This month we were on a mission to find out which new Chevy mill makes for a better performing and towing vehicle, the new Vortec 7400 big-block or the 6.5L turbodiesel. We'd always heard that diesels were torquemasters-but is it true? We figured we'd get the naked truth by comparing two Suburbans with those two engines. Unfortunately, Chevrolet could only supply us with a 1/2-ton diesel with 3.42 gears and a 3/4-ton big-block with 4.10 gears. Not an equal test. However, even though this is not a true head-to-head comparo, we were able to confirm some beliefs and form some opinions on which engine is better for your needs.
It was way back in 1935 that the first Chevy Suburban Carryall took the world by storm. The all-steel wagon was built on a truck chassis and could hold eight passengers-the family truckster. And here we are more than 60 years later, and the Suburban remains the family hauler that GM claims is the world's largest sport/utility vehicle. It can still carry way more than 2.5 kids but does so with some real beef under the hood. For 1997, there's a standard small-block Vortec 5700 V-8 and the optional big-block Vortec 7400 V-8 as well as a 6.5L V-8 turbodiesel engine.
The Vortec line of gas engines was introduced into the Suburban to provide increased power, better towing ability, and longer intervals between tune-ups, while the 6.5L turbodiesel was designed for better fuel economy and improved throttle response, thanks to its Borg-Warner/IHI turbocharger system. Both Suburbans we tested were equipped with the 4L80-E automatic overdrive transmission.
After racking up the mileage during around-town highway cruising, we headed up the long grade to Los Angeles County Raceway while towing 5,000 pounds to see what these 'Burbans could do. And what they couldn't do. Although the common conception is that a diesel is the best choice if you're looking for a tow vehicle, it was during the climb to the dragstrip that the turbodiesel-stuffed
Sub showed its true colors.
While climbing the grade, we had it wide open to maintain 60 mph, and downshifting was required to prevent hunting between overdrive and Third. On steeper sections the trans downshifted to Second and the engine hit the rev limiter. It was kinder to the vehicle to back off and mosey along at three-quarter throttle and 45 mph. So towing performance was not awesome, but we were also hauling the 1/2-ton's maximum trailer weight rating of 5,000 pounds.
Taking the big-block Suburban through the same route was a lot more impressive. We were able to make the entire uphill trip at 65 mph with the trans occasionally kicking down to Third on steep sections. On lesser grades, we were even able to accelerate from 65 to 75, which was impossible with the turbodiesel. The impressive performance was due not only to the Vortec's 410 lb-ft of torque, but also to the lowest available axle gears, 4.10.
Once we made it to the track, we ran acceleration tests on both Suburbans, with and without the trailer. The unloaded diesel Suburban went 68 mph in 19.806 seconds; with the trailer, it peaked at 55 mph in 25.013 seconds-5.207 seconds slower. The gas Suburban ran an impressive 77 mph in 17.722 seconds, while towing added 4.848 seconds for 61 mph in 22.570 seconds. Curiously, both Suburbans were only about 27 percent slower when loaded.
Next question: Does a diesel really get better mileage? Unloaded, the turbodiesel got 18.338 mpg and 11.450 while towing; it averaged 15.493 mpg over the duration of our testing. And remember, that's with many miles of hauling at maximum towing capacity. Not bad when compared to the big-block's 7.41 while trailering at half its max load, and 9.65 when not, for an average of 9.12 mpg.
So what do the numbers mean? If you're after a tow vehicle, the big-block is the better choice. If you're in need of a vehicle that gets good mileage, the diesel would be the better route-its efficiency numbers are particularly good when you remember the Suburban's size. And the diesel can tow-you just need more patience.
But what about the ride? We were surprised to find little ride-quality difference between the 1/2-ton and the 3/4-ton which says a lot for both vehicles, especially since we found them much smaller from behind the wheel than we do as pedestrians. These 'Burbans do not drive like behemoths. Amenities were similar, though there was no mistaking that you were motoring the 6.5L due to the telltale clatter. Thanks to improvements in design over the years, the engine is quieter than old versions, although the big-block cab did have a libraryesque quality after getting directly into it from the diesel. And although the big-block had noticeably more throttle response, the diesel wasn't as sluggish as we'd presumed it would be prior to the test.
If price is a consideration, the big-block is the cheaper option at $600, but the $2,860 for the turbodiesel can be made up for in its better mileage over the long haul.
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 1/2-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 1/2-ton diesel). If you compare 3/4-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 1/2-ton diesel weighed 240 pounds more than the 3/4-ton big-block.
|Vehicle||Engine||Axle ratio||Max trailer weight|
*diesel engines; bold indicates vehicles we tested
Power This was our first chance to try out the 6.5L turbodiesel and the Vortec 7400 V-8 SFI. The Vortec 7400 replaces 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 is available for 3/4-ton Suburbans and makes 290 hp at 4,000 rpm and 410 lb-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).
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.