The Duramax LMM diesel engine is largely like its predecessor, the Duramax LBZ, but with Bosch common rail injection and some minor fuel supply modifications. Many of the upgrades we recommended for the LBZ are also some of the best Duramax LMM upgrades you can make.
Depending on what’s added and altered, the engine can be brought up to about 600 to 650 rwhp (above which point the pistons are prone to crack). If you drive a 2007.5 to 2010 LMM diesel engine, here’s what some of the most recommended upgrades will give you.
Heavier Duty Allison Transmission
If you make any serious upgrades to your LMM, you’ll want to change out the stock Allison transmission for one that’s a bit heavier duty. The stock tranny is capable of handling about 400 rwhp, but it doesn’t take much to get above that mark. A customized Allison transmission can be adjusted to handle much higher outputs — just make sure the work is done by a diesel specialist to ensure reliable performance.
Aftermarket Fuel Supply System
Even though some changes were made to the Duramax LMM diesel’s fuel supply, an aftermarket one is still needed to reach the upper ranges of the engine’s potential power. Switch out the stock fuel supply for an aftermarket bolt-on setup that delivers more fuel at a higher pressure, and your engine will have the supply it needs for greater power.
High-End Variable Geometry Turbo
Of course, the other main ingredient an engine needs is air. Upgrading to a high-end variable geometry turbocharger is a significant change, but it’s one that offers almost unparalleled capabilities. It’ll ensure the LMM has all of the oxygen it needs to combust that extra fuel.
Cold Air Intake
A cold air intake will further infuse the engine with oxygen for combustion. By specifically bringing ambient air into the pistons, the upgrade also helps keep everything operating at cooler temperatures.
When you upgrade a fuel supply and turbocharger, changes should also be made at the other end to usher away the extra exhaust gases that are created. Start by replacing the stock downpipe with a bigger, 3-inch one. This particular change is easy (and inexpensive) to make when the transmission is taken out for customization.
Pair that bigger downpipe with a bigger exhaust that measures 4 inches. Together, the downpipe and exhaust will more quickly usher away exhaust gases and help keep everything a little cooler. As a result, you’ll see improved performance across the engine’s full power range.