John Baxter | January 01, 2012
Mack MaxiCruise MP8

Where it distinguishes itself from its predecessors, though, is where its horsepower peaks. McKenna says Mack dubbed the engine a “hump horsepower engine,” meaning it offers more horsepower when running at cruise speed than at higher rpm levels.

“At 1,400 rpm, you could have 505 horsepower, yet at 1,800 rpm, you might be down to 420,” he says. “This encourages drivers to operate closer to what benefits them. It offers them a lot of power in and around the sweet spot.”

Mack’s engine shares architecture with the Volvo D13, an engine that was beefed up in 2007 with a cylinder block and internal components that were different from the earlier design. The changes were aimed at both Mack and Volvo. Its piston stroke was increased to make it larger, which helped to minimize the mechanical stress that came as EGR levels were increased. The taller cylinders lowered the engine’s sweet spot rpm, which meant an improvement in fuel economy and low speed torque. Modifications also included a more powerful unit injector injection system with the capability to deliver extreme pressure, split injection and, now, rate shaping. This delivers the fuel at a varying rate during each power cycle to help control emissions and optimize fuel economy.

Mack has distinguished the engine from Volvo’s by using different external parts that help serviceability in Mack’s different chassis, and the Mack engines perform quite differently. McKenna says this is because of the MaxiCruise’s torque and power curves.

MaxiCruise MP8

Horsepower: 415, 445, 505

Torque: 1,460 to 1,760 lb.-ft.

Bore and stroke: 5.16 in. by 6.22 in.

Displacement: 13 liters

Lube Oil capacity: 33 quarts

A 2010 change to the engine was its migration from straight EGR to selective catalytic reduction. McKenna says Mack dialed back the EGR rate “substantially,” and increased the engine-out NOx to help keep the diesel particulate filter cleaner.

Mack also offers the MP7, an 11-liter engine with up to 405 hp and 1,560 lb.-ft. of torque, and the MP10, a 16-liter engine with up to 605 hp and 2,060 lb.-ft. of torque.


Paccar’s MX engine has one unusual design feature that enhances low rpm pulling power. It’s called “fractured cap technology” and involves the controlled breaking of a solid connecting rod or main bearing saddle and cap when they are made. The perfectly matched, rough surfaces that result hold rod and main bearing caps in more perfect alignment, allowing the bearings to handle more torque and vibration. This makes it possible to tune the engine to produce a “wide horsepower range and longer torque and power curves for more efficient operation,” says the company.

The engine uses compacted graphite iron (CGI) in both the block and cylinder head. “Using CGI in the block and head reduces weight and provides for a very high strength, reliable and quiet engine,” says Preston Feight, chief engineer for Kenworth Truck Co. The company says this also allows a B10 service life for the engine of 1,000,000 miles, which means 90 percent of engines produced are expected to last that long.

An unusual feature of the design is the single, in-block camshaft, which is driven from the flywheel end of the engine to minimize noise and vibration. The camshaft operates unit pumps to supply the injectors, four valves per cylinder, and the integral engine brake, which is rated at 465 hp. The in-block location allows a low total engine height for easier fit under hoods designed for ideal aerodynamics.

Those unit injector pumps are linked to the injectors through short pipes capable of handling extreme pressure. Electronically controlled valves on both the pumps and the injectors they feed coordinate fuel delivery.


Horsepower: 380-485

Torque: 1,450-1,650 (1,550/1,750 multitorque) lb.-ft.

Bore and stroke: 5.2 in. by 6.5 in.

Displacement: 12.9 liters

Lube Oil capacity: 42 quarts

The MX also features an encapsulated wiring harness mounted directly to the block, said to “protect wiring from the elements and remove stress from connectors,” the company says.

“The oil filter module, which consists of a cartridge-style filter in combination with a centrifugal element, offers a maintenance interval of 40,000 miles. The module is located for easy access during service. When the cover is loosened, oil in the filter module drains back into the oil pan, which reduces the cleanup normally associated with an oil change,” Feight says. strives to maintain an open forum for reader opinions. Click here to read our comment policy.