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.