Is the Gen V Engine’s Combustion System Really New?

Given GM’s pre-introduction hype over the “all new combustion system” on the Gen V small-block, there is likely to be a measure of disappointment within certain progressive gearhead factions. The new Chevy LT1 still runs on the 136-year-old Otto cycle with four strokes of each piston, valves opening at conventional times with respect to crankshaft position and the all-familiar spark plugs igniting a stoichiometric air-fuel mixture. While there is the new-to-GM-V8 direct-injection fuel system, the LT1 lacks other high-profile features often associated with a high-tech engine: turbocharging, dual independent cam phasing, dual overhead camshafts and 4-valve combustion chambers. And the LT1’s scale of boast-worthy features hardly rivals the arousing passion of futuristic blue-sky breakthroughs like the Scuderi, IRIS or opposed-piston/opposed-cylinder concept engines.

In fact, GM didn’t even give its all-new system a clever moniker like SkyActive (Mazda), Multiair (Fiat) or OptiSpark (Oh, wait. GM tried that one with the previous LT1 engine). But still, not even a code name for identity and promotion? “All new combustion” in this marketing setting isn’t any more inspiring than an “all new flavor” for chewing gum.

An EngineLabs technical deep dive into the new LT1 engine, however, reveals just how delicately the the precise combustion dynamics are integrated into the overall powertrain strategy for this engine. Engineers racked up some 10 million hours of computational analysis in developing the LT1, with six million dedicated alone to the combustion. 

Note the difference (left photo) in the piston top design from the LT1, left, and the LS engine. The LT1's eutectic aluminum alloy piston features a direct injection fuel bowl and deep enough valve reliefs to accommodate cam phasing. There's also a skirt notch for the oil jet. Some LS3 piston features that carry over include ring pack and oil drains that are both cast and drilled. The LT1's 6.125-inch connecting rod is a high-strength PM forging and features a tapered pin end. The 59cc combustion chamber (center photo) was redesigned to support direct injection and sized to achieve the 11.5:1 compression ratio. The spark plug was moved closer to center to help reduce knock. The hollow intake valves are 54mm (2.13 inch) while the hollow sodium-filled exhaust valves are 40.4 mm (1.59 inch). Engineers evaluated more than 75 iterations of the combustion system (right photo) that included spark plug placement, valve sizes, piston design and fuel delivery.

“The first two years of this program was based on nailing the combustion system,” stresses Dean Guard, executive director of GM’s North American engine programs. “I will put this combustion in any metric you want to talk about and against any overhead-cam engine in the world. I believe you can do a really good 2-valve combustion system and really bad 4-valve, and any area in between. We will not shy away from what this combustion system will do compared to anybody.”

While GM says the LT1 is a clean-sheet design, some traditional design elements found on every Chevy small-block since 1955 continued into the fifth generation, including a 90-degree V block with a single camshaft, two valves per cylinder and the trademark 4.400-inch bore-center spacing. 

New to the Gen V

From there, very little carries over from the previous Gen IV LS architecture, either physically or in design. The starter bolts and valve keepers are the only common mechanical parts. Dimensionally, the deck height, overall length, rod length, piston compression height and crank-to-cam centerline carry over. The head-bolt centers are the same, although the LT engine has a larger head-bolt diameter. 

Engineers used a sophisticated suite of CAD and simulation programs to first design the engine virtually. More than 75 combustion “iterations” alone were considered before lab testing combinations on a single-cylinder engine. Just what factors made up these iterations?

Here's a comparison (left photo) of the LT1 cylinder head, on the left, and the previous generation LS3. Note that the order of the valves has been reversed and the LT1 intake ports have been raised. The valves stand at 12.5 degrees intake/12 degrees exhaust and are splayed 2.5 degrees. The previous Gen IV heads had 15-degree valve angles. Also, all head bolts are now M12, compared to M11 in the Gen IV. The rocker arms are non-offset and sport a 1.8:1 ratio (right photo). The configuration of the exhaust port allows a small amount of exhaust gas to be drawn back into the cylinder for a more complete burn of the next combustion cycle and reduction of emissions.

“We looked at the momentum of the air, the direction of the air coming into the port,” says John Rydzewski, assistant chief engineer on the small-block program. “We moved the spark plug all over the place. Not only location but also intrusion into the cylinder.”

Power Density

First there was horsepower per cubic inch, then came area under the torque curve. Now the trending measurement is power density, which helps rate an engine’s efficiency and size. GM has made available numbers to compare the new LT1 with a BMW 4.4-liter twin-turbo found in the 5 Series. The BMW is rated at 400 horsepower with 450 lb-ft torque and has a weight of 503 pounds, or in simple terms, .795 horsepower per pound. The LT1 is likely to be rated at 450 horsepower and 450 lf-ft torque with a mass of 465 pounds, or .968 horsepower per pound. According to GM, the LT1 is also 4.3 inches shorter in overall height than the BMW.

With the introduction of direct injection, engineers couldn’t rely on design dynamics learned on GM’s other DI engines that are based on DOHC configuration. According to engineers, the flow field — that is the motion of the air-fuel mixture — is inherently more complex with an OHV configuration since DI requires more mixture swirling.

“DI has a lot of advantages,” explains Rydzewski. “It cools the piston, so we’re able to run a higher compression ratio. It allows you to inject into a piston bowl to help improve cold-start emissions. There’s also more flexibility in designing the intake port, since you don’t have to place an injector in the port.”

In addition to injector placement, engineers manipulated valve size and angles, the size and shape of the combustion chamber and the topology of the piston head — which is crucial to DI. The LT1’s dished pistons feature “risers” at the top to direct the fuel spray.

“We had to look at the direction and speed of the air going past the spark plug,” continues Rydzewski. “If too large or small, it’s not good for proper delivery of spark energy. We had to find that little window.”

Technology has to earn its way in.
         — Jordan Lee

Even then, more iterations were considered to maximize fuel economy and improve emissions. The cylinder-head design steadily came into focus as the different scenarios played out on the computer and in the lab. There were steady-state airflow assessments using CFD and bench testing. Other development tools included one-dimensional cycle simulation, geometric flame propagation analysis and three dimensional combustion analysis. 

Here are four views of the direct injection fuel system. It's rather compact with the injectors under the intake ports and the fuel rails under the intake manifold. The system is fed by a single high-output fuel pump rated at 1.48 cc per rev, and the system operation is rated at 15 MPa or 2,175 psi. It's hard to demonstrate, but the injectors are suspended to help with acoustics. The injectors are rated at 22cc per second at 10 megapascal or about 1,450 psi.

Engineers leveraged lessons learned from racing and raised the port floors to give the air a straighter shot at the valves, but then twisted shape of the ports to maximize the mixture motion of the airflow into the cylinder. And then there’s the return of splayed valves, a feature famous on Chevy’s big blocks and the rowdy SB2 NASCAR race engine.

“Splayed valves are more difficult as it takes a lot of finessing,” says Rydzewski, noting that the valves are splayed 2.5 degrees. “If we had straight up valves, the combustion system would have changed. Also, one of the more radical changes was swapping the intake and exhaust valves. That gave us the best flow field for the combustion.”

A winner!

“We knew we had a winner. We knew we had something that was going to change the way combustion worked in a small-block. We took a mold of the ports and combustion chamber and put it on Dean’s desk. He said not to change it,” remembers Rydzewski. “We then optimized everything with cam duration, overlap, injector spray, airflow vectors, the bore size and 92mm stroke.”

The small 59cc combustion chamber and long stroke resulted in an 11.5:1 compression ratio. GM will recommend premium fuel but not require it.

The cam phaser has 62 degrees of authority compared to 52 degrees on the Gen 4 engines. The cam profiles was optimized to take advantage of the higher rocker-arm ratio and accommodate AFM. Specs for the dual-pattern camshaft include: 200/207 degrees duration @ .050 and .551/.524-inch total valve lift. Lobe separation is 116.5 degrees. Note the tri-lobe in the rear that powers the direct-injection fuel pump. The lift on each lobe is 5.7mm.

“I think this shows you just how good the combustion system is. Typically when you have that much compression you require premium fuel.,” says Rydzewski, noting that overall knock tolerance of the engine improved and is less sensitive to temperature or octane. “That’s significant when you’re driving under aggressive conditions and the vehicle gets hot. The spark isn’t going to be dialed back.”

GM didn’t go with active intake manifold but rather designed a racing-style “box” with and 87mm drive-by-wire throttle body and eight individual runners that encourages consistent air feed between all eight cylinders. The design improved the airflow imbalance factor by 50 percent without sacrificing space under the hood. On the other side of the cylinder, engineers added a tuned exhaust inspired by the short 4-into-1 header design from the LS7 and LS9 engines. 

Valve timing certainly has a significant impact on the combustion. The new Corvette engine get a single phaser to advance or retard the camshaft with “62 degrees of authority.” Relative to the Gen IV truck engines, that’s an additional 10 degrees.

The LT1 block features cast-in-place liners, nodular iron main caps (previously powder metal), new water jackets that allow longer head bolts, new engine-mount bosses and improved crankcase design to reduce windage. There are also provisions for oil squirters under the pistons. Cylinder bore is 4.06 inches. The LT1's forged-steel crankshaft (3.62-inch stroke) rides in polymer-coated bearings and features induction-hardened journals.

“And the combustion system uses all 62 degrees,” says Guard. “In Gen IV we couldn’t go to the limit of the phaser due to combustion stability.”

With GM’s single phasing strategy, the intake and exhaust events are equally phase-shifted relative to the crankshaft as the operating conditions dictate. The obvious question is why GM didn’t utilize a more advanced dual phaser to allow tuning the intake and exhaust valves independently for more precise control of the airflow?

80% of the benefits

“Independent cam phasing would have added mass and could have been a packaging issue,” says Rydzewski. “With an overhead valve engine, that means cam-in-cam. We still get 80 percent of the benefit with our dual equal variable valve timing.”

“Technology has to earns its way in,” echoes Jordan Lee, chief engineer on the small-block program. “We did exhaustive studies on cam-in-cam and dual independent. The benefits just didn’t show it was there.”

The intake is a 4-piece molded and welded composite that is lightweight and thermally efficient. Eight equal-length runners are under the cover and behind the 87mm digitally controlled throttle body. GM says dynamic airflow improved 3.6 percent and the airflow imbalance was reduced by 50 percent with this design.The tuned exhaust is also part of the combustion system. The design is similar to the 4-into-1 short-header manifold found on the LS7 and LS9, although the LT1 version is cast iron instead of stainless steel. The runner geometry was revised following additional CFD analysis. All four runners are nearly equal in length and feed into a wide-mouth collector. This design also helps reduce noise in the Corvette.

The LT1’s displacement was set at 6.2 liters (4.06-inch bore x 3.62-inch stroke) to help optimize the effectiveness of the Active Fuel Management (AFM) function. AFM is GM’s version of cylinder deactivation. A designated low cruising speeds with a light load on the engine, cylinders 1,7,6, and 4 will be deactivated using special valve lifters. Instead of a normal 1-8-7-2-6-5-4-3 firing order, the engine fires in an 8-2-5-3 sequence to save fuel. As soon as the driver step on the gas or engine load increases, the deactivated cylinders seamlessly return to operation.

“We made decisions for this program on the basis of AFM,” says Guard. “In Corvette legacy, we’ve not done AFM.”

“What’s unique is that we’ve applied AFM to a performance valve train,” explains Rydzewski. “This is GM’s highest spinning valve train with AFM at 6,600 rpm.”

Lubrication and cooling systems were also evaluated and upgraded to support working conditions for the LT1’s engine dynamics. There’s a new variable-displacement oil pump, available dry-sump pan, oil-spray piston cooling and offset water pump and thermostat.

The entire operation is managed by a new E92 ECU, which boasts a 65-volt interface to the injectors and very sophisticated programming to control the fuel maps and spark timing.

“There’s a misconception that there’s hidden horsepower in the calibration,” warns Lee, when asked about the potential for aftermarket tuning. “We optimized spark and fuel to make as much power as we can. The ingenuity of the aftermarket is phenomenal, but DI will be an issue. That’s something new and different for the aftermarket.”

Sizzle or fizzle?

The LT1’s combustion system is all new more in terms of precise airflow optimization within an updated mechanical architecture, not all new in terms of ground-breaking combustion revolution. It’s all new because the Gen V LT1 is the first overhead-valve gas V8 to harness the modern technologies of variable valve timing, electronically controlled direct injection and cylinder deactivation. That combination is all new as other DI engines are overhead cam.

Given the challenges of working with just atmospheric pressure and only two valves in an irregular-shaped combustion chamber, GM Powertrain engineers have to be applauded for truly optimizing the components and achieving those power levels while improving emissions and fuel economy. Yet, still, one walks away from talking with the engineers with the sense that they didn’t get all that they really wanted.

“Technology has to earn its way in,” is either a bold statement of conviction to the final engine composition and character, or it’s a euphemism for “Damn, we wanted that but they wouldn’t let us have it!”

GM has a jaw-dropping arsenal of powertrain tricks at its Tech Center. In the past, engineers have shared glimpses of not only exciting future technologies but innovative solutions and improvements to current products. As always, engineers are held back by eventual costs to the consumer. And getting management to sign off on advanced technologies for a low-volume vehicle is even more difficult these days, given the bankruptcy and government bailout situation at GM. 

If history holds true, Chevy will have more powerful versions of the LT1 engine. The Corvette customer will demand enough horsepower to keep up with the over-priced exotic supercars. Since every pony is going to cost more in a naturally aspirated configuration, engineers will have to resort to power adders. Don’t look for a 7.0-liter version anytime soon. But odds are still very strong for a downsized turbocharged version. And that intriguing cam-in-cam technology may just earn its way in to a future special edition Corvette.


About the author

Mike Magda

Mike Magda is a veteran automotive writer with credits in publications such as Racecar Engineering, Hot Rod, Engine Technology International, Motor Trend, Automobile, Automotive Testing Technology and Professional Motorsport World.
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