The new engine package is larger than the version it replaces. The body had to be lifted two inches from the frame to accommodate the upgrade. As a result, the new engine and transmission produce more heat than the old version did. Hummer engineers had to shoehorn in the bigger, more efficient engine, transmission, and power-steering and air-conditioning coolers as well as add a fuel cooler and an intercooler for the turbo. (Hummer says the only reason it didn't tune the Duramax to its maximum-rated 310 horsepower and 610 pound-feet of torque is that the vehicle reached its cooling limitations with the lower-rated engine.)

Other changes to the 2006 H1 Alpha (there are no 2005-model H1s, and all 2006 models are Alphas) include a four-inch exhaust, helical-cut gears in the wheels (to minimize rocking-horse feel at stop signs and road growl), better sound insulation to reduce interior noise, and larger main and auxiliary fuel tanks that now have a 51.5-gallon capacity (9.5 gallons more than the previous H1).

H1 Alphas also have a 200-mile-longer cruising range than their predecessors, or about 570 miles between fill-ups, thanks in large part to the added overdrive inside the five-speed. Real-world fuel-economy estimates have the Duramax getting around 11-12 mpg. Not bad for a 7500-pound chunk of steel.

Other benefits to the new Duramax and Allison package include towing capacity uprated to 9500 pounds--nearly 2000 more than its predecessor's--all of which is due to the stronger engine and transmission. Of course, Hummer upgraded the brakes to 12.0-inch discs (10.7-inch on H1) to handle the heavier loads. During several tests and unplanned panic stops, they brought the big H1 Alpha to a surprisingly fast halt.

The new vehicle's price is higher than the previous H1's. While a 2004 model will set you back more than $110,000, the 2006 H1 Alpha will require a nice year-end company bonus to make up the difference, logging in at a $126,000-plus pricetag.