The New GM L8T Engine Is A 401ci Gen-V That Hot-Rodders Will Love

There’s a brand new pushrod V8 on the GM horizon. The General recently let it be known that they will build a Gen V 6.6L (401ci) engine called the L8T. This is a stroker version of the previous 6.2L direct-injected engine and will be initially deployed in the 2020 Chevrolet Silverado HD and Sierra HD trucks. It appears the main push for this engine was to substantially improve the towing capacity of the 3500-series trucks, but as EngineLabs gearheads – we wanted to know more about the engine.

There are scant few insider details but we can pass along what we currently know. This engine is a displacement bump from the current 6.2L (376 ci) L86 and 5.3L (325 ci) L83 engines, which are direct-injected small-blocks used in the 1500-series trucks. The previous engine in the larger 3500-series trucks was a smaller, 6.0L L96 version rated at 360 hp. This new engine sports the larger 4.065-inch bore (as used in al the 6.2L engines) while adding displacement through a simple stroke increase from the current Gen V 3.62-inch reach by 0.240-inch to 3.86-inches. This calculates out to 400.7 ci (rounded up to 401) all wrapped up in a cast iron block with aluminum heads.

Feel The Power

Power numbers for this new L8T are advertised as 401 hp at 5,200 rpm, while torque is up to 464 lb-ft at 4,000 rpm. The RPO-knowledgeable may question why the horsepower is down compared to the smaller 6.2L L86 engine, which for example, is rated at 420 horsepower and 460 lb-ft of torque.

The explanation can be partially explained by the fact that this new 6.6L engine is delivered with a slightly lower static compression ratio. The Gen V naturally aspirated light truck and passenger car engines – such as the LT1 and L86 versions – enjoyed a static compression ratio of 11.5:1 while the L8T will only squeeze with a 10.8:1 ratio.

Because this L8T is intended for heavy-duty truck pulling applications, GM lowered the static compression in order to operate this engine continuously on 87-octane gasoline. One way to think of this is that the increase in displacement helps to offset the lower compression ratio so torque is still available. It’s also commonly accepted that besides adding displacement, a longer stroke generally improves low-speed torque.

This can be backed up by comparing the torque-per-cubic-inch ratings of the L8T to the smaller L86. The 6.2L L86 light truck engine generates a peak torque of 460 lb-ft from 376ci, creating a 1.22 lb-ft torque-per-cubic-inch rating, while the L8T’s 464 lb-ft from 401ci comes in nearly as strong at 1.16 lb-ft per cubic inch. That is a difference of only four-percent. Interestingly, a full point of compression change can be worth roughly three to four-percent power.

In related news, light-duty 1500-series trucks will use a 5.3L L84 engine (shown) that employs all the latest technology including DI, VVT, and Dynamic Fuel Management, where all 16 valves are controlled by the ECU featuring 17 different active engine cylinder combinations for part throttle operation. The 5.3L L84 makes 355 horsepower and is the successor to the L83. The 420-horse, 6.2L L87 replaces the 6.2L L86.

The Nitty Gritty

Because this engine is aimed directly at low-speed torque generation, GM added a forged crankshaft to its list of inherent advantages. Along with forged rods and hypereutectic pistons, strength is added with the use of an iron block.

It has also become obvious in the last few years that GM’s small-block has been subject to increasing sophistication. Additions such as gasoline direct injection (DI), variable valve timing (VVT), and Active Fuel Management (AFM) are the more significant advancements benefiting the Gen V. This L8T engine incorporates all of these, except that it will not be fitted with AFM (or the latest passenger car and light truck advancement now called Dynamic Fuel Management). The L8T will not include either of these cylinder-deactivation valvetrain control mechanisms.

This offers some interesting potential opportunities for the future engine builder on one of these engines. While it’s far too early to assign any specifics, in the past GM’s iron blocks have offered sufficient cylinder wall thickness to allow taking the current 4.065-inch bore out to somewhere between 4.100 and 4.125-inches. Combined with the 6.6L’s 3.86-inch stroke, this would create a 412ci engine that would not be overly expensive to build. Plus with new pistons, it would be possible to also bump the compression.

GM has not announced any details on the cylinder heads and valvetrain but we can assume the valve sizes to be similar to the direct-injection light-duty engines, and it would not be a surprise to find the cylinder heads are very similar – if not outright identical to the LT1 / L86 engines.

This excellent cutaway drawing by David Kimble of the LT1 Corvette engine illustrates the gasoline direct injection position along with a peek into the VVT hardware. Both DI and VVT, but not AFM (a.k.a. DOD)will be employed on the new L8T engine.

The larger bore does offer the opportunity to increase the intake valve size. Current LT1 and L86 engines offer an impressive 2.126-inch intake and 1.590-inch exhaust valve, so we’ll have to wait to see if the 4.065 bore size offers the same increase in valve diameter as the LT1 and L86.

Keep in mind that the current horsepower rating of 401 hp occurs at an intentionally low 5,200 rpm. This indicates, as with all truck engines, that the 6.6L employs rather conservative (by hot rod standards) cam timing all in favor of increasing torque in the truck’s usable towing engine speed range. If we look at the peak horsepower numbers for the LT1 — rated at 460 hp for the Corvette for example — a switch to a more peak-horsepower-oriented intake manifold and longer cam duration would substantially improve these numbers for the performance enthusiast. What all this really boils down to, is that this engine promises to be popular for a hot rod application — with a few modifications, of course.

As an example, we recently had an opportunity to witness a stock LT1 dyno test fitted with headers at Westech Performance. GM’s rated numbers of 460 hp are created using the current but conservative J1349 standard which raises the standard temperature and lowers the corrected atmospheric pressure. This standard reduces the corrected horsepower numbers compared to the previous J607 SAE correction factor by roughly five-percent.

When Westech ran the LT1, with no tuning, the engine produced an exceptional 507 hp at 6,000 rpm with 509 lb-ft of torque at 4,800 rpm. It’s worth noting this test was performed with an electric water pump which eliminated the entire accessory drive and was fitted with 1-7/8-inch headers.

This new L8T Gen-V small-block will debut in the 3500-series 3/4-ton Silverado HD and Sierra HD trucks, backed by a six-speed 6L90 automatic.

The point of all this is that this LT1 made 47 more horsepower than its conservative GM rating using the traditional “hot rod” correction factor that is commonly employed in the aftermarket. This is roughly a ten-percent increase in power. Given the same aftermarket testing procedure for this new L8T engine with headers it’s likely it would deliver another ten-percent or 440 horsepower. And that’s before any modifications other than the exhaust improvement!

GM has not released any other details on this engine in terms of cylinder heads, valve size and the like so those numbers will filter out as production approaches. But it’s clear that there is substantial potential in this engine and it may quickly assume a position of stature in the continuing horsepower wars. This is a good time to be a hot-rodder and a fan of horsepower from pushrod V8 engines.

About the author

Jeff Smith

Jeff Smith, a 35-year veteran of automotive journalism, comes to Power Automedia after serving as the senior technical editor at Car Craft magazine. An Iowa native, Smith served a variety of roles at Car Craft before moving to the senior editor role at Hot Rod and Chevy High Performance, and ultimately returning to Car Craft. An accomplished engine builder and technical expert, he will focus on the tech-heavy content that is the foundation of EngineLabs.
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