Two years ago, Chevrolet released its new 2.7-liter four-cylinder engine with an all-new “dual-volute” turbocharger design, dubbed the L3B. However, this undersquare (3.630-inch bore and 4.010-inch stroke) inline-four was destined for the 2019 Chevrolet Silverado, as a replacement for the 4.3-liter V6 option. As such, there wasn’t a whole lot of attention paid to it by the performance circles.
However, when Cadillac announced that the 2.7L four-cylinder with twin-volute turbocharger would be powering the new 2020 CT4-V sport sedan, people started to take notice of the engine. Now that we’re halfway into 2020, the performance side of the 2.7-liter turbocharged engine is being realized first-hand, it’s gaining attention. Luckily for us, one of those whose attention it gained was Jason Fenske of Engineering Explained.
The Biggest Currently Produced Four-Cylinder Gas Engine
The L3B engine features an exact displacement of 2,720.2cc, (or 2.72 liters, or exactly 166 cubic inches) which makes it the largest gasoline-powered automotive four-cylinder engine currently in production. However, it’s not the engine’s size that makes it special, but rather, the amount of technology in it. First, it’s a direct-injection engine (would you expect anything else these days?) and features a coil-on-plug ignition system.
It appears that “variable” is a huge theme in this engine. Lubrication comes from a continuously variable oil pump. An electric water pump is paired with an adjustable rotary valve system to vary water flow (both amount and location) in order to regulate the engine’s temperature (in a feature called Active Thermal Management). And the pièce de résistance is the variable camshaft timing system.
A dual overhead camshaft configuration, each camshaft is independently, continuously variable of the other, both in camshaft timing as well as through the use of a three-phase camshaft — one phase of which will make all of you familiar with GM engines hot under the collar — Active Fuel Management. The other two lobes are a high-lift, high-duration performance profile, and a low-lift lobe designed to maximize efficiency. However, the biggest piece of advanced technology is hanging off of the exhaust side of the cylinder head.
The Dual-Volute Turbo Design
Arguably the coolest thing about the L3B is the dual-volute turbocharger. And that is where Fenske comes into play to explain just how the new technology works. Now, before you say, “Twin-scroll turbos have been around forever!” take a breath, and listen up as Fenske explains.
“A dual-volute turbo simply means that it has two volutes [on the turbine housing], one on top of the other,” Fenske explains, simply. “These are fairly similar to twin-scroll turbochargers in principle, except that a twin-scroll has two scrolls side by side, whereas a dual-volute has one volute on top of the other.”
At this point, you might be asking yourself what the difference is. If you allow a bit of a tangent, they are about as similar as the cockpit arrangements of an F-14 and an A-6. Ok, random aviation comparisons aside, what the differences come down to isn’t really based on whether the exhaust gasses are on top of each other or next to each other, but rather on how they exit the volute and act upon the turbine wheel.
As seems to be popular today, the exhaust manifold is an integral part of the cylinder head. The mating flange is a unique three-port arrangement that pairs cylinders one and four, and two and three. “Cylinder two and three are routed through the top volute, while one and four are directed to the bottom volute,” Fenske explains.
“The firing order is 1-3-4-2, so you’re alternating between which volute is getting the exhaust pulse. By separating those exhaust pulses from each other like that, you eliminate the chance of exhaust reverting back into an open cylinder. You get better response, better low-end torque, and overall greater efficiency from the turbo.”
Fenske explains that the theory behind dividing the exhaust pulses are the same for both dual-volute and twin-scroll designs. “The difference is how the exhaust gasses are entering and hitting that exhaust turbine and spinning up the compressor wheel,” Fenske says. “With the dual-volute, each one enters on opposite sides of the wheel, while in a twin-scroll design, the gasses enter together. There’s a little bit of overlap in the twin-scroll design where the exhaust pulse from one can leak back up into the other chamber.”
The dual-volute design eliminates that inefficiency and ensures that all of the gasses from the engine go where they are supposed to. Additionally, with the dual volute design, the clearance from the volute outlet to the turbine wheel itself is less. “You can bring the edge closer to the wheel since you aren’t trying to split the area in the middle,” Fenske explains. “With a twin-scroll, you have about 5mm from the outlet to the blades, while on the dual volute you can get about 1mm away from the turbine blade.”
Any time you can reduce clearances and potential leak paths in a turbocharger you are going to increase its efficiency. According to Fenske, Chevrolet claims that, in a Silverado, going WOT at 1,500rpm, it only takes 1.93 seconds to reach 90-percent of peak torque. Fenske feels that is pretty impressive response, with all things considered.
Whether in a Silverado or the CT4-V, the new L3B sounds like a pretty impressive piece of engineering. However, time will tell whether it is well suited to use in the 1500-series pickup trucks, as well as being worthy of Cadillac’s “V” moniker.