Driving GM's New Six-speed Allison 1000
The light/medium-duty automatic gets a double-overdrive, soon to be joined by a gutsier Duramax diesel.
Tom Berg
Senior Equipment Editor
Pickup trucks are a favorite part of automotive America, and because they are so useful - and fun - these so-called heavy duty pickups are perennial best sellers.
Sales hold up even during spikes in fuel prices, like those we're experiencing now. It's hard to believe, but while buyers of these pickups happily grab for more powerful engines - particularly diesels - and more capable automatic transmissions to go with them, they also are giving more thought to fuel economy.
As a result, many will be happy to learn there are stronger, yet more efficient powertrains on the immediate horizon for the 2006 model year from General Motors.
GM's Fleet & Commercial arm recently highlighted improvements for the popular Chevrolet Silverado/GMC Sierra 2500 and 3500 series pickups. They were part of an all-model car and truck preview for mobs of dealers and customers and a few trade press writers last May in Salt Lake City.
GM executives embargoed news about a new six-speed Allison 1000 transmission and an upgraded Duramax V-8 diesel until Aug. 1st, which is why you're reading about them in this issue of HDT. The trannies and engines themselves will begin appearing in '06 pickups later this fall and early next year.
Let's first have a look at the new Allison.
The new "Generation 4" 1000 adds a 6th gear at the very top of the ratio ladder, making it a double-overdrive like heavier duty Allisons. Sixth is a 0.61 overdrive, which cuts engine rpm by 350 at 60 mph and aids fuel economy at highway speeds. All other ratios remain the same as in a five-speed Allison, including a 0.75 overdrive 5th and a 1-to-1 direct 4th.
At lower speeds, the six-speed Allison is so smooth in operation that you can hardly tell how many ratios it packs or when it's shifting, unless you listen intently to the engine's revving. At least that's true when a truck's empty, as was the case with those provided at this event. With a hefty load it would feel different.
You'll know which gear you're in if you use a new Range Selection feature that lets you lock in groups of gears for hard pulls or braking on downgrades. There's also a new Tap Shift feature, which allows easy manual controlling of shifting - something many cars do with a "slap-shift" selector lever. Tap Shift uses a sprung rocker switch on the selector lever; you press its upper end to call for an upshift and its lower end to get a downshift. If a gear change can be made without lugging or over-revving the engine, the tranny will comply.
To test this, I left Salt Lake City's flat downtown area, where the event was staged, and headed uphill and around Utah's capitol building in the city's east end. Returning toward downtown, I tapped repeatedly for downshifts, going as far as 2nd gear to control downhill speeds without using the brakes. Sometimes a shift happened quickly but more often it took two or three seconds.
As with the current 1000, if you're in cruise control and Tow/Haul mode, the transmission automatically downshifts on downgrades to try to hold speed, which some other car and truck automatics also do while in CC. For most driving, the tranny does fine on its own, but Tap-shifting and the other features give you more control - and are simply fun to use.
Using those features lights up a digital display on the dash, which tells you which gear you're in. If you choose 2nd gear to avoid spinning the wheels on slick pavement or in loose soil, the transmission's controls work with a Duramax diesel's to limit torque. If the engine is cold, the transmission loads it a bit to hurry warm up. To handle all this, the new Allison 1000 has more capable electronic controls, as well as a stronger torque converter and rotating clutch, and more positive lockup of the torque converter.
As with the previous model, the six-speed Allison 1000 mates to GM's 6.6-liter (403-cubic-inch) Duramax 6600 diesel or the 8.1-liter (495-cubic-inch) Vortec 8100 gasoline V-8, both of which produce hefty amounts of power and torque. The 2500HD pickup I drove here had the Duramax with the current rating, 310 hp and 605 lbs.-ft., which is code LLY on an order sheet. Early '06 models will continue with this. Diesel pickups with manual transmissions and medium-duty C4500 and C5500 models get an LLY Duramax with 300 hp and 570 lbs.-ft., while its rating for G series cargo vans with a 4L85-E Hydra-matic is 250 hp and 460 lbs.-ft.
Starting in October, an upgraded Duramax LBZ with the Allison will have 360 hp and 650 lbs.-ft. It'll be the strongest pickup engine available, at least until competitors react. In what I call the "torque war" being waged by GM, Ford and Dodge, the current leader is Dodge's Cummins Turbodiesel, with 610 lbs.-ft. Late last summer, Dodge announced a 600 lbs.-ft. rating, GM trumped it barely a month later with the Duramax 605, and the Dodge boys quickly retaliated with the 610. So far Ford's Power Stroke diesel has gone "only" to 570 lbs.-ft., but let's see what happens now.
The rise in diesel torque has required ever stronger automatic transmissions, and all three companies have beefed up theirs. GM's use of the burly Allison 1000 starting with the '01 model year required a change. Maybe you've noticed that Chevy and GMC 2500HD and 3500 pickups sit higher than 1500s - two inches higher, according to my tape measure - exposing suspension parts in the wheel wells and requiring a healthy climb into the cab. I've wondered exactly why, but wasn't able to get an explanation until the Salt Lake event.
Mounting the body higher on the frame makes room for the bulky Allison without altering the cab's floor to take a bigger transmission hump - a much more expensive proposition - according to Nick Reynolds, a GM Powertrain engineer. Since then I've noticed that even 2500HDs with the Vortec 6000 gasoline V-8 and the more compact 4L80-E Hydra-matic have the high-boy body.
It's probably more economical to assemble them all this way, and customers get a standard-equipment factory "lift."
