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Performance Roadblocks of the LMM Duramax

Easy, tune-only horsepower continues with the LMM Duramax, the version of the 6.6L diesel V8 GM produced for its new body style (GMT900) ’07.5-’10 HD trucks. Like each generation of the Duramax that came before it, 500rwhp can be had with nothing more than a hot calibration uploaded to the ECM. Unfortunately, all the same roadblocks that exist for the ’06-’07 LBZ live on in the LMM platform—along with a lot of the same parts. Granted, the ECM and emissions equipment are different, but the same basic engine architecture is the same. The connecting rods and pistons are identical, and the only difference in the cylinder heads is that they were treated to slight improvements in coolant flow.

In broken-record fashion, however, the LMM’s 500rwhp tune-only capability can’t be reliably enjoyed with the factory Allison transmission behind it. Once again, the pay-to-play phrase applies in that you have to build the six-speed automatic to handle more power (or rather the 950 to 1,000 lb-ft of torque that a 500rwhp truck makes) before you can savor it. From there, freeing up exhaust and intake flow through the install of a larger downpipe and 3-inch Y-bridge helps cool off the engine while adding a few ponies. Somewhere around the 530rwhp mark, the LMM is out of turbo, and soon after that it’s also in need of a larger displacement CP3 and bigger injectors.

For more on the performance obstacles you’ll face with your LMM (as well as how to overcome them), keep reading.

Roadblock #1: Allison 1000

Allison Automatic Transmissions

With the Duramax operating at factory power levels, the commercial-grade six-speed Allison 1000 automatic is hard to beat. But with as little as 120 additional horsepower stacked on top of it (an easy feat to pull off given the engine’s potent Bosch common-rail injection system), the Allison can incur damaged. The Allison’s torque converter and C3 clutches are known weak links with added power in the mix, not to mention it does not like being forced to shift fast.

The Goals of Any Allison Build: Improved Strength and Efficiency

Performance Allison Transmission LMM Duramax

If you’re after making high horsepower while running big wheels and tires, the factory Allison won’t last a week under a heavy right foot. To make sure yours lives, get it to a reputable Allison builder that can fortify it properly. Upgraded clutches, a properly-spec’d triple-disc torque converter, performance valve body and higher line pressure should all be part of an effective Allison build (along with a tuned TCM). And if you have plans to pursue big power, billet shafts, a billet P2 planet and C2 hub and a billet stator converter should all be considered.

Roadblock #2: Restrictive Downpipe

LMM Duramax Factory Turbo Downpipe

With the LLY, LBZ and LMM all essentially utilizing the same variable geometry turbocharger, the Garrett GT3788VA, it stands to reason that the same, restrictive downpipe is employed as well. Because the factory VGT sees plenty of heat with a 500rwhp-capable ECM tune, it pays to free things up on the exhaust side with a larger (non-ovaled) downpipe.

3-inch Downpipe

Duramax LMM Diamond Eye Downpipe

A true, 3-inch diameter downpipe can drop exhaust gas temperature (EGT) as much as 150 degrees F on an LMM. And when you add thermal wrap, such as what Diamond Eye Performance and ProFab Performance do with their product, exhaust gases within the downpipe are kept hotter (for improved scavenging) while radiant heat at the back of the engine is decreased.

Roadblock #3: Choked-Down & Leak-Prone Y-Bridge

LMM Duramax Factory Y-Bridge

Like the LBZ, many LMM owners consider upgrading away from the small-ish factory Y-bridge (the Y-shaped piece that feeds the cylinder heads) for a little something extra before they spring for a larger turbo. To be clear, you can make plenty of power on the factory Y-bridge, but if you’re seeing considerably more boost than stock the two-piece OEM version could split or blow apart at any time.

3-inch Diameter, 1-Piece Y-Bridge

Screamin Diesel Performance LMM Duramax Y-Bridge

For optimum flow, 3-inch diameter Y-bridge kits with one-piece or bolt-together two-piece designs are common upgrades among Duramax owners. The air volume increase that comes from running a larger Y-bridge provides a decent bump in top-end power (20 to 30 hp), most noticeable above 2,500 rpm. Quality Y-bridge systems are available from Screamin’ Diesel Performance, Wehrli Custom Fabrication, ProFab Performance and Pusher Intakes, with optional 3-inch diameter hot-side and cold-side intercooler also available to further open up the intake tract.

Roadblock #4: Stock Turbo, Injectors and CP3

Garrett GT3788VA Turbocharger LMM Duramax

Lifting your LMM out of the 500 to 530rwhp range is going to call for a larger turbo—and injector and CP3 upgrades if you want to see more worthwhile gains. We have to hand it to the stock Garrett GT3788VA though. It’s a tough turbo that can support gains in excess of 200rwhp without throwing in the towel, even though it’s moving a ton of hot air at this point, is on the verge of overspeeding and waved goodbye to its efficiency range long ago.

Drop-In VGT Upgrade

Fleece Cheetah 63mm VGT

To wring every last drop of power out of the factory 26,000-psi injection system, your money should be invested in a drop-in turbo with a 63 to 68mm compressor wheel and an upgraded turbine wheel (Fleece Performance Engineering’s 63mm Cheetah VGT is shown here). By sticking with a factory-based charger, the stock turbo pedestal, intake piping and downpipe can all be retained, but with more horsepower being gleaned. Once you’re ready to break into 600rwhp territory, it’ll be time to install larger nozzle injectors and a stroker or dual CP3 pumps.

Roadblock #5: Head Bolts & Pistons

Head Bolts LMM Duramax Diesel

Disclaimer: before you dive into injector and CP3 upgrades it will behoove you to address the factory head bolts. Not unlike any other Duramax engine, blown head gaskets can occur after prolonged exposure to 40-psi of boost or more, not to mention the fact that variable geometry turbos create more drive pressure than fixed geometry units (and excessive drive pressure can also blow head gaskets). On top of that, the LMM shares the same pistons as the LBZ, which means you’re on borrowed time when making 650rwhp, especially in a street-driven application.

ARP Head Studs and Conservative Horsepower

Dyno Graph LMM Duramax

Just because LBZ’s and LMM’s are known to crack pistons at 650rwhp (give or take) doesn’t mean you can’t play with fire a bit, or go ahead and upgrade the turbo, injectors and CP3—so long as you tune the engine conservatively. Loads of LMM owners have gone the safe-tuning route and enjoyed their setups for years. Just make sure you grab some ARP head studs and that your EFI Live guru knows to keep you out of the danger zone (i.e. limit torque at low rpm torque).

Building power on a budget? Check out these horsepower recipes to help bring your ’07.5-’10 Duramax up to speed.

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