To Smoke A Muscle Car…LMM Duramax Edition

• January 4, 2021
• Story By
  Mike McGlothlin

It’s highly convenient for the aftermarket when the OEM’s stick with the same basic powertrain during a major restyling. Turn back the clock to the GMT900 changeover of 2007 and you’ll see that a very similar version of the Duramax V-8 and Allison six-speed transmission carried over to the ’07.5 model HD trucks. Thanks to the LMM Duramax basically being a twin of the LBZ, virtually everything that can be done to the popular ’06-’07 powerplant can also be done to the LMM. Very little (if any) reverse-engineering was involved in infiltrating the ECM, so big horsepower was once again available with a few keystrokes. The only real difference with the LMM was that you could enjoy your 530rwhp in an updated interior.

However, while the Allison 1000 behind the LMM did feature a few slight improvements over ’06-’07 models, it was largely a carryover item. Long story short, any ’07.5-’10 Duramax owner who wants to enjoy a max effort tune and hunt down pony cars in a reliable fashion has to build the transmission first. Once that mandatory, peace-of-mind expense is made, horsepower can be raised into the mid 500’s with a few bolt-ons and 12-second quarter-miles can be added to your four-wheel drive tow-rig’s long list of capabilities. For the full scoop on how to make it all happen, keep reading.

A Dyno-Proven, 500-Plus HP

If you read the LB7, LLY and LBZ chapters of this chronicle, it’s the same story here: EFI Live tuning and a truck that makes 500rwhp or more (along with 1,000 lb-ft of torque). In best-case scenarios, the LMM’s stock turbo, injectors and CP3 high-pressure fuel pump can support 530 to 540rwhp. The biggest key here is to make sure you choose a quality and highly reputable tuner whose calibrations have been refined for drivability and durability just as much as they have been for making maximum power.

You’ll Have To Put Some Money Into Your Allison

Without a doubt, in factory form the six-speed Allison 1000 is stronger than the five-speed version that preceded it (’01-‘05). This is the case because the six-speed has increased C1 and C2 apply piston area as well as centered apply pistons to help apply those clutches evenly. However, the C3 clutch pack will still slip in fifth gear under big load (i.e. added horsepower and a heavy trailer in the mix). The same fate can happen in sixth gear on account of the C4 clutch pack slipping. Long story short, although a converter upgrade, shift kit and good TCM tuning can help the six-speed Allison hold up under respectable power for a while, eventually you’ll be in the same boat as all other Allison owners: the transmission will have to be built.

3-inch Downpipe & Open Up The Exhaust

One of the biggest choke points on the LMM exists in the factory downpipe. The oblong piece suffices just fine at the stock power level, but it’s a considerable restriction with 200 extra ponies in the mix. Upgrading to a 3-inch diameter downpipe, along with a 4 or 5-inch downpipe-back exhaust system, opens up exhaust flow significantly. Less restriction on the exhaust side can lead to a drop in exhaust gas temperature (EGT) and also relieve some of the stress the turbine side of the turbo experiences with more power in the mix. Note that any aftermarket exhaust system produced for an ’01-’10 Duramax (LB7, LLY, LBZ and LMM) utilizes the same mounts and will fit all Chevy and GMC 2500 and 3500 trucks.

3-inch Y-bridge & Intercooler Pipes

Similar to the exhaust side, you’ll want to open up the LMM’s intake tract for more air volume. Additional, denser air entering the heads will lead to more horsepower potential. The main roadblock here is the factory intake Y-bridge, a restrictive, two-piece component in the lifter valley that is prone to blowing apart under excessive boost. Swap it out for an aftermarket, 3-inch diameter one-piece replacement or a bolt-together two-piece unit. When combined with 3-inch diameter hot and cold-side intercooler pipes, the added air volume can free up as much as 20-30 hp.

Add A Lift Pump, Pronto!

Like its predecessors, the LMM Duramax was void of a lift pump from the factory. Instead, the internal suction valve within the Bosch CP3 high-pressure fuel pump is tasked with pulling diesel from the tank. From there, the CP3 has to pressurize that fuel as high as 26,000 psi before sending it out to the rails (and ultimately the injectors). An entry-level lift pump system from FASS, Fuelab or AirDog can work wonders for the LMM (or any ’01-’16 Duramax). Comprehensive aftermarket systems feature ½-inch feed lines, added filtration and an electric lift pump supplying 8-10 psi to the CP3 at all times. In addition to helping extend your power curve at higher rpm, a lift pump can also extend the life of your CP3 and injectors.

Same Old, Same Old (Garrett GT3788VA)

The same basic Garrett GT3788VA variable geometry turbocharger found on the LLY and LBZ exists on the LMM, which means it too can support 530rwhp (and even a tad more with the aforementioned intake and exhaust mods). And even despite its use of a 270-degree thrust bearing (the LLY and LBZ versions had a 360-degree thrust) the ’07.5-’10 version of the GT3788VA rarely overspeeds or fails at higher boost levels. In conjunction with an aggressively tuned engine, the VGT provides lightning quick response (i.e. instant torque), which is ideal for achieving the low 60-foot times that facilitate an LMM truck’s breaking into the 12’s. With a slightly larger, direct drop-in VGT in the mix, you can add 30-50 hp to a tuned Duramax and go mid 12’s.

At The Track

Taking advantage of four-wheel drive and staging the truck with 8-12 psi of boost on tap, a tuned LMM can easily cover the first 60 feet of the track in 1.8 or 1.7-second intervals, and we’ve even seen some regular cab trucks break into the 1.6’s when launched in second gear. Depending on your truck’s configuration, weight and how aggressive you leave the line, you could go high 12’s in the quarter-mile on tuning alone. With some weight stripped off a regular cab, you could even go 12.50s.

Don’t Forget Tie-Rod Sleeves & Traction Bars

Before you turn 1,000 lb-ft of torque loose at all four wheels, make sure you perform one vital upgrade: install tie-rod sleeves up front. Sleeves can be installed in less than 15 minutes, cost less than $100 and keep the factory tie-rods from bowing during boosted 4x4 launches (which leads to harsh toe-in or even breakage). As for the rear of the truck, it behooves you to install a set of traction bars or Caltracs. Trust us, your U-joints, leaf springs and AAM 1150 will thank you. Plus, without axle wrap or hop you’ll go even faster!