As a piston approaches top dead center on its compression stroke, a spark ignites the fuel-air charge compressed above it. At TDC, the piston has risen as far as it can, and the connecting rod is vertical. Combustion pressure continues rising after TDC, reaching its full-throttle peak at about 11 degrees after top dead center. In the usual motorcycle-sized engine, that 11 degrees ATDC translates to about 3/10 of a millimeter or about 0.012 inch of downward piston travel—not much.
As the crank continues to turn, the big end of the connecting rod is carried forward (in forward-rotating engines), so that the shank of the con-rod is no longer vertical but is inclined. That angularity of the rod, combined with the combustion pressure above the piston, thrusts the piston against the rear—intake side—cylinder wall.
Because the volume above the descending piston is now increasing quite rapidly, pressure in the combustion gas is falling steeply. Yet, as the crankshaft turns, the leverage of the combustion gas driving it continues to increase, reaching maximum when the con-rod and the crank throw—a line through the crankshaft and crankpin centers—are at right angles to each other. This occurs in engines of normal con-rod length at 76 degrees ATDC.
Even though leverage on the crank increases, it is the fall of combustion gas pressure on the piston that is more important, so maximum torque (combustion pressure times leverage) occurs quite early in the power stroke, at around 30 degrees after top center, when the piston has moved just 4mm or almost 0.160 inch.
Probably because it was the French who most quickly adopted the internal-combustion engine, despite its major inventors being German, it was French engineers who looked at connecting-rod angle and the piston side thrust and friction it produced. I suspect one or more of them promptly ran an experiment to offset an engine's cylinder to stand the con-rod up straighter during the power stroke, thereby reducing rod angularity, piston side thrust, and the friction loss it caused. I suspect further that some power was actually saved, for the practice of cylinder offsetting in this way became widespread. The French called this desaxe (say "days axay"), meaning off-axis.
Another way to decrease con-rod angularity during the power stroke is to make the con-rod longer. Hot-rodders speak of “rod ratio,” which is the eye-to-eye length of the con-rod divided by half the stroke. When Japanese manufacturers offered Superbike race kits, con-rods a few percent longer than stock were a commonly included item. This was made possible by deleting the second piston ring and raising the wrist pin as much as possible in the piston, sometimes by enough to require removal of the oil-scraper ring before the wrist pin could be removed.
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Rod ratio is usually in the range of 2.0–2.2, but there can be reasons to use other ratios. When Oldsmobile crammed its truck-sized 455ci V-8 into cars during the supercar era—1965–1970, after which the no-fun EPA insisted on lowered compression ratios—there was little room between the suspension towers; longer rods would have required a taller, wider cylinder block. As a result, the rods were made to a rod ratio of about 1.6.
When John Britten first arrived in Daytona with his homemade 1-liter V-twin, conversation inevitably turned to rod ratio. “I thought to myself, ‘Well, Cosworth put 1.8 rods in its DFV Formula 1 engine,’ ” he said. “That ought to be good enough for me.” But it was common in F1 to change engine shape or dimensions for aerodynamic reasons. Example: to make room on either side of the engine for venturi tunnels to produce downforce.
During the later 20,000-rpm V-10 era in F1, rod ratios had to rise to 2.5 or above to make the piston skirts clear the crankshaft at bottom center.
In the intense era of factory AMA Supersport roadracing beginning in 1988, there was talk of “unscrupulous persons” offsetting separate cylinders to reduce friction, and today big auto manufacturers are using cylinder offset as just another tool in hitting Corporate Average Fuel Economy (CAFE) numbers.
Other things being equal, however, makers of motorcycle engines have tended to end up with rod ratios in the aforementioned 2.0–2.2 range. People talk about mysterious gains to be had from "increased piston dwell" resulting from the use of smaller rod ratios, but the only published dyno data I have seen is from a long-ago issue of Circle Track. Two 350ci V-8s, identical in such variables as piston weight, compression ratio, valve timing, etc. were tested with longer and shorter rods. Although there was one point on partial throttle where the short-rod engine was superior, the test's conclusion was that long rods are best overall.
