To Smoke A Muscle Car…LLY Duramax Edition

• November 5, 2020
• Story By
  Mike McGlothlin

If you read our piece on how to bring the LB7 up to speed, you’re in luck…the LLY Duramax that came in the ’04.5-’05 GM HD’s is very similar in terms of the type of power it can turn out. With one simple mod, your ¾-ton Silverado or Sierra can be sitting at 530rwhp, effectively doubling what it made stock. With a few other additions, you can squeeze a few more ponies out of the factory injection system and turbo and be able to enjoy that power for the foreseeable future.

On top of having a similar injection system and an engine that can support a 100-percent increase in horsepower and torque, these trucks are also the featherweights of the diesel segment. Crew cab short bed models can weigh-in under 7,000 pounds while standard cab versions are even lighter than that. Long story short, a tuned LLY can run neck-and-neck with a more powerful Ford Super Duty or HD Ram thanks to its weight advantage—and it certainly helps when you’re pitted against today’s 12-second-capable factory production cars. To win a fight with either class of vehicle, find out what it’ll take below.

With Custom Tuning, Horsepower Can Double

In the hands of a capable and knowledgeable tuner, 530rwhp can be wrung out of an LLY Duramax-powered truck. EFI Live is typically the weapon of choice in this kind of effort and the top calibrators have things nailed down to a science at this point. To get into the 530rwhp realm (which effectively doubles the LLY’s factory rear-wheel horsepower output), injection timing is advanced, rail pressure is raised and injector duration is lengthened, among other tweaks.

EGT Help: Turbo-Back Exhaust

Because aggressive custom ECM programming—in conjunction with the factory turbocharger—inevitably raises the exhaust gas temperature the engine sees, it pays to help rid the turbo of heat any way you can. By opening up the post-turbo exhaust path, it definitely helps. In the aftermarket, aluminized 4-inch diameter, turbo-back exhaust systems for the LLY typically run in the $300 range. And lucky for LB7, LBZ and even LMM owners, the same systems that work on the LLY will fit any ’01-’10 Duramax (albeit a few minor tweaks).

Add A Lift Pump (Like Now!)

It might not add 50 hp, but it does provide an edge in both horsepower production and injection system longevity. We’re talking of course about a lift pump—something all ’01-’15 Duramax power plants left the factory without the benefit of. With an entry-level fuel supply system from FASS, AirDog or Fuelab in the mix, 8 to 10 psi worth of low-pressure fuel supply is on tap for the CP3 injection pump at all times. Trucks equipped with such systems always pull harder up top and every Duramax we’ve seen break into the 12’s in the quarter-mile was graced with an aftermarket fuel supply system of some kind.

There’s Just One Problem…

Thanks to the tremendous potential wrapped up in the high-pressure common-rail injection system and stock turbocharger, along with heads and a bottom end that will support 530rwhp, means the LLY has a lot going for it. Unfortunately, no vehicle is perfect, and the ’04.5-’05 GM’s suffer from the same problem the ’01-’04 trucks do: the Allison 1000 transmission won’t survive that kind of power. In fact, when subjected to a 500 to 530rwhp tune even the healthiest five-speed Allison will eventually slip. The only real way around this is to pull the commercial-grade Allison, tear it down and have a reputable transmission builder pack it back full of better parts.

Lighter Curb Weight = Less HP Required To Go Fast

In the world of diesel pickups, Fords and Rams were really the only heavyweights back in the classic body Chevrolet and GMC era (’01-’07 as it pertains to the Duramax). The GM’s can best be remembered as the cruiserweight of that particular time period. Simply put, they were light—especially as compared to ’05-newer Super Duty’s. This plays right into your hands if you’re looking to take your mildly-modified LLY to the track. For example, in comparing an ’05 crew cab shortbed 4x4 F-250 to an ’05 Chevrolet Silverado 2500 HD of the same configuration, the Ford needs more than 650 hp at the wheels to break into the 12’s. The Chevy needs 530 to 540 hp…exactly what you can make on an aggressive tune, lift pump and built Allison.

Garrett GT3788VA: One Tough Turbo

One difference between the LB7 and LLY which may help the latter get out of the hole quicker exists in the factory turbocharger. The variable geometry Garrett GT3788VA VVT on the LLY is more responsive than the fixed geometry IHI employed on the LB7. But beyond that, the Garrett VGT on the LLY is also larger than the IHI, especially on the compressor side (62mm vs. 60.6mm). In addition to its larger size likely aiding its capacity to support 530hp, the GT3788VA VVT isn’t notorious for overspeeding like the IHI unit, either. In fact, it’s known to live a lengthy life even with 35 psi of boost in the mix.

You Can Go 12’s

If you happen to be packing a 500-plus hp (1,000 lb-ft) tune on your Duramax and spot a late-model pony car on street tires, get next to it in the staging lanes. With an aggressive launch he or she could spin, while you won’t (so long as you race in four-wheel drive, as you should). Countless LLY-powered trucks running the aforementioned mods (i.e. aggressive tune, lift pump, exhaust, built transmission) have put together high, and even mid, 12-second quarter-mile efforts. It all starts with a boosted launch with the transfer case locked in 4-Hi, which often leads to most GM’s cutting 1.7 to 1.8-second 60-foots.