02-24-2012, 09:00 AM
Join Date: Nov 2004
Twist and Clout: Inside the Heavy-Duty-Truck Torque War
In the shadows of the current horsepower war, Ford, GM, and Ram heavy-duty pickups have been battling over the torque-championship belt for the better part of a decade. Why? Because to tow you need, among other traits, loads of twist, and in the truck world, tow ratings count for a lot.
To get big torque figures, all three rely on diesels with iron blocks, Frisbee-sized turbos, and tried-and-true pushrods operating four valves at each cylinder. But each beats the 750-pound-foot mark very differently.
Ram, with its Cummins inline-six, is the only brand that outsources its diesels. And it has a good partner in Cummins, which has been working on compression ignition for nearly 100 years. The fundamental design has not varied much since the 1980s, but the Cummins got a bit larger in 2007 when the inline-six grew from 5.9 liters to 6.7 (the same size as the current Ford diesel). The engine’s turbocharger, also from Cummins, features a sliding nozzle that varies intake-manifold pressure by controlling exhaust flow to the turbine. This reduces lag at low rpm while still delivering more boost at higher rpm, much as a two-turbo setup would.
During deceleration, the sliding nozzle can also increase engine braking, relieving the truck’s friction brakes of a little heat when towing or hauling a big load.
The Ram remains the only heavy-duty truck offering a manual, but, with it, torque drops from 800 pound-feet to 650.
In the mid-90s, Ford entered into a joint engine program with Navistar, and the Power Stroke series was born. For the 2011 model year, Ford and Navistar severed their ties, and Ford introduced an in-house 6.7-liter V-8.
Still wearing Power Stroke branding, this new brute is nicknamed “Scorpion” because its single turbo sits on top of the engine like a curled stinging tail. The engine is a so-called reverse-flow V-8 with the exhaust runners located in the valley between the cylinder heads. This “Hot V” design reduces lag by minimizing exhaust-plumbing complexity. Honeywell supplies a two-stage turbocharger (one turbine and two compressor wheels on a single shaft) that reduces NOx emissions while still giving the punch of a sequential setup, further reducing what would otherwise be complex plumbing.
For 2012, Ford and Ram are in a dead heat for the torque title at a whopping 800 pound-feet. Twenty bucks says one or both have torque-improving upgrades for 2013.
GM’s diesel is also the product of a joint venture. GM and Isuzu formed DMAX and launched the Duramax brand of compression-ignition V-8s in 2001. Unlike the Power Stroke and the Cummins, the Duramax has maintained its 6.6-liter displacement since its birth, and it led the torque race from 2005 until 2011, when the Ford and Ram offerings hit their staggering *ratings.
The Duramax, like the Cummins and the Power Stroke, has a variable-geometry turbo. It is located in the valley even though the exhaust manifolds are in the normal outboard spots.
GM’s Duramax currently ranks third. But with 765 of anything, let alone pound-feet of torque, it’s difficult to call it a loser.