This situation didn’t have to happen. The issue of piston slap is as old as internal combustion, but JE’s latest piston coating, called Perfect Skirt, can remedy it once and for all.
The misunderstanding at work in the above fictional narrative began with piston noise – often called piston slap. This noise occurs mainly with 2618 alloy pistons because of the necessarily wide cold piston-to-wall clearance. These pistons can demand as much as 0.006-inch of clearance between the piston skirt (gauge point) and the cylinder wall to compensate for expansion.
The large piston-to-wall clearance comes from the fact that 2618 alloy pistons grow substantially more than their cast or 4032 alloy cousins. This accounts for the difference in cold clearances. Once the engine is up to temperature and both pistons achieve their normal operating temperatures, both operate with similar running clearances.
The issue is that in the short time it takes to warm the engine, a 2618 race piston can be somewhat noisy. This noise, or slap, is often misinterpreted as a rod knock or other portending evil engine noise. A classic example of this is the cacophony created by those ‘30s era Offy Indycar engines on cold start. It is an unnerving sound to the uninitiated. Using race-bred pistons on the street with muffled engines, makes piston slap especially noticeable.
With that as a backdrop, JE has found a way to have your race-alloy 2618 pistons and use them quietly. The solution is the Perfect Skirt piston coating. Skeptics will scoff at this as just another piston coating, but that would be selling this process far short of its abilities. Piston coatings are nothing new. They’ve been around for decades and JE has offered its optional Tuff Skirt coating for many years, alongside many other coating options.
Piston coatings are purpose-designed to enhance the base piston material in a specific way. Some coatings increase thermal protection, others, lubricity. There are even coatings to aid in the shed of oil film to theoretically decrease windage. The uniform idea behind each is a that the coating offers some innate advantage raw aluminum does not.
The unique thing about Perfect Skirt is that the coating is compressible. Let’s take a specific example. Let’s use a JE 2618 forged alloy piston for a 4.065-inch bore LS3 engine that demands a piston-to-wall clearance of 0.005-inch. If we examine the Perfect Skirt-coated piston closely, you will see a small window area near the bottom of the skirt where the coating is not present. This is where the piston should be measured for its piston-to-cylinder-wall clearance. The Perfect Skirt coating thickness will be a few thousandths of an inch thicker than the piston diameter. Effectively the Perfect Skirt will reduce the cold start operating clearances by 50 percent or more. That takes a 0.005-inch clearance down to 0.0025-inch or less.
The coating will perform a number of functions after it is installed. First, it has the ability to self-clearance. What this means it will permanently compress in certain spots as the engine is initially started and run. The remaining areas of the coating are free to compress and return to shape as the piston heats and cools. This means it has the ability to deform slightly to accommodate piston thermal expansion.
Once the engine cools, the coating expands slightly allowing it to take up some of that additional clearance required by the 2618 alloy material. The evidence of this is noticeable as soon as the engine starts because the expected piston slap is greatly reduced.
The Perfect Skirt coating does not change the 2618 piston’s aluminum alloy linear expansion characteristics. That can only be changed by altering the alloy. What the Perfect Skirt coating does is essentially create a cushion between the piston skirt and the cylinder wall during that period of time between cold start and when the piston temperature normalizes.
Think of the Perfect skirt coating like your favorite cushioned chair and a non-coated piston as a hard steel chair. The Perfect Skirt coating conforms to the cylinder wall much like that upholstered chair conforms to your body as you sit down. The cold steel chair does not. The Perfect Skirt coating is permanently bonded and will remain in place for the life of the piston.
Another noise advantage offered by the Perfect Skirt process benefits engines equipped with detonation sensors. All late model engines rely on knock sensors to protect against engine-damaging detonation. While these work very well, it is often difficult for a knock sensor to differentiate between piston slap and engine detonation. The addition of JE’s Perfect Skirt coating is that it greatly reduces the chances of false knock readings recorded by the sensors. False knock episodes can reduce ignition timing and cause a loss of power.
Technology has produced huge gains in performance and efficiency for the internal combustion engine. While the Perfect Skirt coating isn’t billed as a power enhancer, it does offer the opportunity to use a strong, race-ready 2618 piston on the street while living without the noise.
Τhe article published on the blog.jepistons site.