|A close up of the forged JWT piston is shown here in place, taken during the up fit up in the sectioned block.|
The stock pistons are cast. Cast pistons have good dimensional stability and can run close piston to wall clearances. This reduces crevice volume and makes for a quieter engine. Cast pistons are also brittle and weaker than forged. These properties make the cast piston more susceptible to ring land damage and breakage of the pin boss.
|JWT pistons feature full skirts because a strut type piston although lighter, may not have enough skirt contact area to avoid scuffing considering the side load of the long stroke, short rod engine. The dome of the piston was coated with Swain Technologies Gold Coat thermo barrier coating and the skirts coated with PC9 anti friction coating.|
JWT’s answer was to develop some strong forged pistons. Forging work hardens and compacts the metal, refining the grain of the metal in an orientation that makes it stronger. Metal like wood has a grain! The forging process also makes the grain finer and removes voids that can cause areas of stress concentration that can be the nucleus of failure points. Forged pistons are tougher and more ductile than cast. JWT’s pistons were designed to lower the compression from nearly 10:1 to 8.5:1 and move the piston pin as high as possible to make room for a longer rod.
|The forged JWT piston has a dish to lower the compression ratio to a more turbo friendly 8.5:1|
The JWT pistons were designed as a prototype for a possible street performance piston, JWT made them from a high silicon forging made of 4032 alloy. 4032 is more dimensionally stable than the tough 2618 low silicon forging alloy which is usually preferable for racing pistons. The 4032 can run a tighter piston to wall clearance but is more brittle although it is much tougher than cast pistons. The pistons were sent to Swain Technologies to be coated with their Gold Coat thermo barrier coating on the dome and PC9 anti friction coating on the skirts.
|Undercrown milling was used to lighten the piston as much as possible to reduce the stress on the rod and crank. The notch in the piston skirt is to clear the piston cooler nozzles that we are going to install later.|
We have always had good luck with Swain coatings. In testing at Nissan Motorsports for Nissan’s IRL program, Swain coatings were found to work the best and be the most durable. Gold coat is a multi layer coating that has heat conductive and barrier layers. The conductive layers helps insure an even temperature distribution across the dome of the pistons while the barrier layers protect the pistons from heat induced damage. The even temperature helps reduce hot spots and chances of detonation as well as reduces the chances of temperature induced damage. Holding heat in the combustion chamber instead of conducting it into the piston also makes for more power.
Swain’s PC9 skirt coating is unique in that it uses tungsten disulfide instead of molybdenum disulfide or Teflon. This is a longer wearing, harder, low friction coating that conducts heat well. Other coatings act like insulators and actually hold heat in pistons while Swain’s skirt coatings are heat conductive so the piston can shed heat to the cylinder walls better. We have found PC9 to be the best wearing of all the skirt coatings we have sampled.
|A pressure equalization groove is used between the compression rings to help them seal better under high piston speeds. Dual pin oiler holes were also used to ensure good lubrication to the piston pin.|
Due to the high side loading of the QR’S poor stroke to rod length ratio, JWT opted to use a full skirt design to spread the load out over a larger area. JWT also specified advanced design features such as a pressure equalization groove between the compression rings, dual piston pin oilers and under crown milling to minimize weight. A thick walled full length steel piston pin was specified to reduce deflection. Thin, racing taper walled tool steel pins often flex in turbo motors and grab the connecting rod small end bushing which causes rod failure. The pin is retained with dual spirolocks.
|Before the final assembly, a scrap QR25DE was cut apart to observe that clearances were correct with a longer rod and a different crank.|
Before machining the block or any sort of assembly was initiated, the prototype parts were test fitted to ensure that there were proper clearances with the inside of the block. A damaged QR25 motor was cut apart to study the internal clearance with the new parts and the fits ruled as acceptable before anything else was done. Follow us in the next installment of our project to see other tricks that Jim Wolf Technology used in the build up of our motor.