As diesel owners, we all know our engines are built with heavy, durable components. We know this because heavy-duty parts are required in order to withstand the kind of stresses a diesel is subjected to. One component forced to handle the brunt of that stress is the piston. From extreme cylinder pressure forcing it down during the power stroke to high heat absorption from unused energy (unburnt fuel), diesel pistons definitely have their work cut out for them—particularly in high-horsepower applications.
| piston Of The Future monotherm Piston
Unfortunately, piston selection is one area where light-duty diesel engines lack what heavy-industry diesels have had for years: steel pistons. Aftermarket, forged-aluminum pistons surfaced a few years ago but lacked the durability of OE cast-aluminum units. Fortunately for us, Mahle Motorsports came up with the Monotherm: a forged-steel, extreme-duty piston. And while they come at a price, they can be used for any purpose and in any application, from daily drivers to tow rigs to competition-only vehicles. The Monotherm piston is as much as 100 grams lighter (in certain applications) than comparable cast-aluminum units thanks to the removal of unneeded material. It’s also much stronger thanks to its high-grade, chromoly-steel construction. Designed to withstand extreme, in-cylinder heat and equipped with oil cooling galleys to reduce thermal stress makes them the best of both worlds. This piston can be part of your million-mile puzzle or your 1,000hp competition killer.
| One very important advantage the monotherm piston has over aftermarket, forged-aluminum units is its use of an oil-cooling galley on the bottom of the piston. A two-piece cover plate (arrow) is clipped to the bottom of the piston, which includes an intake hole for oil to enter, circulate around the bottom of the piston, and then return to the crankcase via the exit hole.
Monotherm Benefits
For decades, forged-steel pistons have been found in Class 8 engines due to their ability to handle extreme, prolonged, in-cylinder heat—and last the equivalent of a million miles. In the pickup market, cast-aluminum pistons always got the nod for their lower cost to manufacture. What Mahle has been successful in doing with the Monotherm is paramount: It has kept the weight of the piston equal to, or lower than, the OE cast-aluminum unit it replaces. This was achieved by removing material in areas where it wouldn’t sacrifice the overall strength of the piston. Another advantage of being lighter is less friction, which yields increased efficiency (power and fuel economy).
| Currently, Mahle offers its Monotherm piston in standard, 0.020-inch-over (shown), and 0.040-inch overbore sizes. At this time, Monotherm pistons are available for the 5.9L common-rail Cummins and all Duramax engines.
Other benefits of the monotherm pertain to its use within an iron block. Under elevated in-cylinder temperatures, the growth rate between a steel piston and the cylinder will expand at a much similar rate than an aluminum piston would. This leads to less risk of the piston expanding enough to scrape the cylinder wall, or worse, melting and essentially sticking a piston in the cylinder. Last but not least, a forged-steel piston can withstand a lot of abuse and resist cracking, and we’re told it can even survive piston-to-valve contact. In essence, the Monotherm piston makes every component around it the weak link.
| You may already know how durable the Monotherm pistons are if you’re familiar with diesel drag racing. Take John Robinson’s Power Service funny car for example: His P-pumped common-rail 5.9L Cummins was fitted with a test set several years ago. Not only were they spun to 5,800 rpm and in the engine for more than 100 passes down the dragstrip, but they were exposed to 2,200-degree exhaust gases on every pass. Upon off-season inspection(s), they checked out perfect.
Piston of the Future?
Because forged-steel pistons expand much less than their cast-aluminum counterparts, tighter piston-to-cylinder wall clearances can be utilized. The direct result of this is more efficiency. A more efficient combustion event, less oil dilution, and less blow-by will contribute to both reduced emissions and increased long-term durability. For this reason, we may start to see the steel piston come standard in future light-duty diesel engines. Luckily for us, and thanks to Mahle, they’re already available in the diesel aftermarket.
| A set of Mahle’s lightweight, high-speed monotherm pistons was also used in Audi and Peugeot 24 Hours of Le Mans prototype race cars, including last year’s winning Audi R18 TDI. If any diesel engine needed a durable, lightweight piston, it was the 540hp 3.7L V-6 powering the R18, which spent 75 percent of the 24-hour race at wide-open throttle.
Optimized Duramax Performance
TTS, which specializes in Duramax technology, worked closely with Mahle during development of the Monotherm and believes enthusiasts serious enough to purchase the steel pistons (it carries a slightly different, lighter-weight version of the standard Monotherm) for a performance engine will also invest in aftermarket, lightweight, billet-steel connecting rods. TTS’ billet rods weigh 1,062 grams each (vs. 1,155 grams for the factory LB7 and LLY connecting rods). When pitting its standard bore, factory compression (16.8:1) LBZ/LMM-style Monotherm and billet rod assembly against the OE components, a weight savings of approximately 117 grams per assembly can be realized. This lessening of reciprocating mass is a huge advantage for performance engines spinning at very high rpm.
