No one can deny the outrageous power being produced by the Mitsubishi EVOs and the EVO X is no exception. Their ability to make power relates directly to cylinder pressure and how the compression is managed. Much of that responsibility rests solely on the piston that needs to be incredibly strong to retain its shape and bore seal under extreme heat and pressure, yet needs also to be as light as possible to reduce power loss.
Ross accomplishes this difficult balancing act by starting with a dedicated forging of 2618 T-61 aluminum. The skirts are CNC-turned to size on a precision Takisawa machining center. They are available in +.020-inch bore size or 3.406-inch (86.5mm). Additional CNC-machining operations are used to cut the ring groove that features a 1.0mm top, 1.2mm second and 2.8mm oil ring. The top ring is placed at least .250-inch down from the crown to provide the strength required to prevent “lifting” the top ring under severe heat. There is also a channel between the top and second ring. It is an accumulator groove that helps prevent pressure buildup underneath the top ring for maintaining good seal.
When you flip the Ross EVO X piston upside down, you can see why these slugs are so bulletproof. The pin bosses are massive pillars (almost one-inch square), which tie the entire forging together. They have dual pressure pin oiling holes on each side that are fed directly from the oil ring. Adjacent to the pin boss, the side reliefs are braced by fillets more than .250-inch thick that help maintain skirt stability and critical ring groove geometry. All of this helps spread the load evenly throughout the forging to maintain optimal ring seal under extreme loads.
Further inspection shows the internal lightening option that removes excess material in non-crucial areas of the piston. The operation trims a significant amount of weight without sacrificing strength or durability. Because the forging has such a strong and rigid design, the lightening can be taken to the safe limit without weakening the piston. Another contributor to the weight saving are the H13 tool steel, tapered-wall wristpins. They are light, strong and durable.
Another optional machining operation is gas porting. Gas porting was first developed in drag racing with small vertical holes that fed combustion chamber pressure from the top of the piston directly to the backside of the top ring. These vertical holes work well for some drag race engines that have minimal carbon buildup and generate heat for short bursts. For road race, Time Attack, and especially boosted all-out drag engines, a better solution is radial gas ports that are machined in the top edge of the ring groove and direct combustion chamber pressure to the backside of the top ring.
The final option that Ross offers is various coatings. Included are thermal barriers for the tops of pistons that reject heat and offer protection up 2100° F. Ross recommends an anti-friction coating for the piston skirts that helps eliminate galling and improve cylinder wall life. Another option is an oil shedding coating for the underside of the piston, that helps the piston shed that heavy layer of oil, which is a plus for engines running very high RPM.
As you can see, these highly-developed pistons are engineering marvels that are one of the key components in the EVO X race engine. Ross Racing Pistons has been providing the highest quality products to Sport Compact and specifically DSM and EVO (4G63) for over a decade. The new 4B11 engine is now benefiting from those years of R&D to help EVO X owners put more power down.
Ross Racing Pistons
Mitsubishi EVO X Pistons