JE Pistons On Piston Design Basics - CHP Insider

Sean Crawford, Stephen Golya, And Alan Stevenson Of JE Pistons Break Down The Basics Of Piston Design

Stephen Kim Sep 1, 2009 0 Comment(s)
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Compression Height
When juggling engine design parameters such as crankshaft stroke, rod length, and block deck height, the compression height of the pistons must be taken into account. Defined as the distance from the centerline of the wristpin to the top of the piston crown, compression height that's too short can conceivably compromise durability in the wake of intense cylinder pressure. According to JE, however, advances in modern alloys and forging techniques mean that "thick" pistons aren't always necessary.

"With shorter compression heights used in many of today's performance engines, the piston design should not suffer as long as there is enough room above the wristpin to provide the proper deck thickness and rod clearance. We have off-the-shelf pistons with compression heights of 1.000 inch that have proven to be very reliable," explains Crawford. "Occasionally, we run into a situation were there isn't enough room to meet our requirements. This is most common on forced-induction engines that require a very large dish and a short compression height. In these situations it is necessary to find a way to increase the compression height."

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Power-Adder Pistons
Some people assume that as long as a piston is forged, it can handle anything you throw at it. Although forged slugs are much stronger than their cast counterparts, they still have their limits. Therefore, JE recommends using dedicated nitrous or blower pistons if you plan on using big-time power-adders on your engine combo. "The major difference between a naturally aspirated and a nitrous or blower motor is cylinder pressures and operating temperatures, so it's important to design a piston for this type of application," says Golya. "Higher pressures call for pistons with thicker crowns and more structural stiffening. In addition, the wristpins should be thicker in diameter and the rings will require more tension, thus making them thicker and more durable. We choose to manufacture most of our racing pistons from 2618 alloy, but also offer 4032 alloy pistons that are compatible with moderate forced-induction and nitrous use."

Design Factors
The two most popular piston designs for internal combustion engines are full round or forged side relief (FSR). Each design can offer unique benefits depending on the desired cost, application, power, and engine speed. A full round has a singular central void with a continuous circular band joining the skirts. An FSR has multiple external voids in addition to the central void. Typically a full round piston is the easiest to manufacture, is more affordable, and offers a greater degree of robustness. The FSR piston is designed mostly for specific applications that require a high level of performance and feature more complicated voids. It can be made lighter and stiffer and offers less skirt area than the full round. This is advantageous for inertial response and reduced friction.

Material quality is equally important to piston design. All of JE's material stock is physically and chemically certified and is traceable back to the producer. We verify physical properties and dimensional integrity in our onsite inspection laboratory prior to manufacturing.--Alan Stevenson

Chamber Volume
The trend in many high-end race motors is the use of small combustion chambers and a dish piston to achieve a relatively high static compression ratio. Compared to a more traditional setup that employs larger chambers and either a flat-top or domed piston, this arrangement yields dividends in efficiency, as the piston crown functions as part of the combustion chamber and the spark plug is positioned more centrally in the chamber. "The combination of a smaller chamber and bigger dish is more thermally efficient and reduces the amount of unburned fuel," Crawford explains. "Shorter flame paths generate higher combustion temperatures and increase fuel efficiency, which all goes to increase the energy output and reduce emissions."

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