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Chevy Duramax LMM Diesel Vs. 6.0L Vortec LY6 Gas Engine- Diesel Vs. Gas

Chevy Engine Shootout: Duramax LMM Vs. 6.0L Vortec LY6

Photography by Courtesy of General Motors, Trevor Reed,

With the introduction of the '07 GMT900 trucks, you can get a 2500HD or 3500HD with an all-new body and interior wrapped around a cleaner and more powerful version of the famous Duramax diesel or a 6.0L small-block gas V-8. What you don't get is the option to buy a big-block gasoline engine or put a manual transmission behind the new Duramax diesel powerplant.

The 6.0L gas engine available in the new 31/44- and 1-ton trucks is rated at 353 hp and 373 lb-ft of torque and uses variable-valve timing along with the new 6L90 six-speed automatic to improve fuel economy. The Duramax LMM is saddled with a bigger EGR system, an intake throttle, catalysts, and a diesel particulate filter but is rated at 365 hp and 660 lb-ft of torque, which is even more than the outgoing LBZ that produced 50 percent more NOX and released 90 percent more soot into the air.

Which one of these modern V-8 engines is the best choice for you? To reveal the differences between the gas and diesel powertrains, we put both trucks on the dyno, ran them down the dragstrip, hooked up a 9,500-pound trailer, weighed them, and logged their fuel mileage while hitting the streets in all driving conditions. We'll report, you decide.

Other than the big difference in price that the Duramax diesel and Allison transmission add, the main distinction between the two powertrain choices can be found in the weight ratings. The 762 pounds of extra payload that can be carried by the Vortec gas truck is the exact difference in the factory curb weights between the two trucks. When it comes to lugging a trailer, the Duramax LMM overcomes its girth and is rated to tow 13,000 pounds using the factory hitch, which is 600 pounds more than the gasser can be counted on to pull.

Other numbers to take note of are the engine displacements. While our Ford test ("Diesel vs. Gas," Jul. '07) compared an unsophisticated 6.8L, V-10 gasser with the brand-new, dual-turbo, 6.4L Power Stroke, in this test, a 6.6L turbodiesel first designed in the late '90s has to battle it out with a high-tech 6.0L small-block with 40 years of GM advancements and a new variable-valve timing system. This should be interesting.

General SpecificationsChevy Silverado 2500HD 4WD,extended cab, shortbed, 4WDBrakesType: Four-wheel disc with ABSFront (in.): 12.8 x 1.5Rear (in.): 12.8 x 1.2

Wheels/TiresWheels: 16-inch steel (standard); 16-inch aluminum, 17-inch aluminum (optional)Tires: LT245/75R16 (standard),LT265/70R17 (optional)

SuspensionFront: Long- and short-arm independent front with torsion barsRear: Solid axle with leaf springs

SteeringSteering ratio: Variable from 13:1 to 15:1Turning circle (extended cab, shortbed): 47.6 feet

Dimensions (extended cab, shortbed)Wheelbase (in.): 143.5Overall length (in.): 230.3Height (in.): 76.8Front track (in.): 68.6Rear track (in.): 66.0Ground clearance (in.): 9.5

Interior Dimensions (extended cab)
Headroom (in.) Shoulder room (in.)
Front: 41.2 Front: 65.2
Rear: 39.2 Rear: 65.3
Legroom (in.) Hip room (in.)
Front: 41.3 Front: 62.5
Rear: 34.3 Rear: 61.8

Manufacturing LocationsPontiac, MichiganFlint, Michigan

Shoulder room (in.)Front: 65.2Rear: 65.3Hip room (in.)Front: 62.5Rear: 61.8

Dyno TestAlthough DC Performance specializes in SRT-10 Dodge Rams and Vipers, the staff was happy to take part in our Chevy slugfest. Both trucks were strapped to the same dyno to reveal the true stock horsepower and torque ratings. Chevy rates the gasoline-powered Vortec LY6 engine at 353 hp and 373 lb-ft of torque, and the Duramax LMM is said to make 365 hp and 660 lb-ft of torque at the engine's crankshaft. The gas truck had trouble staying in Fourth gear when the engine was loaded, so the rear-wheel power and torque ratings are a bit lower than expected. It should also be noted that the new 6L90 six-speed automatic in the gasoline truck doesn't have a direct drive speed (Fourth gear has a ratio of 1.15 and Fifth is 0.85), so the gearing may have affected the final numbers. As expected, the Duramax LMM put down a lot more torque on the dyno, making 244 lb-ft more at the rear wheels.

6.6L Duramax LMM Turbodiesel V-8 Max power: 303 hp at 3,200 rpmMax torque: 507 lb-ft at 2,700 rpm

6.0L Vortec LY6 Gasoline V-8Max power: 279 hp at 5,900 rpmMax torque: 263 lb-ft at 4,800 rpm

Drag RacesDuring our final trip to the now closed Los Angeles County Raceway in Palmdale, California, we raced the gas and diesel Chevy GMT900 trucks along with the diesel Jeep Grand Cherokee seen in our Aug. '07 issue.

One look at the numbers below and you'd think the diesel would win at every stoplight, but in the real world, you don't get to use 4WD and tickle the pedals until full boost is achieved. Of course, the gas truck suffers from annoying throttle interference caused by the computer, which can make it feel sluggish anytime it's spinning below 4,500 rpm and the pedal isn't quickly mashed to the floor. When you try to put your foot through the carpet, the 6.0L Vortec gasser feels like a normal, old-fashioned EFI Chevy with no lag before the acceleration begins.

LACR served the greater Los Angeles area for more than 40 years and there is hope that the Antelope Valley Raceway Committee will be able to persuade local municipalities and Los Angeles County to approve a site for a new dragstrip. For the latest information, check out the Web sites www.lacr.net and www.savelacr.com.

MPG TestIt's not easy to get reliable fuel economy figures while driving the streets and freeways of Los Angeles. Unfortunately, depending on the part of town, the time of day, and the number of high-speed police chases, you can spend more time sitting still with the engine idling than traveling miles for each gallon of fuel burned (a condition also known as zero mpg).

Another way of looking at the data we collected here in the City of Angels is that these may be the most real-world numbers you'll read. We commuted to work through the slop, drove to the dyno shop and made full-throttle runs, towed a trailer loaded with a monster Suburban 4x4, and hit the dragstrip for multiple high-speed passes. We were not babying the trucks and assume you won't be keeping a brick under your GM's pedal.

Are these the best mpg numbers you can expect? Heck no. We got 19.12 mpg from the same Duramax LMM truck during our first test (Aug '07) by using cruise control on the highway and averaged 13.68 mpg in almost 825 miles of driving. This time, we averaged 11.83 mpg with a best tank of 14.23 mpg, even while driving steep mountain roads and with plenty of time spent sitting along with the other sheep stuck on the 405 freeway.

DURAMAX LMM 6.6L TURBODIESEL
Miles Gallons MPG Conditions
36.40 3.48 10.46 City/highway
198.80 13.97 14.23 City/highway
206.10 19.86 10.38 Towing, dyno,
dragstrip, and
highway

Average441.30 miles / 37.31 gallons = 11.83 mpg

VORTEC LY6 6.0L GASOLINE
Miles Gallons MPG Conditions
246.20 21.65 11.37 City traffic
69.60 8.67 8.03 City traffic
148.80 9.98 14.91 Dyno, dragstrip,
and highway
47.10 3.17 14.86 Highway

Average511.70 miles / 43.47 gallons= 11.77 mpg

Noise TestThe diesel particulate filter has nearly eliminated visible smoke emissions from the tailpipe, so the main drawback to owning a diesel may be the added noise of the compression-ignition process. The Duramax line of engines has been famous for low noise since it was introduced in the '01 GMT800 trucks, and the new LMM is the quietest version yet. The diesel particulate filter, multiple catalytic converters, and a bazooka tailpipe all play a role in breaking up annoying sound waves.

The only time the diesel was quieter than the gasser was during wide-open-throttle acceleration that caused the 6.0L engine to growl at 89 dB, while just 85 dB were created by the new Duramax LMM. The interior noise of the diesel at idle was only slightly louder than that of the gas truck, but exterior noise at idle was noticeable (just ask the fast-food attendant on the other side of the intercom).

City speeds kept the Vortec engine relatively quiet around 45 mph, where it beat the diesel by a large, 15dB margin. Plus, the diesel had noticeable turbo whine in medium speed conditions (we love it, but you may not). Out on the freeway, the field was fairly even, with the Duramax registering only 1 dB higher on our unscientific equipment. The numbers are so close, you could call it a tie, but we'll trade that single decibel for the extra 244 lb-ft of torque made by the Duramax any day of the week.

Duramax LMM 6.6L TurbodieselInterior at idle: 69 dB45-mph cruise: 84 dB75-mph cruise: 88 dBWide-open throttle: 85 dB

Vortec LY6 6.0L GasolineInterior at idle: 66 dB45-mph cruise: 69 dB75-mph cruise: 87 dBWide-open throttle: 89 dB

Towing Test: Steep Grade at 15-65 mphTo see how the small-block gasser truly stacks up against the Duramax, we hooked up a 9,500-pound trailer Suburban 4x4 combo and attacked a steep grade at an altitude of more than 3,150 feet. We started by rolling along a freeway onramp at 15 mph and timed how long it took to reach the speed limit of 65 mph. Two attempts were made with each truck, and Editor David Kennedy was able to knock almost 3 seconds off the gas truck's first pass by manually shifting the 6L90. We decided not to try and outsmart the Allison transmission because the shifts during the first pass seemed to be right on when the Duramax pulled the grade.

Duramax LMM 6.6L TurbodieselTest 1: 24.8 secondsTest 2: 24.1 seconds

Vortec LY6 6.0L GasolinePass 1: 33.3 secondsPass 2: 30.6 seconds

SOURCES
Chevrolet Trucks
www.chevrolet.com
Los Angeles County Raceway
DC Performance
Los Angeles
CA
www.dcperformance.com
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