Many engine builders will limit minimum compression height to 1.00 inch. While there are many custom pistons with the wristpin in the ring pack, there are limitations to the minimum piston compression height. At some point, adding a longer rod will push the wristpin too far into the ring package, which reduces piston stability at higher engine speeds. As the length of the connecting rod increases, the piston's compression height shortens and the wristpin moves closer to the ring package. This allows up to 0.025 inch of room to mill the deck to create a zero deck height between the top of the piston and the deck of the block. For example, a typical 383 piston intended for a 5.7-inch-long connecting rod will have a compression height of 1.425 inches to create that same 9.00 deck height. Of course, this requires a custom piston with a shorter compression height. This led to the better idea of combining the longer 5.70-inch 350 rod with the 3.75-inch stroke crank, which made the rod angularity much gentler on the cylinder wall. Compression height is the measurement from the wristpin centerline to the flat portion of the piston. To define everything, the deck height is the distance from the crank centerline to the head surface of the block. The formula is simple: Half the stroke, plus rod length, plus piston compression height needs to fit within the block deck height. Let's add up the numbers to see how this works. The difference is 0.135 inch, which just happens to be exactly half of the added stroke at 0.270 inch. All stock small-block Chevy connecting rods measure 5.70 inches, except for the 400, which measures 5.565 inches. Instead of moving the piston's wristpin location upward, they shortened the connecting rod. Bumping the stroke required changing a few other details as well. When the time came to design the 400, Chevy engineers had to stuff a 3.75-inch stroke into the 265's same block architecture. The original 265ci small-block used a 3.75-inch bore and a 3.00-inch stroke. In the case of the stroker small-block Chevy, it comes down to squeezing a bigger arm into a standard small-block. Stroke, connecting-rod length, and block deck height are the variables the engine designer must play with when creating a new engine. General rule of thumb for acceptable dynamic compression ratio to run safely on pump gas is 8:1 maximum for engines with cast iron cylinder heads and 8.5:1 with aluminum cylinder heads.Stroke Stuffing All production small-block Chevys share the same deck height of 9.025 inches. However, that same 11:1 static compression ratio engine with the radical 259/269 duration camshaft would have a dynamic compression ratio in the neighborhood of 7.5:1, totally acceptable to run on pump gas. Therefore with the mild cam it will have a high dynamic compression ratio, probably 9.5:1+ which would be way too high to run safely on 91 octane gas. If you put a very mild camshaft (194/204 duration this cam will have an "early" IVC (intake valve closing point), and will "bleed off" less compression than a radical camshaft with 259/269 duration and a considerably "later" IVC. To give an example, lets say you're considering only pump gas for your engine, and it has a static compression ratio of 11:1. Unlike Static Compression Ratio, Dynamic Compression Ratio takes into account camshaft timing by considering the intake valve closing point in relation to the piston position. Static Compression Ratio numbers are the ones you hear thrown around the most ("10:1 compression"), and it takes into consideration the full sweep volume of the cylinder in regard to its range of crankshaft stroke. This calculator will calculate both Static and Dynamic compression ratioĬompression Ratio is the ratio of an engine's cylinder volume vs.
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