GM 6.6L L8T Performance Upgrades Guide

GM released the 6.6L L8T gas engine in 2020, producing a solid 401hp and 464lb-ft. of torque. While these are great power numbers, they fall a bit short of the L5P diesel alternative. Fortunately, the L8T has some gas left in the tank, and a handful of performance upgrades can add some decent horsepower gains.

In this guide we’re going to discuss tuning, headers, intakes, and superchargers as a few of the best performance modifications for the L8T. But first, let’s discuss engine power limits as we run into a few weak points starting around 600hp.

Engine Power Limits

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

• 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) ECM Tuning

Tuning is going to offer the biggest power gains without getting into more advanced upgrades. With just tuning, you can add 10-15% horsepower and torque without the need for any other mods. 

GM deliberately under-tunes 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. 

HP Tuners is one of the few solutions available for tuning but is going to be the best – it just gets a little pricey as the L8T requires an ECU unlock for tuning.

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) Long-Tube Headers

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. 

Getting some headers with high-flow cats is going to be the best route for emissions compliance. Some headers will also have have either a Y-pipe or X-pipe as optional for some additional power gains. You can expect gains of around 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) Cold Air Intake Upgrade

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, but bringing more air in never hurts.

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. 

4) Advanced: 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 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 Whipple Supercharger kit is capable of producing 700 lb-ft of torque from a front entry Gen 5 W185ax 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 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. 

Frequently Asked Questions