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Performance Roadblocks of the LB7 Duramax

The 6.6L Duramax is one of the most celebrated diesel engines on the domestic truck market. When it debuted in ’01 model year GM ¾-ton and larger trucks under the RPO code LB7, it introduced the segment to aluminum cylinder heads and high-pressure common-rail injection—two things that make 500rwhp possible with nothing more than a tweaked ECM (think EFI Live). The icing on the cake is that the Duramax remains durable, quiet and fuel efficient while doing it, not to mention the fact that GM’s HD trucks are lighter than the competition (especially the ’01-’07 body style), which makes them faster.

But even though the LB7 Duramax engine’s factory injectors, CP3 and turbo can support 500rwhp on tuning alone, the factory five-speed Allison 1000 will never last under that kind of power. As such, the expense of building the commercial-grade automatic transmission is the first roadblock you’ll encounter when pursuing big power with an ’01-‘04 Duramax. After that, getting beyond the 500rwhp wall calls for a free-flowing exhaust (namely a larger downpipe), opening up the size of the Y-bridge (to feed more air into the heads), upsizing the turbo and eventually upgrading to larger injectors and a bigger CP3 (injection pump).

For the clearest path past the LB7’s performance hurdles, give the checklist below a solid once-over.

Roadblock #1: Allison Transmission

Allison Automatic Transmission

In stock form (and as-intended), the five-speed Allison 1000 will live as long as the LB7 Duramax in front of it. More than 90rwhp over stock (or perhaps more accurately, 650 to 700 lb-ft of torque at the wheels) and you can push this transmission over the edge. The torque converter clutch, C1 clutches, C2 clutches and the C2 clutch hub itself are all weak links. Any time the Allison experiences excessive slippage in any form, it will trigger limp mode, a condition that limits the damage inflicted on internal components and that also affords you the ability to limp the truck home. So while enough fuel and air is available to support an ECM tune that’s good for 500rwhp, the stock Allison will never be able to harness it.

The Parts Every Allison Needs

Sun Coast Allison Transmission Rebuild Kit

To enjoy the LB7 Duramax’s 500rwhp capability, key areas simply have to be upgraded, and the parts required to pull it off come at a cost. Common upgrades include aftermarket clutches and steels with increased clutch disc counts, a shift kit, fresh bushings, thrust bearings and a new internal wiring harness, with billet flex plate, billet P2 planet, billet C2 clutch hub and billet shafts also being available. Of course, things get started with a quality, triple-disc torque converter.

Roadblock #2: Factory Exhaust

Auto Meter Pyrometer Pillar Gauge

Every diesel breathes easier if you can open up the exhaust system. In the LB7’s case, the turbo downpipe is extremely restrictive from the factory. Enlarging the diameter of the downpipe helps evacuate more exhaust gas, which helps cool EGT and aid turbo life.

Aftermarket 3-inch Downpipe

LB7 Duramax Turbo Downpipe

The most common aftermarket downpipe for the LB7 is a 3-inch diameter version, available from manufacturers such as MBRP, Flo-Pro and ProFab Performance. Each one features the unique bolt pattern that’s required to attach it to the OEM turbo’s exhaust housing. Thanks to increasing exhaust flow by as much as 20-percent, the larger downpipe decreases EGT and improves throttle response. Most LB7 owners install a 4-inch or 5-inch downpipe-back exhaust system at the same time.

Roadblock #3: Restrictive Y-Bridge

Y-Bridge LB7 Duramax Diesel

A primary choke point on the intake side of the LB7 (other than the factory turbo, which we’ll delve into below) is the stock Y-bridge. The Y-bridge sits in the valley and feeds boosted air into the Duramax’s cylinder heads. From the factory, the LB7 Y-bridge routes air through 2-3/8-inch diameter openings.

3-inch Aftermarket Y-Bridge

PPE Duramax LB7 Downpipe

When you open the Y-bridge up to 3-inches via an aftermarket version, the added air volume can lead to gains of up to 30hp. Just as the aforementioned 3-inch downpipe is usually installed along with a complete aftermarket exhaust system, many LB7 owners upgrade to larger, 3-inch diameter hot and cold-side intercooler pipes at the same time they add a 3-inch Y-bridge. PPE (shown above) and WCFab both offer proven 3-inch Y-bridge options.

Roadblock #4: Factory Turbo

IHI Turbocharger LB7 Duramax

Even after building the Allison, adding a 3-inch downpipe and installing a 3-inch Y-bridge, the LB7 will still be tethered to the 500 to 530rwhp range. This is because the factory 60mm RHG6 IHI turbo is out of its efficiency range and, to be perfectly honest, may be on the verge of overspeeding once it sees more than 30-32 psi of boost.

Drop-in Turbo Options

Stealth 64 Duramax Tuner Turbo

Direct replacement turbo upgrades abound in the aftermarket, thanks to the 63mm to 71mm options available from company’s like Fleece Performance Engineering, RCD Performance and Duramax Tuner. These drop-in chargers come with high-flow turbine wheels, are the easiest to install and can support at least 625rwhp.

T4 Single Turbo Kits

HSP S300 Single Turbo Kit Duramax

Another option for breaking the 530rwhp barrier is to pull out all the stops and spring for a completely different turbo system. All-inclusive turbo kits that accommodate the popular S300 and S400 chargers from BorgWarner are available from HSP Diesel, Screamin’ Diesel Performance and WCFab, with company’s such as Wagler Competition Products and Fleece Performance Engineering also offering the parts and pieces necessary to make this type of turbo change possible. An S300-based turbo can support more than 700rwhp, with the S400 being proven to support four-digit horsepower.

Roadblock #5: Stock Injectors & CP3

Common Rail Bosch CP3 LB7 Duramax

Though a turbo upgrade can boost your ’01-’04 Chevy or GMC HD’s horsepower to the mid-500’s, the performance capability of the factory injection system is exhausted at this point. In some cases, a mild injector upgrade such as a 30-percent over nozzle can yield 600 to 630rwhp in conjunction with the factory CP3 injection pump (provided that you have a strong CP3 and a good tuner behind you). However, combining a larger CP3 with higher flowing injectors is the best way to get the most out of your turbo upgrade. Full disclosure: once you do that you’ll need head studs and a backup plan in case the factory connecting rods decide to bend.

Stroker Pump and Larger Injectors

Bosch Fuel Injectors LB7 Duramax

A common recipe for the 700rwhp range entails a 10mm stroker CP3 with 60-percent over injectors, but that also includes the addition of an aftermarket lift pump. Remember, the LB7 (and every Duramax through 2010 for that matter) didn’t come with a lift pump from the factory, so installing a 150-gph to 165-gph lift pump, usually in the form of a FASS, Fuelab or PowerFlo system, will have no issue keeping up with moderate injector and CP3 upgrades.

The Duramax revolutionized the diesel truck scene when it debuted 20 years ago. Travel back in time with us to see how GM changed the game forever.

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