GM 6.6L L8T Engine Guide

Are you looking for information on the GM 6.6L L8T Gas Engine? This article will discuss GM and Chevy’s new 6.6L gas engine, covering engine details and features, specs, performance potential, and problems and reliability. 

L8T History & Engine Overview

GM’s 6.6L V8 engine is the gas-engine option for 2500HD and 3500HD Silverado’s and Sierra’s. It is a larger and more powerful replacement for the previous 6.0L Vortec engine. The L8T produces 401hp and 464lb-ft. of torque. This represents an increase of 41hp and 84lb-ft. of torque over the predecessor 6.0L L96 engine. 

Released in 2020, the L8T is a gas alternative to the 6.6L Duramax L5P. The L5P produces similar horsepower numbers but nearly doubles the L8T’s torque, producing 910lb-ft. of torque. This gives the diesel engine option more towing capacity. The L8T engine is about $10k less than the L5P, but it also offers only 14mpg on the highway compared to 19mpg from the diesel L5P.

However, the L8T is a great engine for someone looking for a less expensive truck that still has the capability of towing heavy loads. Additionally, it does not feature active fuel management which is a big plus, considering the issues associated with AFM/DFM. 

The 6.6L V8 L8T is built on a cast-iron small block which provides additional durability compared to aluminum blocks, at the sacrifice of weight and fuel efficiency.  Additionally, it has new aluminum heads and direct fuel injection with a compression ratio of 10.8:1, allowing it to run on less expensive 87 octane fuel. The L8T engine also features forged steel crankshafts as well as powder metal connecting rods providing added strength for demanding performance and durability.

Engine Specs

	6.6L L8T V8 Gas Engine
Production	2020-Present
Cars	2500HD/3500HD
Displacement	6.6L (400.55 cu.in.)
Type	Natrually-aspirated V8
Bore	103.25mm
Stroke	98.0mm
Compression	10.8:1
L8T Firing Order	1-8-7-2-6-5-4-3
Block	Cast iron
Head	Aluminum
Crank	Forged steel
Cam	Billet steel
Rods	Forged powder metal
Injection	Direct injection
Valvetrain	16-valve OHV
Oil Capacity	8.0 quarts
Fuel	87 octane
Redline	5,600rpm
Weight	720lbs.

Performance Capabilities

Producing 401hp and 464lb-ft. of torque, the L8T produces enough power for what it’s built for. The L8T has more displacement, longer stroke, and a forged crank all for the purpose of generating low-end torque. While it can’t tow the 36,000lbs. that the Duramax can, the low-end torque makes the L8T capable of pulling nearly 18,000 pounds. Overall, the 6.6L is built to be a heavy work and fleet truck.

While these power levels are sufficient for the majority of its applications, is it capable of more? Unfortunately, these L8T engines are very new, and the encrypted ECM’s make for limited aftermarket support. Additionally, the market is more limited for L8T mods considering the alternative diesel option. There are likely some gains to be had through headers and exhaust upgrades, but options on the market are limited for now.

Where the L8T will thrive is in the hot-rodding community as a crate engine. L8T crate engines are already available in 500hp+ configurations from various performance shops. Mast Motorsports offers a 500hp and 550hp crate engine option for people looking to swap an L8T into their vehicle.

Ultimately, performance potential of the L8T in the 2500HD/3500HD is going to be limited by aftermarket support and difficulty cracking the encrypted factory ECM. However, there is promise for more performance/aftermarket support for the L8T as a crate engine.

6.6L L8T Common Problems

• Excessive oil consumption
• Exhaust manifold leaks
• Knock sensor failure
• Timing chain issues

1) Oil consumption

One of the most common problems with the GM L8T engine is its high oil consumption. Many owners have reported that their engines consume up to a quart of oil per 1000 miles.

Excessive oil consumption actually affects a lot of different engines and is common amongst direct injected vehicles. On the L8T it is thought that either the piston rings or the PCV valve are the causes of it. Regardless, we haven’t really seen many issues associated with this when properly taken care of. 

Excessive oil consumption can become a bigger problem if you allow your truck to run on low oil levels. We always recommend keeping a few quarts in the truck so that you can fill-up as needed if you get a low oil light during a trip.

A technical service bulletin has been released for this noting that high oil consumption typically occurs while towing heavy loads.

2) Exhaust leaks

This problem hasn’t been very common, but we have heard a few cases of exhaust manifold gaskets going bad. The gasket seals the exhaust manifold to the block’s exhaust ports. When the gasket goes bad it causes an exhaust leak which will noticeably make the exhaust noises louder. You might also notice some exhaust smells inside the cab.

Again, this isn’t really a “common” problem, but the engine is so new that we are resorting to listing problems that we have noticed to occur a few times that could become common down the road.

3) Knock sensor problems

Another common problem with the GM L8T engine is knock sensor problems. The knock sensors are responsible for detecting knocks and pings in the engine, and they help to prevent engine damage. It’s a simple sensor that is cheap to replace and easy to DIY repair. Again, not super common but something that has come up a few times.

A bad knock sensor will give you a check engine light with a P0325 engine code.

4) Timing chain problems

Another common problem with the GM L8T engine is timing chain problems. The timing chain is responsible for keeping the engine’s valves in sync with its pistons, and it can stretch and jump teeth over time.

Any timing chain problems are probably going to be a result of excessive oil usage. If you let your engine run low on oil for a period of time it can starve the timing chain of oil which will cause excess heat to build-up in the timing chain system which can cause the chain to stretch and then jump teeth.

Timing chain failure is few and far between. It isn’t a problem with the engine itself but is more so a problem caused by owners driving on low oil levels due to the excessive oil consumption. If you have a lot of oil consumption, keeps some extra quarts in the car and fill up immediately when the low oil light appears.

Chevy L8T Reliability

The GM L8T engine is so far a very reliable engine. It is built well and the cast iron block and forged crank should make it a very strong and durable engine. The pistons and rods however are not forged which will likely hinder some of the performance potential of the engine, but shouldn’t impact the reliability of a stock 6.6L L8T. The 6L90 transmission in these trucks is really strong so we also don’t have any concerns over longevity of the trans.

Overall, we think this will be a strong engine but it is also too soon to tell. Our caveat here is that this engine is so new that it hasn’t really been around long enough to develop any truly “common” problems. Also, the amount of complete engine failures has so far been very minimal which makes it tough to tell if there are any serious issues plaguing the L8T.

Final Thoughts

While power levels are comparative to Ford’s 7.3 Godzilla, the L8T is a good bit underpowered compared to the upgraded Duramax L5P. The diesel option is better for towing and gas mileage, but the L8T does run on 87 octane which is a good bit cheaper than diesel, helping mitigate some of the fuel economy downsides. Coming in about $10k less than the L5P, it’s a good budget option for those not needing maximum towing capacity.

The L8T is a completely fresh engine design, replacing the previous 6.0L engine option in the HD trucks. The cast iron block and forged crank are important components that will help contribute to solid reliability and performance. However, considering this is a new engine design there are likely to be some flaws, but we have yet to see any major issues plague these engine yet. With that being said, these aren’t the most popular engines and therefore there just aren’t a lot out on the road yet.

One of the biggest, or most important, changes to this engine over the prior 6.0 is that active fuel management was not included. AFM, or DFM as it is now called, has led to a myriad of mechanical problems with the Vortec and EcoTec3 engines. So it is a breath of fresh air that it wasn’t included in the 6.6L gasser.

All in all, we expect this to be a solid engine that will offer good durability and a nice balance between budget and towing capabilities.