Tow Test - Big Blocks VS. Diesels

The Tow Truth
We'd always heard that diesels rule for tow duty, but do they? If you look at the chart of Chevy's 4x4 Suburban tow ratings, you'll find that a 11/42-ton (K1500) diesel has the exact same tow rating as the Vortec 5700 small-block gas engine when both vehicles are equipped with 3.42 axle gears (which are the only available gears for a 11/42-ton diesel). If you compare 31/44-ton Suburbans, notice that a 4.10-geared turbodiesel is rated at 2,500 pounds less than a 7.4L big-block gas engine with the same gears. This tells you two things: first, the diesel has a lower tow rating than a big-block in every configuration. Second, axle gears are crucial to towing performance. Also remember that vehicle weight and stopping power affect tow ratings, so the added weight of the diesel engine itself could contribute to the lower ratings. Believe it or not, our diesel weighed 240 pounds more than the big-block

*Diesel engines; bold indicates vehicles we tested

The 6.5L turbodiesel gets great fuel economy, thankfully, since the cost of filling up can definitely come back and bite you in the buck. Most stations charged an average of $2.65 per gallon, but we paid as much as $2.95, whereas gas is right around $2.49. Both Suburbans have 42-gallon tanks, so the cruise range was around 650 miles for the diesel and only 380 for the big-block.

Power This was our first chance to try out the 6.5L turbodiesel and the Vortec 7400V-8 SFI. The Vortec 7400 replaced the throttle-body-injected L19 7.4, which made 230 hp at 3,600 rpm and 385 lb-ft of torque at 1,600 rpm. The 7400 was available for 3/4-ton Suburbans and makes 290 hp at 4,000 rpm and 410lb-ft of torque at 3,200 rpm. The turbodiesel has more peak torque (430 lb-ft at 1,800 rpm) but less horsepower (195 hp at 3,400 rpm).

The Suburban came with a choice of rear panel doors or a tailgate with lift glass (shown). There's ample cargo space, even when you've got a full house.

With diesel engines, there's a delay time between when you turn the key on and when you should crank the engine so that the glow plugs can heat up. Chevy's instrument panel features a "Wait to Start" light (arrow) that goes off when the system is functioning properly and the engine can be started. We waited as long as three seconds for the light to go off when warm and as long as 15 seconds when cold.

Chevrolet was big on safety, and the '97 had a standard front-passenger airbag. Carryovers are daytime running lights, four-wheel ABS, a driver-side airbag, and brake/transmission shift interlock, which prevents the truck from shifting out of Park until you step on the brake.

How Diesel Works
Back in 1893, Rudolf Diesel produced the first diesel engine, primarily to be used in industrial applications. Because diesels were very fuel efficient, they were in demand, and that holds true today. Many improvements have been made to the powerplants since Mr. Diesel's early design. With the advent of indirect injection (IDI) and turbochargers, acceleration has improved, and manufacturers have also eliminated a lot of the clattering diesel noise.

Jack Blanchard of Diesel Engineering at the GM Powertrain Group gave us the scoop on how four-stroke diesels work. With a gas engine, fuel mixes with the air that enters a cylinder during the intake stroke. However, with a diesel engine, only air is taken in during the intake stroke. Therefore, only air is compressed during the compression stroke. Diesel fuel is injected into the cylinder at the end of the compression stroke.

When the air compresses it gets really hot, and that's what ignites the fuel when it's injected into the system. Since the compression ratio is so much higher (a GM diesel has a 20.2:1 compression ratio as compared to 9:1 for a gas engine), the temperature is high enough to spontaneously ignite the fuel injected into the cylinder. Therefore, it doesn't need a spark to ignite. So the biggest difference between a gas engine and a diesel engine is that a diesel doesn't use spark plugs. Rather, it has a compression ignition system.

All diesel engines are fuel-injected-they inject fuel right into the cylinder. The GM diesels are IDI engines-fuel is injected into a prechamber that sits right above the piston. With direct-injected engines, fuel goes into the cylinder atop the piston. IDI is quieter and has better luck meeting emissions at the low end. That's why a diesel can go into the C/K 1500 series, according to Blanchard.