This month we were on a mission to find out which Chevy mill makes for a better performing and towing vehicle, the Vortec 7400 big-block or the 6.5L turbodiesel. We figured we'd get the naked truth by comparing two Suburbans with those two engines. Unfortunately, Chevrolet could only supply a diesel with 3.73 gears and a 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 Carryall Suburban took the world by storm. The all-steel wagon was built on a truck chassis and could hold eight passengers-the family truckster. And the Suburban remains GM's largest sport/utility vehicle more than 70 years later. It can still carry way more than 2.5 kids, but does so with some real beef under the hood. In 1997 there was 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 the Los Angeles County Raceway while towing 6,000 pounds to see what these 'Burbans could and 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 6.5L maximum trailer weight rating of 6,000 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.
Chevrolet was a step ahead of the competition when it introduced the short/long-arm front suspension on its pickups in 1988; Suburbans have had IFS since 1992. Four-wheel-drive fullsize Chevys have torsion bars, and the front diff is mounted to the frame with rubber bushings to reduce ride harshness off-highway. | 
The enormous Suburban is designed for carrying the entire fam and all their stuff, and the rear suspension compensates for variances in load via two-stage, multiple leaf springs. The longer leaves flatten out as load increases, while the shorter, stiffer springs act as support, so the ride remains smooth regardless of how much you're hauling. Both Suburban K2500's use a 14-bolt (shown). | 
Our gas Suburban weighed in at 6,560 pounds, while the diesel topped the scales at 6,800 pounds. Because of the higher compression ratio and higher internal pressures of a diesel engine, it needs to be stronger and therefore requires heavier components. |
Next question: Does a diesel really get better mileage? Unloaded, the turbodiesel got 18.338 mpg and 11.450 mpg 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 mpg while trailering at half its max load and 9.65 mpg 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 diesel and the gas 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 older 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 was 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.