While accommodating the longer H22 rod, we also wanted to bump the compression ratio by one point and deepen the pistons’ valve reliefs. The deeper valve reliefs were needed to provide additional clearance for the increased lift and duration of the SR16VE N1 camshafts and also to leave room for optimizing the camshaft timing on the dyno. The end result was a true 11:1 compression ratio piston that weighed in at a mere 271 grams by utilizing an innovative design twist.
The typical custom piston utilizes skirts that are symmetrical about the wrist pin axis; however, for our SR20VE build JE Pistons decided to utilize their new series of asymmetrical forgings. During the firing case (or power stroke), it is common for one piston skirt to experience more side load than its counterpart. This skirt is often referred to as the major thrust side. In the case of the SR20VE, the major thrust side experiences approximately 6,700 newtons of force while the opposite skirt only experiences around 1,900 newtons of force. JE Pistons’ asymmetrical skirt design takes advantage of this disparity in loads and optimizes each skirt accordingly. The end result is a set of piston skirts that are staggered in size or “asymmetrical” in design.
|This picture demonstrates how the major thrust surface has more load.
Two key benefits are unlocked by optimizing the area of a piston skirt according to side loads. The first benefit is a reduction in weight. Since one skirt experiences less load, that skirt panel can be smaller in area and its supporting struts can be thinner in cross section. This results in significant reduction in overall piston weight. The next advantage in decreasing the area of the less loaded skirt (or minor thrust side), is achieving a reduction in friction that occurs between the cylinder wall and the piston skirt. This ultimately translates into more power.
|The calculated difference in side load in an SR20 with the standard length rod vs the H22 rod. Less side load equals less friction and less wear.
|The shaded areas are the only net surfaces left on the forging. The white surfaces are machined areas that were machined for proper bore clearance or where FEA has determined that extra material can be removed without weakening the piston. JE’s sophisticated 3D milling can remove material in complex geometries. It also shows the great amount of machine time spent on a JE piston to optimize it. Few other piston companies sweat the details like this.