High Tech Pistons for the Masses – Part 4

Wayne Scraba

13 Jun 2019

Click Here to Begin Slideshow Here’s our final segment on high performance pistons from the folks at MAHLE Motorsports. We’ve covered a lot of ground over the past three issues (1, 2, 3). This time around, we’ll finish our look at ring lands and grooves. There’s huge technology here, and MAHLE includes that high-end technology in a wide range of pistons they have on the shelf, ready to go for race and street applications. Check it out: Certain aftermarket high performance and race pistons include a groove that is machined into the land area between the top and second ring. Some manufacturers ball mill the groove – MAHLE Motorsports uses a v-shaped groove. This groove is often referred to as an "accumulator groove." The idea behind it is to reduce pressure buildup between the top two rings. Theory has it that the accumulator groove will stop the top ring from lifting at the bottom of the groove. The groove effectively maximizes ring seal and increases engine vacuum. Two different types of oil drain back holes have commonly been used in pistons: Drilled holes and slotted grooves. The use of grooves is old technology, typically found in cast pistons from way back when. The vast majority of race and high performance pistons are constructed with drilled drain back holes – MAHLE included. The primary reason is strength, since the slotted system makes for a more flexible skirt. MAHLE takes the drain back holes a step further: The round drain back holes machined into their pistons are actually lower in the ring groove than on many other piston designs. In fact, the drain back holes are lower than the lower oil ring scraper. The idea here is to force some of the oil to drain back on the cylinder wall. The clearance between the ring groove and the piston ring is extremely important. Basically, the ring grooves must be smooth – and the smoother and flatter the better. The ring grooves in MAHLE pistons are machined ultra-flat. MAHLE points out: “A flat ring groove is essential in achieving and maintaining the best seal possible. The quality of the ring grooves is so critical to peak and lasting performance, that MAHLE designed and built the machinery used to machine the ring grooves for its MotorSport piston assemblies. Combined with ultra flat MAHLE rings that are included in every PowerPak piston kit, MAHLE delivers the formula for the best sealing power cell on the market.” Having a piston with an ultra flat groove is especially critical for the compression ring. What makes the clearance so crucial? During the compression stroke the ring drops to the bottom of the groove, eventually sealing against the machined surface. During the power stroke, the piston moves down in the bore. The ring then moves up in the groove and eventually seals against the top of the machined surface. As you can well imagine, a poor finish in the groove will not allow the ring to seal tightly. Because of this, the pressure will leak past the rear (or back) portion of the ring. Evidence of this leakage can be found in excessive heat discoloration or carbon buildup in the land area between the top and second rings. One more factor that affects the ring seal is the vertical clearance of the ring. Typically, a race piston never exceeds 0.002-inch vertical clearance (production line street engines often have clearances of between 0.002-0.004-inch). More than that and the ring will leak. Less than that figure and you run the risk of seizing the ring in the groove. A ring that can't turn freely in the groove will not clean carbon out of the groove, and will not be free to expand when combustion pressures enter the groove. If a piston has tight vertical clearances, the ring back clearance figures become even more important. Gas pressure enters the area behind the ring during combustion. This gas pressure forces the ring out, against the cylinder wall. If there is too much back clearance (which in turn creates too much volume for the combustion gasses to fill), it takes too long for the pressure to build and force the ring outward. Naturally, a smaller amount of back clearance will increase the speed and the force at which the ring will exert pressure upon the cylinder wall. Typically, a production line engine piston will have as much as 0.040-0.050-inch of back clearance. In a race or high performance application, the backside clearance figure can be reduced to as little as 0.020-inch or less (depending upon the ring configuration and the piston design). Of course, the back clearance can't be so small that the ring protrudes past the ring land. There's one exception when it comes to vertical clearance, and that's when gas ports are used in the piston. The bottom line with gas ports is simple. They provide a method of supplying combustion pressure directly to the backside of the piston ring. Because of that, the ring vertical clearance can be reduced and at the same time, ring flutter can be reduced significantly. In the end, ring seal goes up and so does horsepower. Unfortunately, gas ports are only suited for use with engine combinations that see frequent teardowns (and that means gas ports don't see use on engines found in street-driven vehicles). Carbon can plug the gas port holes and because of the nature of a pressurized ring, ring and cylinder wall wear is much faster than with a more conventional set up. Conventional gas ports consist of twelve to sixteen 0.040-0.060-inch holes drilled vertically through the piston deck that intersect with the backside of the compression ring groove. Keep in mind that gas porting is most advantageous when a narrow face, lightweight piston ring (such as the 0.043-inch ring) is used. While MAHLE’s competitors rely upon vertical gas ports, MAHLE uses lateral gas ports, 8 to 12 horizontal holes drilled into the top flank of the top ring groove allowing combustion gases to the backside of the ring groove. They can do vertical on custom line but they primarily provide lateral gas ports. In general however, gas ports are an option on only a few of the PowerPak listings. In the end, MAHLE’s super flat ring grooves provides for consummate ring sealing, oil control and ring longevity. Each PowerPak piston kit includes either a low drag 1.0mm, 1.0mm, 2.0mm ring set, a 1.5mm, 1.5mm, 3.0mm ring set, or a 1/16-inch, 1/16-inch, 3/16-inch high performance ring set, dependent upon the application. In terms of applications, MAHLE Motorsports offers PowerPak pistons for a very wide range of engine families - American made, Japanese and European. The best feature, however, is that these high tech piston kits are not made to order (as many are). Instead, all of the applications are on the shelf and ready to go. That means you don’t have to endure downtime waiting for a custom piston to be manufactured. What you’re getting is a readily available piston with many of the critical features found in a custom piston, but without the delay or the cost. They’re perfect for sportsman racing applications or general high performance use. That definitely works for us. For a closer look, check out the accompanying photos:

High Tech Pistons for the Masses – Part 4 — Click Here to Begin Slideshow Here’s our final segment on high performance pistons from the folks at MAHLE Motorsports. We’ve covered a lot of ground over the past three issues (1, 2, 3). This time around, we’ll finish our look at ring lands and grooves. There’s huge technology here, and MAHLE includes that high-end technology in a wide range of pistons they have on the shelf, ready to go for race and street applications. Check it out: Certain aftermarket high performance and race pistons include a groove that is machined into the land area between the top and second ring. Some manufacturers ball mill the groove – MAHLE Motorsports uses a v-shaped groove. This groove is often referred to as an "accumulator groove." The idea behind it is to reduce pressure buildup between the top two rings. Theory has it that the accumulator groove will stop the top ring from lifting at the bottom of the groove. The groove effectively maximizes ring seal and increases engine vacuum. Two different types of oil drain back holes have commonly been used in pistons: Drilled holes and slotted grooves. The use of grooves is old technology, typically found in cast pistons from way back when. The vast majority of race and high performance pistons are constructed with drilled drain back holes – MAHLE included. The primary reason is strength, since the slotted system makes for a more flexible skirt. MAHLE takes the drain back holes a step further: The round drain back holes machined into their pistons are actually lower in the ring groove than on many other piston designs. In fact, the drain back holes are lower than the lower oil ring scraper. The idea here is to force some of the oil to drain back on the cylinder wall. The clearance between the ring groove and the piston ring is extremely important. Basically, the ring grooves must be smooth – and the smoother and flatter the better. The ring grooves in MAHLE pistons are machined ultra-flat. MAHLE points out: “A flat ring groove is essential in achieving and maintaining the best seal possible. The quality of the ring grooves is so critical to peak and lasting performance, that MAHLE designed and built the machinery used to machine the ring grooves for its MotorSport piston assemblies. Combined with ultra flat MAHLE rings that are included in every PowerPak piston kit, MAHLE delivers the formula for the best sealing power cell on the market.” Having a piston with an ultra flat groove is especially critical for the compression ring. What makes the clearance so crucial? During the compression stroke the ring drops to the bottom of the groove, eventually sealing against the machined surface. During the power stroke, the piston moves down in the bore. The ring then moves up in the groove and eventually seals against the top of the machined surface. As you can well imagine, a poor finish in the groove will not allow the ring to seal tightly. Because of this, the pressure will leak past the rear (or back) portion of the ring. Evidence of this leakage can be found in excessive heat discoloration or carbon buildup in the land area between the top and second rings. One more factor that affects the ring seal is the vertical clearance of the ring. Typically, a race piston never exceeds 0.002-inch vertical clearance (production line street engines often have clearances of between 0.002-0.004-inch). More than that and the ring will leak. Less than that figure and you run the risk of seizing the ring in the groove. A ring that can't turn freely in the groove will not clean carbon out of the groove, and will not be free to expand when combustion pressures enter the groove. If a piston has tight vertical clearances, the ring back clearance figures become even more important. Gas pressure enters the area behind the ring during combustion. This gas pressure forces the ring out, against the cylinder wall. If there is too much back clearance (which in turn creates too much volume for the combustion gasses to fill), it takes too long for the pressure to build and force the ring outward. Naturally, a smaller amount of back clearance will increase the speed and the force at which the ring will exert pressure upon the cylinder wall. Typically, a production line engine piston will have as much as 0.040-0.050-inch of back clearance. In a race or high performance application, the backside clearance figure can be reduced to as little as 0.020-inch or less (depending upon the ring configuration and the piston design). Of course, the back clearance can't be so small that the ring protrudes past the ring land. There's one exception when it comes to vertical clearance, and that's when gas ports are used in the piston. The bottom line with gas ports is simple. They provide a method of supplying combustion pressure directly to the backside of the piston ring. Because of that, the ring vertical clearance can be reduced and at the same time, ring flutter can be reduced significantly. In the end, ring seal goes up and so does horsepower. Unfortunately, gas ports are only suited for use with engine combinations that see frequent teardowns (and that means gas ports don't see use on engines found in street-driven vehicles). Carbon can plug the gas port holes and because of the nature of a pressurized ring, ring and cylinder wall wear is much faster than with a more conventional set up. Conventional gas ports consist of twelve to sixteen 0.040-0.060-inch holes drilled vertically through the piston deck that intersect with the backside of the compression ring groove. Keep in mind that gas porting is most advantageous when a narrow face, lightweight piston ring (such as the 0.043-inch ring) is used. While MAHLE’s competitors rely upon vertical gas ports, MAHLE uses lateral gas ports, 8 to 12 horizontal holes drilled into the top flank of the top ring groove allowing combustion gases to the backside of the ring groove. They can do vertical on custom line but they primarily provide lateral gas ports. In general however, gas ports are an option on only a few of the PowerPak listings. In the end, MAHLE’s super flat ring grooves provides for consummate ring sealing, oil control and ring longevity. Each PowerPak piston kit includes either a low drag 1.0mm, 1.0mm, 2.0mm ring set, a 1.5mm, 1.5mm, 3.0mm ring set, or a 1/16-inch, 1/16-inch, 3/16-inch high performance ring set, dependent upon the application. In terms of applications, MAHLE Motorsports offers PowerPak pistons for a very wide range of engine families - American made, Japanese and European. The best feature, however, is that these high tech piston kits are not made to order (as many are). Instead, all of the applications are on the shelf and ready to go. That means you don’t have to endure downtime waiting for a custom piston to be manufactured. What you’re getting is a readily available piston with many of the critical features found in a custom piston, but without the delay or the cost. They’re perfect for sportsman racing applications or general high performance use. That definitely works for us. For a closer look, check out the accompanying photos:

This groove is often referred to as an "accumulator groove." The idea behind it is to reduce pressure buildup between the two top rings. The groove effectively maximizes ring seal and increases engine vacuum.

The round drain back holes machined into MAHLE PowerPak pistons are actually lower in the ring groove than on many other designs. The idea here is to force some of the oil to drain back on the cylinder wall.

The ring grooves in MAHLE pistons are machined ultra-flat. A flat ring groove is essential in achieving and maintaining the best ring seal possible. To get there, MAHLE actually designed and built proprietary ring groove machining equipment.

Pistons with tight vertical ring clearances such as these also have tight back clearance dimensions. Gas pressure enters the area behind the ring during combustion. This gas pressure forces the ring out, against the cylinder wall. A smaller amount of back clearance will increase the speed and the force at which the ring will exert pressure upon the cylinder wall.

Each MAHLE Motorsports PowerPak piston kit includes either a low drag 1.0mm, 1.0mm, 2.0mm ring set, a 1.5-mm, 1.5-mm, 3.0-mm ring set, or a 1/16-inch, 1/16-inch, 3/16-inch high performance ring set, dependent upon the application.

The PowerPak kits also include wrist pins and, as mentioned in our last segment, a complete set of round wire wrist pin retainers.

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