Chrysler's all-aluminum 3.6L DOHC 60-degree Pentastar V-6 earned a Ward’s 10 Best Engines award for its performance, refinement and versatility in the ’11 Jeep Grand Cherokee and Dodge Avenger. It won another trophy in the ’12 Chrysler 300S.
This year, it bagged a third straight 10-Best Engines award in the ’13 Ram pickups for the same reasons. Can an engine that excels in a luxury car really be that good in a pickup truck and vice versa?
Yes it can.
“The supremely smooth Pentastar has impressed us in muscle cars, SUVs, cross/utility vehicles, minivans and luxury sedans, and now it turns in another stellar performance in the fullsize Ram pickup,” WardsAuto editors say.
“Despite (the Ram's) 5,073-lb. (2,301-kg) curb weight, the Pentastar summons a boatload of low-end torque and can tow 6,500 lbs. (2,948 kg), all the while sounding burly and confident and delivering best-in-class fuel economy.”
The engine is so versatile that it can shine in Chrysler, Dodge, Jeep and Ram vehicles with minimal changes for transverse front-wheel drive to longitudinal rear-, all- and 4-wheel drive, and it replaces seven older V-6s to power about a third of current Chrysler products. There are minor external differences among them: The inlet manifold is on one side for north-south installations, the opposite side for east-west, and the alternator switches sides in some cases. But internal parts are identical. Its architecture also is flexible enough to accommodate displacements from 3.0L to 4.0L, including a new 3.2L version debuting in the ’14 Jeep Cherokee.
Mated with a new 8-speed automatic transmission in ’13 Ram trucks, it delivers a best-in-class 305 hp, 42% more than the 3.7L V-6 it replaces, and its 269 lb.-ft. (365 Nm) torque peak arrives earlier for strong low-end response. It can burn E85 or regular unleaded gas and offers stop/start operation for better city fuel economy in an HFE model.
Non-HFE Rams are rated at a best-in-class 18/25 mpg (13-9.4 L/100 km) city/highway with RWD and 16/23 (14.6-10.2 L/100 km) with 4WD. During testing, WardsAuto editors averaged 18 mpg in 325 miles (523 km) of mostly suburban commuting in a well-equipped Quad Cab test Ram.
A new thermal-management system elevates engine and transmission oil temperatures more quickly to reduce parasitic losses and improve efficiency, and a variable oil pump delivers 2.5 bar (36 psi) of gallery oil pressure below 3,000 rpm in low mode, then shifts to high mode to generate more than 4 bar (58 psi) at higher engine speeds.
Yet it makes do (at least for now) with port fuel injection instead of more complex and costly direct injection. “DI is an enabler for turbocharging,” says Pentastar chief engineer Steve Gorgas. “For a naturally aspirated engine, the benefit is rather small, so we are very competitive without it.”
Among the Pentastar’s other key technologies are piston cooling jets, dual variable-valve timing and integrated exhaust manifolds fully contained within the aluminum cylinder heads, with close-coupled catalysts bolted directly to them. “The integral exhaust manifolds really shine in high-energy-demand vehicles like pickup trucks where you can realize their full potential over vehicles with lighter energy demand,” Gorgas says.
Because they live within the cylinder heads' cooling water jackets, the exhaust gases exit to the catalysts at lower temperatures. That eases thermal stresses on the catalysts and other exhaust-system components, especially at higher engine speeds and loads, reducing the need to enrich the fuel mixture to control exhaust temperatures.
“It allows you to delay fuel enrichment,” Gorgas says. “That lowers emissions and provides real-world efficiency improvement, especially for customers who use their trucks for towing and hauling. At a given speed and load, if you're pulling any kind of load at all, it reduces catalyst temperatures 15% to 20%.”
Another benefit is that heat generated through the integral exhaust manifolds is used by the thermal-management system to more quickly heat the engine and transmission lubricants to their most efficient operating temperatures for additional fuel-economy gains.
The only significant differences between the Ram truck Pentastar and most other versions are its intake manifold, oil pan and front cover; all driven by packaging needs. “The Pentastar goes into 13 different vehicle lines,” Gorgas says. “We're able to get by with only two different intake manifolds, one for Ram truck and Wrangler, the other (with a different throttle body location) for all other applications.”
Some changes accommodate the HFE model's stop/start system, including a new crankshaft sensor, a hardened-tooth ring gear and minor modifications to the idler shaft's surface finish and idler sprocket bushing lubrication path, all to improve durability. It also has polymer-coated bearings.
“If you're inching down the freeway in rush-hour traffic or sitting at a railroad crossing for a long time, those restarts are tough on your cam and main bearings because you don't have sufficient oil pressure at engine start. The polymer coating provides a dynamic film at the bearing surface that is bulletproof in those stop/start scenarios,” Gorgas says.
Will the cost/benefit ratio of DI begin to look better as corporate average fuel economy requirements accelerate in future years?
“We've been studying that for quite some time,” he says, “and we still haven't seen the fuel economy benefit. Also, there are new particulate emissions requirements on the horizon that will be challenging for DI engines.”
For the ’17 model year, the Environmental Protection Agency is proposing a Tier-3 rule to reduce by 70% the particulate matter limit for light-duty vehicles weighing up to 8,500 lb. (3,856 kg) GVW. The EPA also wants to add a first-ever PM standard on the high-speed/high-load US06 emissions test cycle for vehicles less than 6,000 lbs. (2,722 kg) GVW and for vehicles from 6,001 to 8,500 lbs. GVW.
Because DI tends to produce more soot than port-injection does, Gorgas says DI engines will be challenged to meet these new requirements. “I know that several manufacturers are looking at adding port fuel injection to their DI engines for that reason,” he adds. EPA plans to finalize Tier-3 emissions and low-sulfur gasoline rules by the end of this year.
Gorgas breaks down potential future improvements into three potential areas: frictional, thermodynamic and pumping. “Some people are running Atkinson Cycle-like engines with very late intake-valve closing along with variable-valve lift and/or MultiAir to reduce pumping losses,” he says.
“The thermal-management system can use heat generated by the engine to help in other areas of the vehicle to get things up to their optimum temperatures as quickly as possible. It's a process of adding all these different devices and looking at it from a full-vehicle systems approach, not just an engine approach.”
