A Plexiglas plate is sealed...
A Plexiglas plate is sealed to the chamber with some grease, and a burette is used to fill it with fluid from a hole positioned to chamber's edge. Just fill it up, and measure how much fluid volume it takes to fill. These chambers measured 65cc's.
Let's look at 360 engine combos, probably the most popular small-block for performance applications. With a stock-replacement dished piston, the compression ratio works out to be 7.9:1 with a stock chamber size of 68 cc and a standard .039-inch-thick gasket. Now, chasing compression by milling the heads .060 inch and using a thin head gasket will get 8.7:1. That's better, but it's far off the mark for serious performance-and be prepared for the typical problems that arise from heavy milling. Again, choosing a more performance-minded piston, such as Speed-Pro's 116 CP flat-top-which can be set up to zero deck when building the engine-provides an easy 10.2:1 ratio with a stock 68cc chamber head, and a .039-inch-thick head gasket.
|318 2V castings||1968-84||64cc|
|318 #302 swirl port||1985-91||59 cc|
|MP Magnum R/T|| ||60 cc|
|MP Aluminum Magnum||53 cc|
|MP W2 closed chamber||55 cc|
|MP W-5-race||59 cc|
|MP W-5-street||60 cc|
|MP W-9||62 cc|
|Edelbrock 6077||63 cc|
|Edelbrock 6017||65 cc|
|Indy 360-1||63 cc|
|Indy 360-2||63 cc|
|Brodix B1BA||65 cc|
Getting the volume on domed...
Getting the volume on domed pistons that have been radically altered, either by profiling or fly cutting, will require a direct measurement. To do that, place the piston down in the bore a precisely measured amount (usually .500 inch), and add grease to the ring land area to seal the piston to the bore. Now measure the total cc's it takes to fill the cylinder to the deck with a burette. Compare the measured volume (cc's) to the mathematical volume of the cylinder at .500-inch depth. The difference is the dome volume or clearance. In this case, a 4.350-inch bore at .500 inch down has a volume computed mathematically of 121.7 cc. With the piston deck set at .500 inch down, we measured 114 cc of fluid to fill it. The missing 7.7 cc is the net dome volume.
Quench And Compression
Pre-Magnum small-block heads were a fully open-chamber design, with a recessed relief of over .100 inch cast into what is normally the quench side of the chamber. Modern performance and production heads are typically a quench design, in which the head's flat deck extends over a substantial portion of the cylinder bore. As the piston approaches TDC, the space between the quench portion of the head and piston rapidly closes up to the designed minimum clearance. As the piston is approaching TDC, the combustible mix in this portion of the chamber is rapidly displaced, creating combustion-promoting turbulence.
Secondly, a cooling effect is imparted on the remaining gasses in the closed quench area, since the hot gasses are in close proximity to the cooler surfaces of the chamber. This cooling of the end gasses-which are normally the last gasses reached by the flame front and the most likely to detonate-is properly referred to as the quench effect. The quench effect materially enhances detonation tolerance.
Quench heads are sensitive to the clearance between the head and piston at TDC. As quench clearance is lessened, the velocity of the gasses pushed out of the cylinder goes up. Cut the quench clearance in half, and the mixture is expelled at four times the velocity. Though we've heard some guys brag about running the quench so tight the pistons kiss the chambers (this will occur somewhere between .030- and .020-inch clearance), .040 inch is considered a safe and effective quench clearance. Open the quench gap over .060 inch, and most of the benefit is gone, and at some critical point the detonation tolerance will be substantially reduced.
Beyond enhancing the detonation limits, a tight quench clearance is credited with measurably enhancing torque production. Building an engine with an effective quench comes down to selecting a piston-and-head combination with the required clearance. Since about .040-inch clearance works so well, a flat-top piston at zero deck combined with a closed chamber head and a typical .039-inch head gasket is the easiest and most straightforward route to building effective quench into an engine.