Top L8T Performance Upgrades Guide

Even though it’s a relative newcomer to the scene, the GM L8T 6.6 liter V8 engine is already making a name for itself. GM introduced the engine in 2020 in the Silverado and Sierra HD trucks, and it is a fantastic power plant. From the factory, the L8T makes 401 horsepower and 464 lb-ft of torque. While it is already very formidable stock, compared to its 6.6 liter diesel counterpart the L5P, it is severely underpowered. That’s why almost from its debut, drivers have had their eyes on L8T performance upgrades. Read on to find out the top 4 best L8T performance upgrades, from intakes to superchargers. 

GM L8T Engine Info and Vehicles

GM has had the L8T in production since 2020 inside the Chevrolet Silverado HD and its GM counterpart the Sierra HD. In 2021, GM added it to the Chevy Express/GMC Savana full-size vans. The L8T is essentially a larger version of the 6.2 liter L86/87 V8s, which GM had based on the LT1/LT2 inside the Corvette. The L8T uses direct injection fueling and is 400 cid. It uses a 16-valve, pushrod actuated, overhead valve train (OHV) with two-valves per cylinder. It uses variable valve timing to improve performance and fuel economy.

The compression on the L8T sits at 10.8:1, which is down from the 11.5:1 of the LT1 on which it was based. This is due to a larger combustion chamber on the L8T, but also so that it can run 87 octane gasoline. Unlike other truck engines, the L8T does not utilize Active or Dynamic Fuel Management. AFM/DFM is also known as “cylinder deactivation,” and it allows the engine to “turn off” half its cylinders at low engine loads to improve fuel economy and emissions. 

While the L8T is already a powerful engine at 401 horsepower and 464 lb-ft of torque, it pales in comparison with its diesel counterpart: the 6.6 liter L5P V8. The 2023 L5P puts down more horsepower with 445 ponies, but also doubles the torque with 910 lb-ft. Obviously, you won’t hit those torque numbers easily with the L8T, but you can definitely make it more formidable with some bolt ons.

GM L8T Performance Upgrades: Power Limits

Before we talk about modding, let’s look at how robust the L8T engine is and how much power it can handle. Within a few years of its release, famed engine builder Scoggin Dickey put on a twin-turbo kit and pumped out more than 2,000 horsepower from the L8T. However, they had to make some serious internal upgrades to handle the strength. 

GM built the L8T to be a very strong engine capable of taking some serious punishment. The block is cast iron, the crankshaft is forged steel, and the rods are forged powdered-metal. The pistons are hypereutectic aluminum. The L8T is a very sturdy engine, but it’s not indestructible. The cast iron block is very strong and robust, and should easily make more than 800 horsepower without having any issues. 

The forged powdered metal connecting rods are also very strong, and should be good through similar power limits. The pistons being hypereutectic aluminum will not have the same resistance. Those should be upgraded closer to 600 horsepower with the L8T. The crankshaft is forged steel, and again should be able to withstand similar power as the block and connecting rods. We’ve already seen that upgraded blocks and internals can handle 2,000 horsepower. So you can pretty much build this engine to make whatever you want, provided you have the funds. 

Top L8T Performance Upgrades

Our top 4 L8T performance upgrades are:

• Tuning and Ethanol
• Long-tube Headers
• Cold Air Intake
• Superchargers

For our 5 best L8T GM 6.6 gas performance upgrades, we’re looking at tuning, headers upgrade, camshaft upgrade, cold air intake upgrade, and even superchargers. The L8T is still pretty much a brand new engine, so the aftermarket community is playing catch up. Still, you can add some solid performance to the L8T with a few bolt-ons. As of now, there are no available aftermarket L8T camshaft upgrades, so they unfortunately did not make the cut. 

With tuning and bolt-ons you can get a solid power bump out of your L8T. Or, if you really want to shoot for the skies you can always slap on a blower. Let’s take a look at the top GM gas 6.6 performance upgrades.

1) L8T Performance Upgrades: Tuning

For the first of our L8T performance upgrades, we’re looking at L8T ECU/ECM tuning. Usually, we recommend tuning as one of our top mods on pretty much any platform, and the L8T is no different. With just tuning, you can add 10-15% horsepower and torque without any other bolt-on mods. 

From the factory, even though the L8T already makes more than 400 horsepower and torque, there is still room for so much even with the stock parts. GM/Chevy deliberately under-tune their vehicles from the factory for a myriad of reasons. Some of them are necessary trade-offs for things like emissions and fuel economy, but not all of them. 

With an aftermarket tune, you can increase things like camshaft timing, ignition timing, fuel pressure, and other variables, to increase horsepower and torque. If you add other mods, like the headers, camshaft, intake, or blowers we cover below, then you need some sort of ECU tune to help your engine compensate for the new parts. 

GM 6.6 Gas Performance Upgrades: Best Tuners

As is the case with many newer vehicles, it’s not the easiest thing in the world to tune the L8T. The L8T is powered by the GM 93 ECM, and GM/Chevrolet locks the ECM from the factory.  The primary reason for this is that GM doesn’t want you to increase emissions or add more performance than stock. GM tested the engine with only their stock calibration, and poor tunes can lead to catastrophic failures. 

That’s why it’s important to make sure you only have reputable and respected tuners touch your L8T. With the relative dearth of E-tuning options, unfortunately, L8T owners do not have a lot of tuning solutions. Due to the GM ECU being locked, the only real solution is to use HP Tuners (L8T tuning from HP Tuners). Through HPT, you can either have them unlock your ECM or you can buy a new one from them (HP Tuners L8T ECM unlock). 

The only other solution is a completely stand alone ECM. But those are prohibitively expensive (for most people) and are really only for heavily modified builds with substantial mods. As of this writing, EFI Live does not support the 6.6 liter L8T, making HP Tuners really your only option for serious tuning. 

We suggest you stay away from things like pedal commanders or other gimmicky “chip” upgrades. They only change the throttle mapping and do not actually add any power, despite some claims otherwise. In addition, most handheld tuners will not work on the L8T due to the locked ECU. 

L8T Ethanol Tuning

While you can stick with pump gas for your L8T, if you have access to E85 you have the opportunity to make a whole lot more power. E85 effectively has an octane rating over 100. This means it is incredibly knock resistant, which allows tuners to run leaner air-to-fuel mixtures without encountering detonation. Leaner AFR mixtures means better power, and ethanol provides the opportunity to run more advanced ignition timing (or boost). 

Since there are no flex fuel kits available, you can either source your own or calculate the mixture yourself at the gas pump. Most direct injection systems are ethanol compatible but struggle to run mixtures greater than E60. Still, even with an E60 mix and tuning you will see much bigger gains in horsepower and torque vs gas. In addition, it’s cheaper at the pump and more environmentally friendly. You will however suffer worse gas mileage. 

2) L8T Performance Upgrades: Headers

Next up we’re looking at our first bolt-on for the L8T: Long-tube headers. Headers in their OEM form are known as exhaust manifolds, and they are the first step in the exhaust system. Headers connect directly to the cylinder heads on the L8T, and they coral exhaust gasses as they exit the engine. The stock exhaust manifold is made from cast iron, and is not only heavy but is also restrictive. 

The main causes of restriction on the OEM headers are too small diameter of piping and the OEM catalytic converters. From the factory, GM wants to put the smallest exhaust manifold possible that still makes 400 horsepower, so that it can control emissions. A bigger exhaust means more emissions, and it’s too much for OEMs to make them federally compliant. In addition, the factory catalytic converters are very restrictive due to the high cell count. They are extremely effective at filtering out negative emissions, but that causes them to restrict power. 

Upgrading to long-tube headers removes much of the restriction from the OEM headers. Not only is the diameter of the headers larger, but they replace or remove the factory catalytic converters. The best option is replacing them with high-flow catalytic converters, as these retain emissions components for street legality. 

GM Gas 6.6 Performance Upgrades: Best Silverado/Sierra 6.6 Headers

There are not a ton of options for long-tube headers on the L8T, but we recommend the L8T Stainless Works long-tube headers. These headers use high-flow catalytic converters, and connect to both the stock cat-back or Stainless Works cat-back. Compared with stock, the SW headers have larger primaries and a larger merge collector. The merge collector is a full 3”, and the primaries can be had as either 1 ⅞ “ or 2”. 

These headers have either a Y-pipe or X-pipe as optional, as they are manufactured from high quality 304 stainless steel. They are high quality headers, and will deliver gains of 10-20 horsepower and torque, and possibly more with tuning. Tuning is highly recommended for long-tube header upgrades, as the ECU has the potential to run lean and detonate without. 

3) GM Gas 6.6 Performance Upgrades: Best Cold Air Intake

For our third mod we suggest cold air intakes. We’ll be honest, cold air intakes will not gain a ton of horsepower or torque. The OEM intake is already pretty good as is technically a cold air intake due to the filter placement. It can flow well past 500 horsepower without any issues, making intakes more of an aesthetic mod.

Aftermarket intakes will have slightly larger intake tubing and a larger cone-style filter. That allows for greater airflow into the engine, so you can see a small bump in performance if you combine the intake with tuning. Still, things won’t be drastic, and will likely fall in the 10-15 horsepower range, even with tuning. 

Best Silverado 6.6 Intakes

There are already several options on the market, and we recommend the L8T S&B cold air intake. S&B is one of the biggest names in truck intakes, and their L8T unit looks to be solid. It comes with an airbox and either an oiled or dry filter. It flows 16-18% better than the OEM unit, with the oiled filter flowing a bit better. Still, we always recommend dry intake filters to eliminate any potential MAF issues. 

4) GM Gas 6.6 Performance Upgrades: L8T Supercharger Upgrades

For our final L8T performance upgrade, we’re looking at superchargers. While superchargers are pretty big mods to make, with a big-block like the 6.6 liter (400 cid) L8T they are cost effective ways to get big power. The engine can more than take the power put out by modest supercharger kits, and all that really needs to be upgraded is the fueling – but only once you really turn up the boost. 

So far, the only supercharger kit available was just released by Whipple in June 2023. The L8T Whipple Supercharger kit is capable of producing 700 lb-ft of torque from a front entry Gen 5 W185ax (3.0 liters/3000cc) Whipple twin-screw supercharger. It comes with a large air-to-water, bar & plate intercooler, as well as a bypass system to reduce parasitic loss. 

Incredibly, the L8T Whipple supercharger is actually CARB approved with an EO number, making it 50 states emissions legal. You can get the cover in either red, orange, white, or black, and the kit looks fantastic. You will definitely need custom tuning for an L8T supercharger upgrade, and boy can you make some serious power. 

Top GM 6.6 Gas Performance Upgrades FAQ