How many top-ranked V-6 engines wear two single camshafts atop 4-valve heads? Of our 2009 10 Best Engines, only Honda's 3.5L fits this description.
Volkswagen AG's 2.0L I-4 turbodiesel has just one overhead cam. Another, Chrysler Group LLC's 5.7L OHV Hemi V-8, hides a single cam within its block with old-school pushrod architecture. All seven other 2009 award winners — one V-8, two V-6s, two I-6s and two I-4s — sport twin-cam heads.
Why doesn't Honda follow the DOHC crowd, like fellow awardees, General Motors Co.'s 3.6L and Toyota Motor Corp.'s 3.5L V-6s?
“Cost, complexity, weight,” says Craig Brazeau, principle engineer for Honda R&D Americas. “We are able to meet our requirements with the SOHC, and it gives us space for a very flexible valvetrain design. We also wanted this engine to be as compact as possible, and going to double overhead cams would substantially change that.”
Ward's judges didn't seem to miss those two extra cams.
“Crack open the razor-sharp throttle at any speed, and this engine…springs to life like a tiger hiding in the brush, pouncing on helpless prey,” wrote one Ward's editor after evaluating this delightful 271-hp SOHC V-6 in an '09 6-speed manual Honda Accord coupe.
“It's a rocket ship,” said another. One reason, perhaps, why they voted it a 10 Best Engines winner for the second straight year.
Great as it is, Honda's J-Series V-6 engine family has been around a while. Its design and development began in Japan around 1994, Brazeau says.
It debuted as a 3.0L in the '97 Acura CL coupe, then powered the '98 Accord sedan. It expanded to 3.2L for the '99 Acura TL and has proliferated and continually improved ever since.
Today's displacements are 3.5L and 3.7L in a variety of Honda and Acura products.
What was the design team's top priority? “Probably flexibility — an engine that would be adaptable to multiple platforms, both passenger vehicles and light-duty trucks,” Brazeau says.
“Then there's always the struggle between maximum power and maximum efficiency. Durability, of course, and manufacturability are also important attributes. We consider very carefully the factory impact and have good factory involvement (in the design).”
Two more key priorities were mass and refinement. “We're always trying to use more lightweight materials wherever it's cost effective. And for every vehicle we develop, there is extensive NVH (noise, vibration and harshness) testing of the engine, as well as at the vehicle level,” Brazeau says. “One thing I really like about this engine is its sound, especially during hard acceleration.”
Among the toughest design challenges, he says, was achieving the flexibility necessary to meet the requirements in a variety of vehicles.
“Recently, we've been able to adapt some different valvetrains that can fit within the same heads, which is one way to differentiate between some Honda applications and Acura,” he says.
“It was quite a challenge to come up with a valvetrain that could accommodate intake-only VTEC, intake/exhaust VTEC and VCM (Variable Cylinder Management, the auto maker's cylinder deactivation technology). That came with a lot a stress.”
Honda's 3.5L V-6 is the industry's only OHC engine with available efficiency-enhancing cylinder deactivation. Just as VTEC uses an oil-pressure-driven pin to engage either a high valve-timing rocker arm or a lower rocker arm depending on conditions, VCM engages and disengages cylinders by switching between a single (intermediate) rocker arm and none at all.
VCM I, the first version that debuted in 2005, shut down three of the engine's six cylinders under light loads. The second iteration debuted in 2008 and now is available in several models including automatic-transmission Accords. It chooses among 3-, 4- or 6-cyl. modes by disengaging either the front bank of three cylinders or one on each side.
Not surprisingly, another challenge with VCM engines is managing noise and vibration. “Some auxiliary devices had to be added to compensate for that,” Brazeau explains.
“The first one is an ANC (Active Noise Cancellation) system that operates like noise-canceling headphones. It puts out a signal 180 degrees out of phase in both 3- and 4-cyl. modes automatically through the sound system, so the radio doesn't have to be on, to cancel out unwanted low-frequency tones.”
When two or three cylinders are deactivated, the exhaust tone also changes significantly, Brazeau says. “We tune the exhaust mostly for 6-cyl. (operation), since that's the mode you're in most often, then make up for any deficiencies with the ANC.”
Active engine mounts also absorb some of the physical vibration by changing damping based on signals from the engine computer telling it which mode it's in. There also is a special 2-stage spring in the torque converter to damp vibrations and prevent them from being transferred through the driveline.
While Honda tries to commonize components among different members of an engine family, there are a few differences, mostly in valvetrain and controls tuning, between Honda car and truck and Acura brand iterations.
“This engine was developed for a wide range of vehicles with different maximum speeds and requirements,” Brazeau says.
When the conversation turns to the potential for future improvement, especially for increased efficiency, Brazeau concedes the addition of direct gasoline injection is high on the list.
“We're watching CAFE (corporate average fuel economy) very closely and the unknowns are more stressful than the knowns,” he says. “That is an emerging technology, so of course we're investigating it.”
Beyond DI, one thing that can provide “pretty good bang for the buck” in the near term is reducing internal friction and operating losses. “VCM does that, reducing pumping losses in the 3- and 4-cyl. modes,” Brazeau says.
“And we're looking at other ways to reduce both friction and accessory losses. There are still quite a few things we can apply that will allow this engine to remain competitive while meeting future CAFE requirements.”
Another area for improvement is lightweight materials. “We've started developing a lot more magnesium parts for this engine, beginning a couple years ago with a magnesium intake manifold and head covers.
“As the cost of magnesium die casting has come down, we've been able to roll some magnesium parts into these engines to make them more lightweight.”
Will fairly large V-6s such as Honda's 3.5L and 3.7L have to yield to smaller V-6s and even fours in midsize (and up) vehicles as CAFE requirements become more stringent?
“It's hard to change the laws of physics,” Brazeau responds. “With the coming CAFE regulations, as strict as they are, there will have to be some downsizing of engines.”
Some will be turbocharged, he says, adding Honda, not known for forced-induction, has learned a lot from the turbocharged 2.3L I-4 in the Acura RDX cross/utility vehicle.
And what about those lonely single cams on J-series heads?
Would jumping on the twin-cam trolley potentially add some needed efficiency a few years further down the CAFE road? “I can't say,” Brazeau says.
ENGINE SPECS
HONDA MOTOR CO. LTD.
3.5L SOHC V-6
Displacement (cc): 3,471
Block/head material: aluminum/aluminum
Bore × stroke (mm): 89 × 93
Horsepower (SAE net): 271 @ 6,200 rpm
Torque: 251 lb.-ft. (340 Nm) @ 5,000 rpm
Specific output: 77 hp/L
Compression ratio: 10.0:1
Assembly site: Anna, OH
Application tested: Accord Coupe(6-speed manual)
EPA city/highway (mpg): 17/25
