The relationship between connecting rod length, piston compression height and compression ratio is often misunderstood, largely due to the misuse of the term “compression”. In all honesty it probably shouldn’t be applied to piston terminology at all except as it relates to the shape of the piston crown surface. Compression is a volume related term that refers to compression ratio. It bears no relationship the mechanical link created by a specific crankshaft stroke and connecting rod center-to-center length or the pin position that brings the piston crown essentially even with the top of the bore.
Compression Height (compression distance) The dimension from the flat top of the piston crown (not inclusive of dome or dish) to the centerline of the piston pin.
This should always be referred to as the pin height to avoid confusion.
We often say an engine has some specified compression ratio, such as 10:1 compression for example. But it is not an appropriate usage when referring to the mechanical interaction of crank stroke and rod length. Pin height is the preferred term and you can see the relationship in the accompanying illustration. With a fixed stroke length, changing the rod length affects two things, neither of which is the compression ratio. It dictates the required pin height to bring the piston crown flush with the block deck at TDC. It also influences the approach and departure speed of the piston relative to TDC and to some degree the piston’s dwell time at TDC.
If you study the accompanying diagram you will note there are four core dimensions governing the crank, rod, piston relationship. They are:
- Block deck height
- Stroke length
- Rod center to center length
- Pin height
The crank throw, connecting rod and piston must all fit within the block height dimension so that the piston deck comes nearly flush with the deck surface at TDC. Because the crank throw rotates about its own center at the main bearing you can see that only half the stroke length is used when the piston is at TDC. The rest of the distance is taken up by the rod length and the pin height of the piston. So, the reciprocating assembly final dimension is calculated as:
½ stroke length + rod length + pin height
Since block height is fixed within a narrow window available for deck milling, the combination of stroke length, rod length and pin height must add up to the same height with a small tolerance for desired deck height and piston to head clearance which also incorporates gasket thickness. A common practice in performance circles is to zero-deck the block. That means that the combination of one half the stroke length plus rod length and pin height equals the fixed deck height of the block. The flat portion of the piston top is exactly even with the deck surface of the block. This forces the builder to select the appropriate compressed gasket thickness to control piston to head clearance. Not surprisingly, most performance head gaskets are .039– to .042-inch thick when compressed. The commonly accepted minimum piston to head clearance with steel connecting rods is .035 inch.
The stroke length is almost always chosen first as it relates to the bore and stroke combination for the desired displacement. Rod length is usually specified next based on the application. Theory on this is widely debated and often contradicting, but as a rule, shorter rods are usually chosen to gain a more rapid departure from TDC as the piston starts down the bore. This opens a larger cylinder filling space more quickly so that a high-velocity intake system can start filling the cylinder faster. It is often used to enhance throttle response.