What Makes the GM L8T So Special?

I think at this point, it’s no secret that the GM LS and LT are very popular engines, and for good reason. They’ve packaged very small thanks to their cam-in-block design OEM heads that have incredible flow, a simple cam swap can pick up massive power, there are tons of different levels of displacement to choose from, and so much more.

For the longest time, the Gen III, Gen IV, and now the Gen V small block engines were available in only a handful of displacements, including 4.8L, 5.3L, 5.7L, 6.0L, 6.2L, and 7.0L. That is, until GM released the newest LT variant in 2020, which is the 6.6L variant, dubbed the L8T, which was designed as their new heavy-duty alternative to the 6.6L Duramax diesel engine.

Why Did They Create It?

While the previous 6.2L L86 engine was pretty good, GM ultimately wanted to offer a gasoline engine with more torque for their truck customers who want to tow a lot but don’t want to opt for a diesel truck.

And I know you might be wondering, who wouldn’t want to opt for the diesel if they intend to tow? And while I agree with you, there are a handful of things that make a gas truck a more attractive option for very specific buyers, including the price of the truck when new, the price of 87 octane compared to diesel, availability of diesel fuel in their area, and much more.

So, what they ended up doing is taking their Gen V small block design that we’ve seen since the LT’s debut in the 2014 C7 Corvette and simply making it better for heavy-duty use. This involved a new block, new intake, more displacement, eliminating some efficiency features, and more.

All of this led up to a very impressive 401hp and 464lb-ft of torque on 87 octane fuel. But, at the end of the day, it’s still a Gen V small block, which is incredibly similar to the Gen III and Gen IV LS engines we see being swapped into everybody’s project cars. With that in mind, let’s start from the bottom and work our way up.

GM L8T Cylinder Block and Internals

The basis of the L8T is interesting for the simple fact that up until this point, every Gen V LT engine GM has produced uses an aluminum block with aluminum heads, but the L8T uses an iron block with aluminum heads. If you look back at Gen IV and Gen III, iron blocks were used in the majority of truck engines, so it’s certainly interesting to see them return to an iron block with the L8T.

The most obvious reason for doing this would be the extra strength, but that being said, we’ve seen aluminum block LS engines reliably produce over 1000hp and 1000lb-ft of torque in the aftermarket world for decades now. So, you could argue that the extra strength isn’t really needed for a naturally aspirated truck engine, but ultimately that’s why they switched back.

With that new burly iron block, they kept the bore the same as the L86, LT4, LT1, and so on, at 4.06 inches, but that being said, the cylinder liners were extended in length to help support the new 3.85-inch stroke, which is the biggest stroke any OEM Gen III, Gev IV, or Gen V engine, excluding the performance-oriented LSX engines.

The block retains many of its Gen IV/Gen V characteristics, with six fasteners for the cross-bolted mains and a center-placed thrust bearing. Interestingly enough, it’s been reported that the L8T block is only 12lbs lighter than the Chevy Performance LSX blocks, which is definitely an indicator of how strong this block is and what it’ll be able to take when aftermarket parts crank the power up.

Another change they made is the lack of AFM or DFM, which is active fuel management or dynamic fuel management; it’s also void of the start-stop system. All of this means the hydraulic circuits for the lifters are not drilled, but the stands remain inside the lifter valley.

The pistons have a 10.8:1 compressions ratio, with bore and stroke measuring 4.065 inches and 3.86 inches, respectively. For anyone not familiar with the LT engine series, one of the biggest changes GM made going from Gen IV to Gen V is the addition of direct injection, which is ultimately what allows the L8T to run on 87 octane while using a 10.8:1 compression ratio, which would be high for a Gen III or Gen IV engine running 87 octane, but its no problem for the L8T.

Again in the pursuit of engine strength, GM is using a forged steel crankshaft, which is pretty much standard across all LT engines. The crank does differ from its previous smaller iterations of the LT in the fact that it has an additional center counterweight to improve durability.

The connecting rods retain the 6.098-inch length of other LS engines, but unfortunately, they are not interchangeable. Because the L8T has a longer stroke with the stock 9.240-inch deck height, they used a smaller wrist pin diameter compared to the normal Gen V LT engine, at 0.863-inch compared to the standard 0.927-inch.

This was done to allow the standard rod length while avoiding the issues of the wrist pin intruding into the oil ring while still clearing the counterweight with the piston at bottom dead center.

Another interesting point on the L8T is the variable volume oil pump, which was put in place for improved efficiency. This works by decreasing oil volume and pressure at low rpm or idle, and then oil volume is increased as engine load and RPM increase. While this is a great feature for efficiency, it can become a problem at high RPM.

So, for a stock engine, it’s no concern, but if you plan on turning your L8T into a high RPM, 1,000hp engine, it can become an issue in the long run and require switching to a more traditional style oil pump.

GM L8T Cylinder Heads

As I mentioned earlier, the L8T is a direct-injected engine, and because of that, the heads are very similar to any other Gen V cylinder heads. All LT heads have the valve order different as compared to the earlier LS engines, which you can see here with the LS head on the right and the LT head on the left. Since the L8T is a direct-injected engine, it uses heads similar to other Gen V family engines.

One change with all direct-injected heads is the valve order. The older heads placed the exhaust valve at the front of the head, repeating an exhaust-intake pattern. The LT engines reverse this, placing the intake valve forward and repeating an intake-exhaust sequence. This, of course, also changes the lobe arrangement on the camshaft.

Compared to other LT heads, the L8T heads offers an ever-so-slightly larger combustion chamber to help lower the compressions ratio since again, this engine was designed with 87 octane fuel in mind because no one wants to put expensive fuel in their work truck. The larger combustion chamber paired with, the deeper pocket on the pistons drops the compression ratio down to 10.8:1 compared to an engine like the LT1 at 11.5:1.

The L8T heads, like other Gen V heads, feature surprisingly large intake ports with near line-of-sight to the intake valves. The valves on the L8T are the same size as the LT1, with a 2.126-inch intake valve. On the exhaust side of the head, it’s again pretty much identical to any other Gen V head and offers raised exits for improved flow.

The Aftermarket World

Something that is worth noting on the L8T as compared to other LT engines is the dual alternator system, which is generally specific to only heavy-duty applications, which again, makes sense considering the whole point behind the L8T was to put in work reliably and be a real viable alternative to the 6.6L Duramax diesel engines.

On the aftermarket side of things, this is probably the most promising LT engine yet, thanks to its longer cylinder liners and iron block, which ultimately means it can hold up to a lot of power and has the ability to be bored and stroked to a massive amount of displacement, upwards of 7.0L.

For reference, we haven’t seen a 7.0L LS engine since the LS7, so the L8T having the ability to get up that high in displacement is really promising. On the high-end, we’ve already seen engine builders reach upwards of 2,000hp with the L8T, so it’s definitely going to be a favorite in the LT engine family, at least as far as engine building potential.

Summary

And before we end this article, I do want to mention that on paper, the L8T barely appears to be any better than the existing 6.2L L86, with the power outputs at 401hp and 464lb-ft and 420hp and 460lb-ft, respectively. So, on paper, the L8T is making 19 less horsepower and only 4 more lb-ft of torque, but that peak torque is delivered 100RPM sooner.

But that’s just a paper measurement of peak power and torque. What really matters is the area under the curve which is where the L8T’s extra 400cc of displacement will really show. And compared to the old 6.0L truck engine, the L8T is a mile ahead. Although I will say it would’ve been even cooler to see a new rendition of the Vortec 8100 as their alternative to the Duramax, the L8T will have to do for now!