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When it comes to building a performance diesel engine, whether it’s for towing your race trailer to the track or for a dedicated drag race or pulling build, arguably one of the most important aspects is choosing the right piston material and design for your intended use. Just buying whatever piston can handle the highest power output for your engine may not be the most economical, reliable, or best performing choice for your engine.
In an effort to spread the knowledge around to enthusiasts, Trey McFarland of Mahle Motorsports breaks down the advantages and drawbacks of each diesel piston design they offer.
When working with diesel platforms, there isn’t a spark plug to ignite the fuel mixture. As the diesel fuel is injected into the combustion chamber and atomized at a very high pressure, you rely on the piston to produce enough compression as it reaches TDC to start the combustion process reliably – without the piston failing under these extremely harsh conditions.
As those of you familiar with modifying common rail 3/4-ton and one-ton diesel platforms already know, most of these engines are equipped with fairly stout factory pistons; especially with manufacturers like Cummins that borrow parts from their industrial market. These OE cast slugs are capable of handling a decent amount of power and boost, but the designs primary goal is to reduce emissions and increase longevity in the environment created by a factory output engine.
An OE cast aluminum piston failure exacerbated by high pressure (up to 44,000 psi common rail systems) diesel fuel being injected and eroding the surface of the crown, causing structural failure when exposed to extended periods of high heat and pressure.
This limits the pistons ability to withstand extended periods of high heat and pressure; which is generally increased as power levels move upward with the addition of power adders and ECU recalibration, and can quickly lead to piston related failures.
Performance Cast
Mahle’s performance cast PowerPak piston for a 5.9L 12V Cummins (6BT). Notice the proprietary skirt coating designed to reduce drag and increase scuff resistance as the piston rocks as it changes direction at the bottom of the stroke.
Mahle’s performance cast series of PowerPak pistons are machined using the factory casting, which makes it the perfect option for the enthusiast who enjoys participating in drag race or pulling events on the weekend and then driving it to work comfortably on Monday.
While it does use the factory casting, Mahle’s engineers make a few distinct tweaks to the design to improve its strength and longevity in high power builds. In addition to the proprietary Grafal skirt coating, the bowl is redesigned to reduce erosion commonly caused by the high pressure fuel injection on cast aluminum pistons, as well as a compression ratio reduced by as much as 1.5:1 to improve detonation resistance while increasing power when tuning.
This piston is recommended for street machines making up to 800 horsepower.
Performance Forged
Mahle’s performance forged PowerPak Duramax piston; featuring their piston skirt coating, optional valve pockets in the crown, and redesigned bowl.
The performance forged PowerPak series pistons were designed with the needs of purpose-built drag racing and sled pulling applications in mind. Mahle’s engineers replaced the wear friendly top steel ring groove found on the factory and performance cast version with a hard anodized groove for increased durability at higher power ratings.
The performance forged piston series shares some features with its cast sibling, such as the redesigned bowl and piston skirt coating, but quickly leaves it in the dust when it comes to overall strength. This design starts its life being CNC machined from Mahle’s own M142S alloy concoction to better withstand higher heat and pressure, where it then receives a custom crown design with valve pockets (if needed), a custom compression ratio, and a hard anodized top ring groove in place of the steel groove to further increase durability.
Mahle’s forged pistons are definitely a step above the cast design in strength, but this series is best suited for competition engines that don’t see any street miles and have a somewhat frequent teardown schedule.
Extreme Duty Monotherm
The extreme duty monotherm all steel piston is expected to become the industry standard for the future of the diesel market. Since the body is forged steel, this design is able to handle much higher loads and also integrates steel ring inserts for long service life, which has enabled some manufacturers using this design to offer a million mile warranty on their engine.
The steel bowl completely eliminates any erosion related concerns common to its aluminum alloy based siblings when exposed to high pressure diesel injection; and in an effort to further increase durability and maximize its service life, Mahle’s team even integrated a cooling gallery to keep the piston as cool as possible.
Like the performance cast and forged versions, this piston also includes valve pockets as needed, a performance designed bowl, Grafal skirt coating but with phosphate added to further reduce drag and increase scuff resistance. Amazingly, the extreme duty monotherm model is compatible with anything – from a tow vehicle making 500 lb-ft of torque to a dedicated sled puller making 5,000 lb-ft.
