Everybody Likes to be Stroked GM's 3.5L Twin Cam basics are best

WARREN, MI - I walk into the headquarters of General Motors Corp.'s Powertrain Group here, and Max Freeman, Powertrain's assistant chief engineer, is in top gear.

He and Allen Cline, from Powertrain product engineering, are ready to talk engines - specifically, GM's all-new 3.5L Twin Cam V-6, often dubbed Shortstar owing to its modular-type derivation from DOHC all-aluminum 4.6L Northstar and 4L Oldsmobile Aurora V-8s (see WAW - May '98, p.155). So for both of these horsepower-heads, talking engines is pretty much like hitting the lottery.

Mr. Freeman says his passion is flying, but that's only half true. He must like automobile engines a lot, too: The 3.5L Twin Cam is the 17th all-new GM engine in which he's had a hand. And Mr. Cline, who coordinates production of the Premium V engine family at GM's Livonia, MI, assembly plant, appreciates anything that moves, from "massaged" jet skis and dirt bikes to snowmobiles.

Okay, I begin, everybody's saying the 3.5L Twin Cam is a Northstar V-8 with two cylinders knocked off, right?

"We wanted to keep the best features of the Northstar. But you can't just cut two cylinders off," chides Mr. Freeman. "There's a split-pin crankshaft for even-firing (the 3.5L Twin Cam shares the Northstar and Aurora's 90-degree cylinder layout) and the bores are restaggered because of the crank."

This is an important point, the new engine sharing the 90-degree Northstar design: 90 degrees is ideal for a V-8, but a 60-degree arrangement is best for a V-6. Yet Mr. Freeman asserts that the 90-degree architecture is crucial to the Premium V philosophy of long-stroke torque production.

"Instead of variable valve timing (or other power/torque-enhancing add-ons), we added stroke to surpass competitive engines in torque and power across the speed range." Given his choice of how to "do" an engine, he says, "I'll package as long a stroke as I can get in there. We think an ideal combination for a passenger car that doesn't run 10,000 rpm is long stroke and four valves per cylinder. If we put a more 'typical' stroke in this engine, it would be 3L. But we'll outperform the others because of the stroke."

I mention that competitive DOHC V-6's like Ford Motor Co.'s 3L Duratec V-6 (200 hp), Nissan Motor Co. Ltd.'s 3L VQ V-6 (190 hp) and Toyota Motor Corp. 's 3L V-6 (194 or 200 hp) have GM's 215-hp 3.5L Twin Cam beat on specific horsepower. But Mr. Cline heads me off, noting that the 3.5L Twin Cam, with 230 ft.-lbs., beats all those engines on torque. And we all know the new saying: People buy horsepower but drive torque.

Moreover, Mr. Freeman points out that the long-stroke, 90-degree 3.5L Twin Cam employs a single balance shaft, rotating opposite the crank at the same speed, to dampen primary imbalance always found in a 90-degree V-6. And that secondary imbalance actually is slightly better than that of a 60-degree V-6.

In the end, Mr. Freeman says the 3.5L Twin Cam's design is nearly an ideal comprise of an engine's conflicting trade-offs: fuel economy, inertia weight of the vehicle and performance goals.

Mr. Cline points out a primary 3.5L Twin Cam attribute, one that satisfies the Premium V team's first goal for customer satisfaction: reliability. Like its 4L and 4.6L stablemates, the 3.5L Twin Cam has a "limp-home" ability to run in the event of a total loss of coolant.

Along with that reliability, Mr. Freeman says the other two prime customer-driven attributes the Premium V designers shoot for are pleasability and value. The 3.5L should deliver on those fronts, too.

The two discuss dozens of other design features and philosophies that space doesn't permit detailing, yet Mr. Cline articulates this engine team's overriding development axiom:

"We want the engine to take care of you."