The basic rule when choosing a rod for a stroker motor is to go for the longest one practical. This 5.4-inch D.S.S. billet rod is paired with a D.S.S. piston. It's light, cost-effective, and a good choice for most stroker 5.0 applications.
Piston Side Loading
The single biggest factor affecting side loading is the rod length to stroke ratio (rod/stoke ratio). The shorter the rod is in relation to the stroke, the greater the angularity it goes through. The negative consequence of this is that the piston experiences greater side loading as it approaches its maximum angularity. The bad news is that when rod/stroke ratios approach the 1.6 mark, you really need to do something about it. The good news is that some of the power-producing moves we need to do help offset the negative issues here. A prime one is that as the compression ratio is raised, the negative effect of a short rod becomes less in terms of overall percentage of power lost to additional side load friction. In a nutshell, this means your 10.5:1 (or more) fairs better with the shorter rod than does an 8.5:1 motor. With higher compression ratios, it's possible to successfully utilize rod/stroke ratios down to the 1.55 mark. However, those lower rod/stroke ratios are not so good in supercharged engines, as the frictional losses escalate with lower compression ratios and big boost numbers.
Allowing that it's not that practical to have pin heights (the distance from the center of the wristpin to the deck of the piston that comes flush with the top of the block at TDC) less than about 1 inch puts a limit on the stroke that will fit into a given block. For instance, if we assume a 1.55:1 minimum rod/stroke ratio and a 1-inch piston height, the maximum stroke that will fit (assuming no other clearance problems) in a 5.0 block having an 8.2-inch deck height is a few thousandths over 3.5 inches. For typical street motors, most of the top engine builders attempt to keep the rod/stroke ratio's lower limit to about 1.58:1. With a small margin, this allows for a stroke of 3.4 inches and gives a workable 1.1-inch pin height with a 5.4-inch rod. It's this combination that delivers the popular 347 inches in a 0.030-inch-over 5.0 block.
This Mahle piston comes with all the bells and whistles you'd expect of a company that makes pistons for 20,000-rpm Formula 1 engines.
Minimizing Friction
We've looked at one half of the equation in terms of minimizing the potential to lose power through the use of a long stroke and the consequence of a short rod. Now let's consider the other half-friction. If we could eliminate friction between the skirt and the cylinder wall, then a short rod actually begins to have an advantage over a longer rod.; but we can't, so the next best thing is to minimize it. Fortunately, piston manufacturers are always looking for ways to cut skirt friction. Millions of dollars have been spent on researching the best skirt shape to cut friction, and recently we've seen off-the-shelf pistons with low-friction coatings applied to the skirts. There's not a lot you can do to a piston's skirt, but you can get it coated. In addition, you can buy pistons with the lightest and thinnest rings your budget will allow. Another factor is to have the machine shop size the bore to give the biggest clearance recommended by the piston manufacturer.
Also, watch the weight at the pin end of the rod. Anything to lessen this, such as a lighter-than-normal piston and wristpin, is a help. When it comes to lubrication, run the best oils out there at the lowest viscosity your engine builder may recommend.
As the crank rotates and receives forces from the firing pulses, it advances and retards from its true nondeflected position. This is called a torsional vibration and must be adequately damped as it will lead to an early crank breakage. Additionally, such torsionals also upset the cam dynamics leading to spurious valve bounce. | 
This is a piston that Ross built for one of your author's Dart-blocked, 382-inch stroker motors. Note the narrow ring grooves. The top ring is only 0.043 inch wide and intended for a Total Seal gapless ring. The piston is also super light. | 
Shown below are the measured principle torsional vibrations of a 3-1/2-inch stroke crank. Curve No. 1 is what we see with a simple aluminum hub with no damping. Curve No. 2 is about the worst-case scenario for a new stock damper. In the good case, the peak could be down 30 percent on this. Curve No.3 is what we see with a well-sorted damper, although one specifically tuned to the system can do even better than this. |