Putting The Engines To The Test
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.