One of the more remarkable aspects of the modern performance era is how stout some of the factory engines are. In the past, you couldn’t take a stock block small-block Chevy and expect it to live pushing near four-digit horsepower under an 8-71 blower pumping in a mix of oxygen and alcohol. But in recent years, modern engines like the LS and LT platforms from GM have been able to do it with the stock block, crankshaft, and heads without breaking a sweat. Is this just internet hype and smoke and mirrors with dyno queens trying to push product, or something attainable for the blue-collar weekend racer? We decided to take GMs latest pushrod V8 offering, a 6.6-liter truck engine, and conduct an L8T dyno shootout.

What in the world is an “L8T”?

Since the 2020 model year, GM has offered a 6.6-liter V8 engine as the heavy-duty base gas engine for its heavy-duty pickups under the RPO code “L8T.” It is part of the direct-injected Gen V LT engine family and is the only LT-based production engine that utilizes an iron block. In stock form, the L8T is rated at 401 horsepower at 5,200 pm and 464 pound-feet of torque at 4,000 rpm, perfect for a 3/4-ton or 1-ton truck that needs to tow heavy loads with ease on low-octane pump gas.

When you order your Chevrolet Performance L8T crate engine, this is how it comes. Complete from intake to oil pan, but no accessory drive, exhaust, or engine wiring.

The L8T shares the same 4.065-inch bore as its 6.2-liter brother, but with a 3.858-inch stroke versus the 6.2’s 3.622-inch stroke to gain the extra 24 cubic-inches. The iron block is cheaper to manufacture than aluminum and is better at handling the additional heat and load a heavy-duty Silverado will see in its lifetime. The block shares the same architecture as the rest of the Gen V small-block family, like a 9.240-inch deck height, 4.400-inch bore spacing, main bearing diameter, and even the bell housing bolt pattern out back.

The 6.6 liter L8T also shares the same cylinder head casting as the 6.2 liter L86 and LT1 engines, but with different materials for the valves, like Inconel exhaust valves, to handle the rigors of heavy loads and high heat for long periods of time. The pistons are also made out of hypereutectic aluminum alloy but with a different design to lower compression to 10.8:1, which when combined with the direct-injection EFI, allows the L8T to be run on low-octane gasoline.

What Makes This Engine So Tough?

Usually, when it comes to picking out your next LT engine build, you keep your eyes peeled for marketplace listings of well-used 6.2s out of a sixth-generation Camaro or C7 Corvette, so why bother with a truck engine? Just like the LS truck engines, there can be some serious performance benefits to using a “non-performance” Gen V LT. Manufacturers know that the owner of a gas-powered heavy-duty truck plans to abuse it, so they design it to live with that mistreatment and abuse.

For starters, the L8T iron block is not just a carbon copy of the LT1 cast in iron. The deck is thicker, which translates to better head gasket retention under boost. The cylinders are also cast with thicker walls. Shops are starting to report that after sonic checking the block, there can be enough material to bore these blocks to a monster 4.125-inch bore or more. This means you can easily use the 6.6L block as a base for a big-inch LT, or for us, leave the bore at or near stock for a bulletproof cylinder that can handle all the boost we can throw at it. While this also equates to a hefty weight penalty compared to the aluminum blocks, we can more than make up for that with the ability to make more power.

If you’re towing over 15,000 pounds up a long mountain pass, your right foot is beating on the crankshaft like Mike Tyson. Because of the torture the rotating assembly can see during high-load situations, GM decided that the L8T crankshaft needed more than just a little extra stroke. It is a forged unit that features center counterweights. The advantages of a center counterweighted crankshaft are that it will be stronger with less flex and reduced harmonics, which help during those high loads, but also help at higher RPM.

We will also be testing the capability of the factory direct injection EFI system throughout our dyno testing. We will be utilizing an engine harness from BP Automotive. Note the high quality braided wiring loom and how each connector is properly labeled, making install clean and easy.

When LSX Magazine and Chevrolet Performance reached out to us for the L8T testing, we were excited to be involved! We knew the L8T could withstand big horsepower, and we looked forward to finding its limits. We were also interested in seeing how it would respond naturally aspirated to camshaft changes. – Brian Tooley

Time To Put The Theory To The Test

For our L8T dyno shootout, we teamed up with the crew at Brian Tooley Racing to put these factory parts to the test. Our goal is to find the limits of what stock parts, like the block and crankshaft, can handle under boost. But we also want to see how much power we can make with bolt-on parts naturally aspirated. We will start with a brand new, all-stock L8T crate engine from Chevrolet Performance, strap it down to the engine dyno at BTR with a stock ECU wired to the 6.6L via a BP Automotive standalone engine harness to make some baseline horsepower and torque figures.

After a baseline has been established, we will start testing different camshafts, intake manifolds, and cylinder heads in naturally aspirated form to see if GM’s latest workhorse can also deliver some tire-shredding performance. Then we will end with seeing what the block and crankshaft can handle under some boost. Can factory components handle our torture testing? Or will we hit the limit like small-blocks of old and have to shell out the big bucks for an aftermarket block, crankshaft, and other parts? We will start answering those questions in the next phase of our L8T dyno shootout!

Our bone-stock L8T crate engine strapped to BTR’s engine dyno for some baseline numbers.