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Performance Roadblocks of the LLY Duramax

Many diesel enthusiasts know that every Duramax offered between 2001 and 2016 can support 500rwhp on tuning alone, but not everyone knows the specific hurdles that face each version of the 6.6L V8 in getting there, or moving beyond it. Case in point, while an LB7 owner can enjoy 500 ponies without touching the engine, the LLY is notorious for overheating thanks in large part to a major airflow restriction. If you want to relish the same tune-only performance the LB7 does, adding a freer flowing turbo mouthpiece is one of the key components in making that possible. After that, you can take full advantage of the LLY’s larger turbocharger and squeeze as much as 530rwhp out of your setup.

However, the LLY Duramax’s being equipped with a bigger, 62mm turbo is of little use until you address the Allison transmission. That’s right, the same five-speed automatic that was bolted to the LB7 engine resides behind the LLY—and it needs to be properly prepped for big power before you try to send big power through it. Then, and preferably while you’ve got the transmission out of the way, it pays to ditch the factory turbo downpipe for a larger version (to lower exhaust gas temperature) followed by a high-flow Y-bridge in the valley (to lower air intake temps). Once you’ve had your fill of a reliable 530rwhp, it’s time to invest in a larger turbo, injectors and CP3—but not before you add an electric lift pump and get serious about adding head studs.

Sound like a lot? It’s really not that bad. For the low-down on how to break through each of the LLY’s performance roadblocks, keep scrolling.

Roadblock #1: Allison Transmission

Allison 1000 Transmission

Though there are a few subtle differences between the Allison 1000 found behind the LLY vs. the version employed with the LB7, for all intents and purposes they are the same five-speed automatic. And this means the Allison is still only capable of handling 90hp more than stock reliably and roughly 650 lb-ft of torque at the wheels before limp mode or considerable internal damage occurs. Full disclosure: if you leave your LLY completely stock or adhere to a slight 50-90hp increase the Allison will last forever. The problems arise with aggressive, high horsepower ECM calibrations and especially when you go from light throttle driving to full throttle racing on a whim.

Performance Allison Transmission Build

Performance Allison Transmission Parts Duramax

Being that the Allison used in conjunction with the LLY is basically the exact same as the one parked downwind of the LB7, the same upgrades are on the table when fortifying the commercial-grade slushbox to harness more power. Common internal upgrades include aftermarket clutches and steels with increased clutch disc counts, a shift kit in the valve body, a new internal wiring harness, fresh bushings and new thrust bearings, and in higher horsepower applications a billet flex plate, P2 planet, C2 clutch hub and shafts. In any performance transmission build, power transfer begins with a reputable triple-disc torque converter. Some big aftermarket names in the Allison transmission game are Sun Coast, Goerend Transmission, Inglewood Transmission Service and Merchant Automotive. The Allison building bench at LinCo Diesel Performance, a shop that swears by Sun Coast parts, is shown above.

Roadblock #2: Restrictive Factory Turbo Inlet Manifold

LLY Duramax Factory Turbo Inlet Manifold

It can be argued that the turbo inlet manifold on the LB7 Duramax was an airflow choke point, but on the LLY there is absolutely no denying it. In restricting airflow into the compressor side of the LLY’s turbo, it forces the charger to work even harder to produce sufficient boost. As a result, the LLY produces warmer intake temps, exhaust gas temperature (EGT) and coolant temperature, which often leads to the engine overheating.

Aftermarket Turbo Inlet Manifold

S&B Turbo Inlet Manifold LLY Duramax

Believe it or not, upgrading to a higher flowing turbo inlet manifold (also known as the mouth piece) solves most of the LLY’s infamous overheating problems—along with cooling off intake temps and EGT. The aftermarket version offered by S&B outflows the factory mouth piece by a whopping 78.6 percent. Needless to say, it’s an extremely popular (and invaluable) add-on in the Duramax world.

Roadblock #3: Bottlenecked Downpipe

OEM Downpipe LLY Duramax

Another choke point on the LLY can be found on the backside of the turbo in the form of a flow-prohibitive downpipe. We actually feel bad for the Garrett GT3788VA, an otherwise stout  and efficient variable geometry turbo. It’s starved for fresh air on the intake side thanks to the aforementioned OE turbo inlet manifold and it can’t expel exhaust gases adequately on the other side of things thanks to the factory downpipe.

Aftermarket 3-inch Downpipe

Diamond Eye Performance LLY Duramax Downpipe

Installing a larger downpipe on a Duramax is a bigger deal than you think. With a common aftermarket 3-inch diameter unit in place, LLY owners see as much as a 150-degree F reduction in EGT. Diamond Eye Performance’s downpipe is pictured here, which features CNC-machined end forms for a precise fit to the back of the turbo and it also comes pre-wrapped in thermal header tape. For further improvement in exhaust flow, most Duramax owners add a 4-inch or 5-inch downpipe-back exhaust system at the same time.

Roadblock #4: Factory Y-bridge

Factory LLY Duramax Y-Bridge

Allowing the engine to breathe better is a common theme when upgrading the LLY—but it’s something that yields solid results. Take the restrictive OEM Y-bridge for example, the Y-bridge being what sends boosted air into the cylinder heads. If you open up this passageway to 3-inches in diameter you can gain as much as 30hp. Combine that with the fact that the stock, two-piece Y-bridge is known to leak or even blow apart under elevated boost pressure and you start to realize why so many LLY owners ditch it as soon as possible.

3-inch Y-Bridge Upgrade

3-inch Duramax LLY Y-Bridge

Moving forward with the opening up of the LLY’s intake tract, 3-inch Y-bridge systems are highly popular upgrades. They lower intake air temps, provide a 20-30hp bump in horsepower and can be had fairly affordably. One well-known name in this field is WC Fab, or Wehrli Custom Fabrication. Their fabricated 3-inch diameter Y-bridge is a one piece design and comes with a bead rolled on the inlet so the silicone boot won’t slip and the pipe itself won’t distort when the provided T-bolt clamp is cinched down.

Roadblock #5: OEM Turbo, Injectors and CP3

Garrett GT3788VA Turbo LLY Duramax

At 530rwhp, give or take, the stock Garrett VGT, common-rail injectors and CP3 high-pressure fuel pump are all out of gas. We’ll admit that an upsized turbo will free up a few more ponies and that slightly larger injector nozzles coupled with spot-on tuning (to keep rail pressure supplied by the stock CP3 happy) can wring 600 to 630rwhp out of an LLY, but all three of these areas need to be upgraded to move the ball forward in a meaningful way.

Blown Head Gaskets

Blown Head Gaskets LLY Duramax

Speaking of moving the ball forward, this deserves an honorable mentioned before we get ahead of ourselves. Due to LLY’s running warmer than other Duramax mills, they’re much more prone to failed head gaskets. A lifetime of running on the hot side and then being forced to fuel harder and experience more than 35 psi of boost thanks to aggressive ECM tuning can push them over the edge. When the heads do finally lift, most performance-minded owners opt for ARP head studs to make sure it never happens again.

Drop-in Turbo Upgrade, Larger Injectors & Stroker CP3

Turbocharger Fuel Injectors CP3 Duramax LLY

Things get expensive at this point, which is why more than 50-percent of LLY enthusiasts decide that making horsepower in the mid-500’s is plenty. But should you decide to escalate things further, the Duramax aftermarket is more than willing and capable of helping you reach 650, 750, even 1,000rwhp, provided you build the engine first. In this order: start with a rebuilt rotating assembly with stronger rods, go with a 150 to 165-gph lift pump, throw a larger turbo on and then up-size the injectors and install a stroker CP3. Then re-tune and have fun.

Trying to glean more power out of your LLY Duramax without breaking the bank? Check out our budget mods list right here.

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