Thrifty Honda V-6 Marries Two Technologies, Spurns Another

The Ward’s 10 Best Engines competition has recognized outstanding powertrain achievement for 19 years. In this installment of the 2013 Behind the 10 Best Engines series, WardsAuto looks at the development of Honda’s new 3.5L V-6.

A growing number of auto makers are dropping optional V-6s in favor of turbocharged 4-cyl. engines in midsize sedans, but Honda proudly offers its latest 3.5L V-6 in ’13 Accords. And while many are investing in direct fuel injection to achieve an optimal balance of performance and efficiency, Honda stays with simpler, less costly port injection.

WardsAuto editors can’t find much to criticize about these choices. “This engine positively storms. It pulls like a freight train at hard throttle,” one editor gushes. And despite the V-6’s abundant power, they collectively averaged an impressive 29.4 mpg (8 L/100 km) over 590 miles (950 km) of driving – well above its 25-mpg (9.4 L/100 km) combined rating. Editor Christie Schweinsberg saw more than 31 mpg (7.6 L/100 km) on a weekend trip to northern Michigan. In an era when many auto makers are being hammered for not coming close to their official fuel-economy ratings, this is exceptional.

Previous versions of this SOHC all-aluminum 60-degree V-6 were Ward’s 10 Best Engines winners in 2005, 2008 and 2009. This new one tops those in every conceivable way. Tested by WardsAuto editors in a ’13 Accord sedan with a 6-speed automatic transmission, it pumps out 278 eager horses and 252 lb.-ft. (342 Nm) of tire-twisting torque while achieving segment-leading fuel economy of 21/34 mpg (11.2-6.9 L/100 km) city/highway.

The most significant difference between the much-improved ’13 V-6 and its predecessor is the first-ever marriage of Honda’s variable valve timing and lift electronic control (i-VTEC) to variable cylinder management (VCM) cylinder deactivation.

Efficiency-enhancing i-VTEC hydraulically shifts the overhead cams back and forth between two intake valve lobes. The first optimizes valve timing and lift for low-rpm torque; the second is a high-lift, longer-duration profile for increased power above 5,150 rpm.

Honda’s VCM uses the same system to shift between a more midrange profile and a second lobe that gives zero lift to keep that cylinder’s intake valve closed to deactivate it.

While the previous VCM shut down two or even three cylinders to save fuel, depending on driving conditions, this new system transitions between 6- and 3-cyl. modes only (by shutting down the rear bank). This simplification allows the addition of i-VTEC.

“With the VCM engine in the previous Accord, we had 6-cyl., 4-cyl. and 3-cyl. modes,” explains Mike Dunn, principal engineer and manager for the Engine Design Group at Honda’s Ohio R&D Center. “With this VCM, we focus on just 3-cyl. and 6-cyl. operation, then add the i-VTEC for high-end performance.”

Each of the ’13 engine’s six cylinders has one intake lobe for low-pm torque, another for high-rpm power, and the rear-bank camshaft also has zero-lift lobes that deactivate those three cylinders. “We can only have three lobes and three oil circuits to control that,” Dunn explains. “So taking away the 4-cyl. mode allowed us to add the high-valve timing.”

While General Motors and Chrysler deactivate intake pushrods to shut down cylinders in their cam-in-block V-8 engines (and GM’s new CIB V-6) during light-load conditions, Honda claims to be the first to use its (patented) zero-lift cam lobe system to do the same in overhead-cam engines.

Volkswagen introduced a zero-lift OHC cylinder-deactivation system on its 1.4L TSI engine at the 2012 Frankfurt auto show, while other OEMs and suppliers with OHC deactivation are believed to use a switchable bucket or valve-lash adjustment mechanism.

Development of this 2013 “Earth Dreams Technology” V-6 started in Japan in 2011 as the next generation of the previous version. The following year, all V-6 engine responsibilities were transferred to Dunn’s Ohio team. “The V-6 engine is more mainstream in North America, so it's easier for us in the U.S. to follow those developments and understand the market needs,” Dunn says. “And that freed up our Japan counterparts to work on other technologies.”

The updated engine’s top two priorities were fuel efficiency and performance, both of which were helped by a new “tumble-port” cylinder head design with integrated exhaust manifolds and friction reduction measures that include a new method of cylinder honing and coating of the pistons.

Even though direct injection is gaining momentum in mainstream powertrains, it did not make sense for the new Accord, at least not yet.

“Considering the vehicle the engine was going into (port injection) is a very good match for that car,” Dunn says. “DI adds a lot of cost and complexity, and we were able to meet our performance objectives and still get top-of-class fuel economy without it at this time.”

Cylinder deactivation typically brings noise, vibration and harshness challenges, so a new vibration-smoothing mount system and active noise cancelation were developed to allow longer 3-cyl. operation to achieve better fuel economy. “We end up balancing what we feel is acceptable to the customer vs. how long we want to stay in the 3-cyl. mode.”

Dunn says his team’s toughest challenge was getting it all done in time for the new Accord launch.

“The i-VTEC-plus-VCM idea originated within our U.S. team as the engine was being developed in Japan, so that valvetrain system was jointly developed between Japan and our U.S. team,” Dunn says.

Having engineers in two locations was a challenge to begin with, but Dunn says the biggest test for engineers on both sides of the Pacific was Japan’s devastating earthquake in 2011. “This engine’s development began right after the market downturn and continued through the earthquake. Even though the devastation caused by the earthquake was a huge impact to Honda, we did not have to delay the production start-up of this engine.”

Dunn is tight-lipped about the future, but he won’t rule out adding DI to the V-6 sometime down the road.

“We’re always looking at cost vs. benefit,” he says, “and if you look at our other engines, we have other technologies that we could apply to this engine, DI, being one of them.

“Better burn, better EGR (exhaust gas recirculation) tolerance, other friction-reducing technologies, even going to double-overhead cams with CVVT (continuous variable valve timing) like our 4-cyl. already has…those are always possibilities.”

He adds that as customer and government fuel-economy demands ramp up over the next several years, adding DI may make sense. If so, the DI version of the 3.5L V-6 that now powers the new ’14 Acura RLX sedan and MDX cross/utility vehicle likely will find its way into Honda Accords as well.

Electrically driven accessories are another possibility but not likely in the near term. “We have investigated all of those types of applications,” Dunn says, “But as you increase electromechanical devices that adds to the load on the vehicle’s electrical system. I think the VTEC/VCM valvetrain is our winning strategy.”