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.

Head Games

HeadProductionVolume
318 2V castings1968-8464cc
340/360/3184V1968-9168 cc
318 #302 swirl port1985-9159 cc
Magnum1992-up60 cc
MP Magnum R/T 60 cc
MP Aluminum Magnum53 cc
MP W-270cc
MP W2 closed chamber55 cc
MP W-5-race59 cc
MP W-5-street60 cc
MP W-962 cc
Edelbrock 607763 cc
Edelbrock 601765 cc
Indy 360-163 cc
Indy 360-263 cc
Brodix B1BA65 cc

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.