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A Closer Look at the Chevy LT Engine

Jeff Smith May 26, 2017
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Back in the smog-addled years of the '70s, a base 1975 Corvette 350 wheezed through a Q-jet carburetor on its way to 165 hp with a claimed 8.5:1 compression ratio that was really closer to 8.2:1. How far have we come? Today, the Gen V LT1 base engine for the Corvette snaps to with 11.5:1 static compression ratio and 2-1/2 times the power with 420 hp.

It's a brave new performance world out there. While power has admittedly come at the price of complexity, GM engineers have managed to produce truly impressive gains while also enhancing driveability, emissions, and mileage. If that LT1 doesn't impress you, how about an emissions-legal supercharged LT4 that can make 650 hp on pump gas? That's cause for celebration and perhaps a look into "How'd they do that?"

Accompanying this flood of creative combustion engineering is an avalanche of GM acronyms, some of which you may already know. Let's start with GDI (gasoline direct injection). This is the heart of Gen V. Next is AFM (Active Fuel Management), which is GM-speak for displacement on demand that transforms V-8 engines into V-4s at light throttle. ETC (electronic throttle control) is one you should already be familiar with. Finally, VVT (variable valve timing) has also been around for a while. GM started swinging the cam in the Gen IV small-blocks back in 2005 in search of better emissions, performance, and mileage.

Before we dive inside the Gen V, we're gonna need a scorecard just to keep track of all the variants. There are more family members than just the headliner LT1 and LT4. The littlest member of the Gen V family is the LV3 4.3L V-6, but there's also a truck L83 5.3L and L86 6.2L versions. All these engines are GDI and the V-8s are loaded with AFM and VVT. The 5.3-liter makes 355 SAE horsepower while the truck 6.2-liter is rated at 420 hp with 450 lb-ft of torque.

Keep in mind the current SAE horsepower rating system uses a more conservative correction factor, which reduces the power by roughly 5 percent compared to the traditional "hot rod" correction factor used in the performance industry. That puts the 6.2L L86 closer to 440 hp using our traditional correction factor. Just savor that number for a moment, 440 hp from a small-block workaday truck engine. Adding 5 percent to the LT1's 460 rating pushes it over 480 hp.

The Gen V engines are evolutionarily different, even from their Gen IV predecessor. While some parts interchange, it's best to think of these engines as a whole new branch on the small-block Chevy family tree. That's all because of requirements dictated by GDI. A trio of lobes located at the rear of the camshaft drives a mechanical fuel pump located on top of the lifter valley. This generates a maximum of 2,200 psi of fuel pressure plumbed directly into each combustion chamber.

When the fuel hits the top of the piston just a few milliseconds before spark occurs, the fuel is instantly vaporized. This gives the engineers the ability to finitely control both the arrival time and amount of fuel to be mixed with the air in the cylinder. The advantage to this approach is not only precisely metered air/fuel ratios, but the opportunity to increase efficiency.

This efficiency advantage is clearly announced by the LT1's 11.5:1 static compression ratio. This is unquestionably the highest compression ratio small-block GM has ever built for a production V-8. Even going back to the original L-79 11.0:1 compression 327 from the mid-'60s, compression never exceeded 11.0:1. Those older engines took advantage of this compression because of the availability of excellent, high-octane leaded pump gas. Today, 93-octane is the best you'll see and that's available mainly east of the Rockies. Despite this, GDI allows an engine like the LT1 to make outstanding power using this higher compression ratio.

The combustion gurus have accomplished this by actually reducing the area in which the initial combustion occurs. If you look closely at the piston top on the LT1 engine for example, you'll see a small trough or concave area where the fuel is aimed as it exits the injector. Off to the side of this trough is a smaller but deeper area concentrated in the piston underneath the exhaust valve. This depression is actually where a majority of the combustion occurs. All of this enhances efficiency at the beginning of the combustion process.

Combine this with the ability of VVT to move the camshaft to its ideal location for throttle response and power. Camshaft position is one way to improve engine performance but until digital control arrived, the engine tuner was forced to choose a single position that was a compromise between low-speed torque and high-rpm peak power. But with VVT, GM can now move the cam over an incredible 62-degree range of authority.

Veteran readers might recall the Vari-Cam that was advertised in all the car magazines back in the '60s. The idea was to advance the cam at low speeds and gradually retard the intake centerline as engine speed increased. While the theory was good, the execution suffered. Now fast-forward to the 21st century and we find computers can control a gear on the front of the cam on the VVT engines to accomplish the same task far more accurately.

There are certain limitations to this ability because as the cam position moves, this also affects the timing of the intake and exhaust valves relative to the piston. Advancing the cam tightens the intake valve-to-piston clearance while retarding reduces the clearance between the exhaust valve and the piston. Over such a wide number of degrees, VVT engines must incorporate plenty of valve-to-piston clearance so contact never occurs.

AFM is another computer-control system that also employs engine oil pressure to disable a set of four intake and exhaust valves in alternative firing cylinders effectively turning the engine into a V-4 during light throttle conditions. The system works by first deactivating the exhaust valve in each of the alternate cylinders. The exhaust gas is trapped in the cylinder and then the intake valve is subsequently disabled to prevent adding additional air into the cylinder. This trapped air acts like an air spring to cushion piston upward movement and is offset by the trapped pressure in an alternate disabled cylinder pushing down on the crank to balance the effect. The valves can disable and re-enable in microseconds and the effect is literally seamless to the driver. Of course, fuel and spark are also disabled in these same cylinders.

The key to this system is a set of what GM calls switching lifters that, when triggered, still follow the cam lobe but become what can be called a lost motion device where the lift from the cam lobe and does not transfer lift motion to the pushrod. Instead, the internal piston merely rides in a bore inside the lifter. This is more efficient than merely disabling the fuel and spark to the cylinder because it eliminates pumping losses inside the cylinder that would occur if the valves continued to open and close.

AFM has been in LS engine service since the 2005 model year and there have been some issues with lifter failure in performance engines since the AFM lifters are more complex and heavier than their standard counterparts. Service complaints regarding failures to reactivate are most often tied to oil feed issues due to clogged inlet screens. This is generally tied to poor or non-existent oil change service intervals.

The benefit from AFM is supposed to be in the neighborhood of a 5-7 percent increase in fuel mileage. Given that many performance enthusiasts are willing to forgo that minor mileage gain, there are kits available to disable the AFM feature by replacing the eight lifters and removing what is called the lifter oil manifold assembly (LOMA) and blocking off the AFM pressure passages.

Moving past AFM's complexity, we've saved the best for last to look at the horsepower leader of the GDI pack the 6.2L LT4. This beast is rated at 650 hp at 6,400 rpm and 650 lb-ft of torque at an amazing 3,600 rpm. That puts its powerband, the spread in rpm between peak torque and peak horsepower, at nearly 3,000 rpm. That's impressive because most normally aspirated engines are doing well to manage a 1,500-rpm power band.

Perhaps the LT4's most endearing aspect is the amazing amount of torque this motor generates at normal driving speeds. Employing a small, 1.7L Eaton twin-rotor supercharger (TVS-1740, or 1,740 cc's per revolution), this motor makes 90 percent of maximum torque beginning at 2,500, maintaining that level until 5,400 rpm. That torque is what you will feel when you plant your foot in the throttle.

One reason this engine can make such outstanding power over such a broad band is the combination of its high static 10.0:1 compression ratio, a small supercharger that spins faster than larger blowers, combined with VVT to take advantage of the blower's ability to pack air into the cylinders at lower engine speeds. Keep in mind that these are power numbers accomplished with pump gas octane fuel.

Some of that power comes from a built-in intercooler located between the supercharger's outlet and the intake ports. An external pump, coolant reservoir, and small intercooler help minimize the heat created from the conservative 9.7-psi boost. This is critical because even on a relatively cool day, blower discharge temperatures can reach 220-plus degrees F. The intercooler is capable of mediating this down to a much more manageable 120 degrees F.

There's a package for both the LT1 and LT4 engines that offers either a four-speed 4L65-E automatic or even the eight-speed auto along with a factory-matched torque converter, wiring harness, electronic throttle pedal, and a few other parts. If you need further convincing, Chevy also ties it together with a 24-month/50,000-mile warranty.

In the not-so-distant past, finding a street engine that could make 500 rwhp demanded compromises in driveability, throttle response, fuel mileage, and acceptable street manners. Frankly, normally aspirated engines like that were generally huge and rude. Today, we can have all of that power combined with late-model attributes that are actually a pleasure to plant your foot into. While the technology may be a bit imposing at first, that brave new world is also plenty powerful!

This is a first in a series of stories on the LT family of Chevrolet Performance motors. The series will deal with how the LT works and what it will take to drop it into your hot rod no matter what year it is.

01 Chevrolet LT1 LT4 Engine 2/37

A Gen V engine might still pass for its older cousin. But there's much more to this book than its cover. Thanks to GDI, this latest generation small-block is more powerful with higher compression, while still more fuel efficient.

02 Chevrolet LT1 LT4 Engine 3/37

The biggest change with Gen V is direct fuel injection into the cylinders. With the LT1's intake manifold removed, the rear-mounted high-pressure fuel pump sits atop the lifter valley cover and is driven by fuel pump lobes on the camshaft. Hard lines are used to transfer the fuel to the in-cylinder injectors because the fuel pressure can exceed 2,200 psi. Also note the large rectangle intake ports.

03 Chevrolet LT1 LT4 Engine 4/37

While everybody talks about the LT1 and LT4, there's a whole family of GEN Vs, including 5.3L and 6.2L truck engines. This is the 6.2L L86 version that could easily become the future underdog deal of the decade. Despite its journeyman-like outward appearances, it is internally identical to the LT1 right down to its 11.5:1 compression ratio and cam timing.

04 Chevrolet LT1 LT4 Engine 5/37
05 Chevrolet LT1 LT4 Engine 6/37

At first glance, when it comes to the LT1 cylinder heads, the reversed valves and their splayed relation are one of the most noticeable differences when compared to the LS series. The reality is that every square inch of the LT1 head has been redesigned to support the direct-injection fuel system, from the piston topography to the combustion chamber to the valve locations to the intake and exhaust ports to the spark plug and fuel injector location, all yielding a very efficient 11.5:1 compression.

06 Chevrolet LT1 LT4 Engine 7/37

This combustion chamber view clearly reveals the position of the mechanical fuel injector nozzle opposite the spark plug. It's hard to see but each nozzle contains six smaller holes that help the transition from liquid fuel to vapor, which burns much more completely.

07 Chevrolet LT1 LT4 Engine 8/37

The heart of GDI is using extremely high pressures to inject fuel directly into the cylinder just before ignition. This allows increasing the static compression ratio, which also increases efficiency and power. We've come a long way from Flatheads and Stromberg carburetors.

08 Chevrolet LT1 LT4 Engine 9/37

There's no doubt the direct fuel injection system played a part in the design of the combustion chamber of the LT1. From the valve size (2.130-inch intake/1.590-inch exhaust) to their layout (reversed as compared to the LS) to their 2.5-degree splay to the spark plug and fuel injector location, everything was engineered to meet the criteria of the LT1 engineering team. Over 6 million hours of CPU time was dedicated to analyze the hundreds of initial iterations of the combustion system. So advanced was the development that an SAE paper was written covering the process.

09 Chevrolet LT1 LT4 Engine 10/37

The billet steel Gen V camshaft is unlike its predecessors, using a front-mounted variable valve timing control mechanism while at the rear employing three lobes to drive the mechanical fuel pump. All Gen V engines are built with variable valve timing that offers an enormous swing of over 60 degrees of authority over the camshaft position. This is carefully engineered to improve low-speed torque and top-end power so it's not necessarily a bad thing.

10 Chevrolet LT1 LT4 Engine 11/37

The Gen V front accessory drive is completely different than the LS. The biggest change for passenger car engines like the LT1 and LT4 is the missing hydraulic power steering pump. The Gen V truck engines, however, do retain the hydraulic pump.

11 Chevrolet LT1 LT4 Engine 12/37

The heart and soul of the LT1 engine lies in the completely redesigned, CNC-machined 391 precision cast-aluminum block. It still retains the traditional 4.40-inch bore centers and cam-in-block design, but that's where the similarities end. Cast-in-place cylinder liners result in an all-aluminum deck face with induction liner heating utilized for dimensional control. This results in an exact placing of every cylinder in every block. Nodular iron main caps, retained via six cross-bolts, replace previous efforts where powder metal was the norm.

12 Chevrolet LT1 LT4 Engine 13/37

The Gen V aluminum block maintains the bore spacing and main dimensions, but makes minor changes to items like the motor mount bolt pattern as well as moving the top's bellhousing bolt location.

13 Chevrolet LT1 LT4 Engine 14/37

In addition to the redesigned main caps, every cylinder features oil-spray piston cooling, where eight oil-spraying jets in the block drench the underside of each piston and the surrounding cylinder wall with an extra layer of cooling, fiction-reducing oil. This reduces piston temperature, promoting extreme output and durability.

14 Chevrolet LT1 LT4 Engine 15/37

The LT1 crate engine will be offered as a "wet-sump" setup as well. A redesigned windage tray will be utilized as well as a new oil scraper designed to enhance performance and efficiency by improving oil flow control and bay-to-bay crankcase breathing.

15 Chevrolet LT1 LT4 Engine 16/37

Another evolutionary step is the Gen V exhaust port flange is different enough from the previous LS family that headers won't interchange.

16 Chevrolet LT1 LT4 Engine 17/37

In addition to stock crate engines, Chevrolet also offers the LT376/535 crate engine, which is a hopped-up LT1 with CNC-ported heads and the LT1 HOT cam.

17 Chevrolet LT1 LT4 Engine 18/37

The LT4 is the hero version with its 1.7L supercharger and rated at a thumpin' 650 hp in the Corvette version a dry sump oiling system. It's available in a crate engine package but requires some skill to package in older cars due to dry sump connections, all the extraneous control and wiring harnesses and external coolers for the supercharger.

18 Chevrolet LT1 LT4 Engine 19/37

Swapping in the LT4 into older cars is very popular. This is an LT4 nestled into a 1967 Chevelle with the swap performed by the fabricators at Galpin Auto Sports in Van Nuys, CA.

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