GM expands power, torque

GM expands power, torque

For 2008, General Motors' TopKick models offer more power, more torque, six-speed Allison transmissions on most models, and other enhancements.

Mike Eaves, product manager for medium-duty trucks, said that while the displacement specs match those of the previous version, the available Duramax 6.6-liter V-8 turbo-diesel powering the 2008 TopKick differs greatly from its predecessor:

  • New fuel injectors to precisely control combustion.

  • More durable variable-geometry turbocharger. The compressor is changed to a “boreless” type, increasing durability at high RPM.

  • A larger exhaust gas recirculation chamber to help control combustion temperatures, contributing to lower emissions.

  • A closed crankcase ventilation system. The crankcase ventilation system is a sealed chamber between both cylinder banks to ensure that emissions from the crankcase are kept to a minimum.

  • A diesel particulate filter (DPF) system. To reduce particulate matter, the ceramic honeycomb channel DPF captures sulfate particles and soot. Exhaust gases are directed through the channels and through a porous material that traps the particulates.

  • An intake throttle for precise combustion temperature control increases the amount of load on the engine during low-load conditions, such as idling for extended periods and unladen low-speed driving. The throttle is operated by the engine control module (ECM) as an additional control for combustion temperatures

  • An electrically variable engine cooling fan. The engine cooling fan is now controlled by the ECM, allowing the engine cooling system to be assisted by the fan only when necessary. This results in more precise control over the engine temperature.

  • Increased engine control processing. The E35 ECM now controls the new injectors, the data from the diesel particulate filter pressure sensors, the new intake throttle operation, and the electrically variable engine cooling fan.

The new Duramax 6.6L is available in two horsepower and torque configurations: 300 horsepower at 3000 rpm and 520 lb-ft at 1600 rpm; and 330 hp at 3000 rpm and 620 lb-ft of torque at 1600 rpm.

The standard engine in C4500 and C5500 models is the Vortec 8.1L gasoline V-8, which delivers 325 hp at 4000 rpm and 450 lb-ft of torque at 2800 rpm.

C6500, C7500, and C8500 models are powered by one of three engines: a Vortec MD (8.1L V-8) delivering 295 hp and 440 lb-ft of torque; an Isuzu 6H (7.8L diesel I-6) delivering horsepower ranges from 215 to 300 and torque ratings from 560 to 860 lb-ft; and a Caterpillar C7 (7.2L diesel I-6) with horsepower ratings from 207 to 250 and torque ratings from 520 to 800 lb-ft.

Allison six-speed transmissions are the norm for 2008 TopKick models. C4500/C5500 models are mated to 1000 or 2200 series transmissions, and a new six-speed 2350 series Allison with 13,600 kg (30,000 lb) GCWR and park pawl is available for heavier towing applications. All Allison six-speed models in C4500 and C5500 models are close-ratio, fully automatic, and electronically controlled transmissions with double overdrive ratios.

C6500 and C7500 diesel engine models also use Allison six-speed transmissions in place of former five-speed Allison transmissions. The 2500 Series offers a 14,969 kg (33,000 lb) GVWR/GCWR without a park pawl, while the 2200 Series and a new 2550 Series offer a park pawl with maximum GVWR/GCWR of 11,793 and 13,600 kg (26,000 and 30,000 lb).

Six-speed transmissions are also standard on C4500/C5500 gasoline models; C6500/C7500 gasoline models will continue to be equipped with five-speed transmissions as standard equipment.

Additional enhancements

TopKick models receive additional enhancements for 2008, including:

  • New dual 150-amp alternators, delivering more amps for specialist applications, such as ambulances and shuttle buses for C4500/C5500 diesel engines.

  • 8,165-kg (18,000-lb) capacity front axle for C8500 models.

  • New quiet rear axle on C5500 8,618-kg (19,000-lb) rear axle model for cutaway and motor home models only.

TopKick tandem models are available with new Hendrickson suspension systems for 2008:

  • HMX 400-Series 18,144-kg (40,000-lb) capacity tandem rubber suspension is now available across a wider range of wheelbases, up to 308".

  • HMX 460-Series 20,865-kg (46,000-lb) capacity suspension (GPB). It's available with a 20,412-kg (45,000-lb) axle and for wheelbases ranging from 170" to 308".

  • HAS 400-Series tandem air suspension. It's available for Regular and Crew Cab models, with 15,422-kg or 18,144-kg (40,000-lb) axles and in wheelbases ranging from 188" to 308".

All C4500 and C5500 Series models are designed off a medium-duty chassis featuring a more durable foundation frame that is constructed with higher 50,000- and 80-000-psi-strength steel.

C6500 and C8500 models use 50,000-, 80,000-, and 120,000-psi heat-treated frames. Compared to C4500 and C5500 models, C6500 and C8500 feature larger-sized frames and offer a wider selection of wheelbases and single and tandem rear-axle configurations to accommodate larger-sized bodies. Single-axle models can easily accommodate bodies up to 30' in length without requiring any extra-cost frame extensions. TopKick is available in GVWRs ranging from 7,484 kg (16,500 lb) Class 4 to 28,576 kg (63,000 lb) Class 8.

All C4500 and C5500 trucks use standard four-piston front caliper brake design and Hydro-Max brake systems to optimize performance for their particular GVWR requirements. Heavier GVWR C5500 and C6500/C7500 models feature fixed-caliper brake design with diameters sized to particular GVWR load requirements. Premium, high-capacity 70-mm, four-piston brake systems are standard on both the front and rear.

The Hydro-Max brake booster for brake actuation uses the power steering pump to supply the booster power. A tandem (split) master cylinder provides the independent front and rear circuits for the hydraulic brake system. Sensors warn the driver of “low fluid” or secondary brake failure.

All models share a stamped, straight-section, C-channel frame, providing maximum strength and a clean back-of-cab design for easy body and equipment installations. Cross members are web-mounted to the frame, providing additional torsional stiffness and rigidity.

The stronger frames allow wheelbases ranging up to 308“ on a C8500, with plenty of rear overhang for upfitter applications. Fuel tanks, available in multiple capacities, are mounted between the frame rails on C4500 and C5500 models. C6500, C7500, and C8500 models feature outboard step tanks with up to 100-gallon capacity provided by dual tanks. The C4500/C5500 4×4 regular cab diesel models offer a driver-side, under-cab, 29-gallon fuel tank.

Silverado, Sierra

Dan Tigges, product manager for full-size trucks, said the following are new for 2008:

  • LD integrated trailer brake controller (JL1). It's not available with the regular cab or 4.3L V6, and it requires Stabilitrak.

  • XM standard (UE1). Fleet delete is available (UEO). OnStar fleet delete requires XM fleet delete.

  • Fast idle feature (UF3) on HD models. It's not available with PTO (diesel chassis cabs). It uses a cruise-control switch, the DIC message when active. The variable mode allows the user to change speeds.

  • B20 capability for Duramax diesel. It requires SEO 5F4, and is planned to be standard for the 2009 model year.

He provided the following upfitter issues:

  • Tail light calibrations sense either two or zero bulbs. Bulb outage warnings are required for bodies under 80“ wide if not equipped to the trailer. He said few, if any, upfitters use dual bulb tail lights. They have developed a new calibration to sense one bulb, and it is now orderable as SEO 9U5.

  • Integrated trailer brake controller (JL1). It's not available with box deletes, and it requires new wiring harnesses and calibrations.

  • Center of gravity limit. A higher CG limit was carried over from GMT800 on two-wheel-drive 2500HD models. An upfitter request was made to expand eligible models, and a crash test is required to verify these models meet requirements.

  • Perimeter lighting. It causes an aftermarket backup alarm to sound when the key fob is used. The SEO 8S3 backup alarm has a calibration that turns the perimeter tail lamp lighting off.

  • Tire pressure monitoring. The TPM system has the capability to handle tires up to 100 psi but requires different clam-in sensors. The system needs to have the baseline reset to 90 psi by the dealer. The TPM system will then accurately report over- or under-inflation.

  • The 2500HD and 3500HD SRW FGAWR is 4200 lb. Classic models were 4410 to 4670 lb. A higher FGAWR is required for Hyrail applications. The VYU snowplow prep package provides 4800 lb FGAWR on four-wheel-drive models ($285 MSRP). A change request has been submitted to create an orderable option to get 4500 FGAWR on two-wheel-drives and 4800 lb FGAWR on four-wheel-drives.


For the 2008 model year, the Savana delivers a higher level of safety, a freshened interior, and the performance of the small-block V-8 engine family.

Savana's roster of safety features is enhanced for 2008 with the inclusion of a standard tire-pressure monitoring system and new flat and convex exterior mirrors that offer a wider field of vision.

Other Savana safety features include four-wheel antilock disc brake system with Hydroboost, dynamic rear proportioning, StabiliTrak, driver and front-passenger air bags, and daytime running lamps.

A three-piece frame features fully boxed sections for greater strength and rigidity, while extended and tapered front frame rails are used for improved crashworthiness. Additional tubular frame cross members are used to increase torsional stiffness.

Savana is available in regular (135“) and extended (155“) wheelbase lengths, and can accommodate eight, 12, or 15 people. The cutaways, which can be converted to various commercial, school bus, and RV requirements, are available in 139“, 159“, and 177“ wheelbase lengths.

The 1500 Series vans include light-duty models rated at 7200 lb GVW. The 2500 Series includes heavy-duty models rated at 8600 lb GVW. The heavy-duty 3500 Series includes full-bodied cargo/passenger vans rated at 9600 lb GVW, and commercial and RV cutaway chassis, with GVW ratings of 9900-14,050 lb.

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