L What? Determining Which Duramax Is Best

• January 1, 2019
• Story By
  Mike McGlothlin

After exhausting the pros and cons of each Duramax, it’s time to pick a favorite—but it’s anything but an easy decision. The truth is that each variant of the iconic 6.6L V8 diesel has its strong points. The LB7 was the simplest in terms of emissions controls, while the LLY came with the largest turbocharger ever offered. The LBZ packed a host of improvements, including considerably more power, and the LMM was nearly a mirror image of it aside from its use of a diesel particulate filter. The LML brought higher fuel pressure, cleaner emissions and nearly 400hp into the equation, while the current L5P upped the ante even further and has 600rwhp potential in ’17-newer GM trucks.

To touch on what was covered in our Duramax History Lesson series, let’s run through the ups and a few of the downs associated with each mill. Then we’ll reveal the one we prefer most and why. While you may be partial to another RPO code Duramax, we don’t blame you. No matter the generation, you can’t really go wrong with any rendition of this engine. Hard-part wise, they’re pretty solid. When properly cared for, they can easily give you 300,000 miles worth of use, but are definitely capable of going 500,000 miles (or more) if you’re willing to keep one that long. The Duramax platform has made GM a viable contender in the diesel pickup game since the turn of the century and there is no indication that that will change in the foreseeable future.

LB7: The O.G.

It doesn’t get any simpler than the original 6.6L: the LB7. With the exception of California models, the first Duramax was effectively free of any of the problematic emissions control devices that would be introduced in the years to come. There was no exhaust gas recirculation (EGR) and certainly no diesel particulate filter (DPF) yet, just a catalytic converter. The LB7 also made use of a fixed geometry turbocharger—the only Duramax to do so—which meant no sticking turbo vanes or actuator issues.

LB7 Drawback #1: Injectors

Blazing the trail of common-rail injection in the diesel truck segment didn’t come without its quirks for GM. Cracked and/or leaking injectors could fill the crankcase full of diesel fuel, yield excessive smoke out the tailpipe and cause irreparable engine damage if neglected long enough. Today, the LB7 injector issue has all but been resolved (thanks to improved injector designs and materials), but if we were in the market for a pre-owned Chevrolet or GMC HD, we would be a bit uneasy about buying a low-mile ’01-’04 model.

HP Potential: LLY

With the largest version of the factory Garrett GT3788VA turbocharger bolted to it, the LLY (found in ’04.5-’05.5 trucks) has arguably the most tune-only horsepower potential of any Duramax produced between 2001 and 2016. Thanks to its 62.6mm inducer compressor wheel and the tallest turbine vane height (15mm) of any GT3788VA, enough intake and exhaust flow exists to support 530rwhp in 2500 and 3500 model Chevrolet Silverados and GMC Sierras. On top of that, the LLY-spec’d GT3788VA sports a 360-degree thrust bearing for optimum turbo longevity.

Toasty Coolant Takes Its Toll

Unfortunately for the LLY, it’s fairly notorious for overheating in towing situations. Primarily due to a combination of a restrictive turbo mouthpiece, an undersized radiator and a dirty cooling stack blocking airflow, it’s not uncommon to see coolant temp crest 230 degrees when working this engine. Over time, and with a lot of the LLY-powered trucks being tuned most of their lives, purchasing a 200,000-mile version could mean you’re pulling the heads for new gaskets shortly thereafter.

LBZ: The Pre-DPF Era LMM

With a stronger block, meatier rods, taller main bearing caps, a revised, higher pressure common-rail injection system and 360hp right out of the box, it’s hard not to like the LBZ, available in GM trucks during the ’06-’07 model years. While working to meet much more stringent NOx and particulate matter emissions regulations set to go into effect in 2007, GM made the changes necessary in ’06 that would essentially allow the LBZ to become an LMM (’07.5-‘10) with very few changes. As a result, the LBZ is basically an LMM without the troublesome exhaust aftertreatment system (i.e. the DPF, and the mileage-and-longevity-killing regeneration cycles).

It’s All Fun and Games Until You Lose a Cylinder…

There is little doubt that GM’s use of wrist pin bushings (and a thinner wall wrist pin, to boot) weakened the LBZ pistons’ ability to handle excess cylinder pressure (i.e. torque), but the quality of the castings have also been suspect for some years now. If you’re in the market for an LBZ, just know that the failure rate of the factory pistons gets higher the closer you get to 650rwhp. At power levels close to stock, we liken cracked piston scenarios in an LBZ to the killer dowel pin striking in a 5.9L Cummins. It could happen at any time unless you dig into the engine and eliminate the possibility—but it probably won’t.

LML: Significant Bottom End Improvements

To stand up to 765 lb-ft of torque in stock form, GM once again improved the Duramax’s block strength with the LML, offered in GM HDs from ’11-‘16. As such, this crankcase is now used as the foundation in many performance engine builds in the aftermarket. But not only that, higher horsepower requires improved oil flow—and an oil pump that flows 11-percent more than ’01-’10 engines is employed on the LML. To free up horsepower (it dished out 397hp, stock), the LML also sports the lightest rotating assembly of any Duramax.

CP4.2: A New Achilles Heel

For emissions purposes, GM was forced to move away from the Bosch CP3 on the LML, which makes use of Bosch’s CP4.2 instead. While the smaller pump cranks out 29,000 to 30,000 psi and packages easier, it’s prone to catastrophic failure any time a lack of lubrication exists or debris infiltrates the high-pressure fuel system. The pit-in-the-stomach double-whammy is that, when the twin-piston CP4.2 self destructs, it often takes the injectors out with it. The first thing to do when picking up a used LML is to ditch the CP4.2 in favor of a CP3 (if possible) or install a lift pump (for added filtration and low-pressure fuel supply).

L5P: Nothing Else Like It

No other Duramax in history has offered what the L5P does. The Bosch CP4.2 has been ditched in favor of a Denso HP4, Denso state-of-the-art solenoid-style injectors replace the Bosch piezoelectric units used on the LML and for the first time ever the injection pump is supported by a lift pump. Radically different cast-aluminum heads flow quite a bit more air, the variable geometry turbocharger is manufactured by BorgWarner (not Garrett), sports a 61mm billet compressor wheel and is both controlled and actuated electrically. As a result, the latest and greatest Duramax on the market turns out 445hp and a mountain-moving 910 lb-ft of torque right off the showroom floor. Check out the table-top torque curve!

L5P: Into the Unknown

So far, the biggest drawback for the L5P is that it’s so fresh. While we know the new Denso injection system has great performance potential, we don’t yet know how reliable it will be. The same can be said for the cutting-edge, all-electric BorgWarner turbocharger sitting in the valley. Though everything looks good on paper, it has yet to prove it can be reliable, day-in and day-out, like its predecessors were. Only time will tell.

If It’s Our Money, We’re Going LBZ

For its use of the tried and true Bosch common-rail system based around the durable CP3, pre-DPF emissions system and its tune-only performance potential, we’re big fans of the LBZ Duramax. It improved on the LB7 and LLY platforms with a more robust block, stouter connecting rods, higher pressure injection system (26,000 psi vs. 23,000 psi) and came bolted to a stronger, six-speed Allison automatic transmission. With an LBZ-powered ’06-’07 Chevy or GMC HD you gained double overdrive, saw an uptick in fuel economy, enjoyed great reliability and still had the lightweight (and arguably better-looking) classic body style. Without any turbo or fueling mods (other than tuning), you could dip into the 12s in the quarter-mile with a 4x4 crew cab ¾-ton. Trust us, there is a reason these 13-year-old gems still bring $20K in the used truck market.