is, for instance, 0.025", then a gasket with a thickness of 0.015" (Fel-Pro FPP-1094) would put the squish at 0.040". If the piston deck height is, for instance, 0.012", then a gasket with a thickness of 0.028" (GM 10105117) would put the squish at 0.040". Mix and match the piston deck height with the gasket thickness in your particular application to achieve this ideal 0.035" to 0.045" squish. |
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| + | Another consideration is the type of engine, naturally aspirated, supercharged and so on. It's been said that the COMETIC mls (multi-layered steel)gaskets, although relatively expensive, negate the need to "O" ring the block to accomodate higher boost numbers. I use COMETIC's in my blown 383 SBC with 10 pounds of boost and will be increasing boost to 12 - 15 pounds for this racing season, and have had no problems. | ||
''(need to expand on this section)'' | ''(need to expand on this section)'' | ||
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