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It seems that the death knell for the internal combustion engine may be a bit premature. GM has not yet given up on our favorite means of making horsepower with the announcement of the new LT6, the latest supercar powerplant for the 2023 Chevrolet Corvette Z06. And there’s plenty to talk about when it comes to this engine.
Let us be clear: This is not a compromised production engine that has been breathed on. This is a purpose-built competition engine that has been brought to the street. There is no other way to say it.
The LT6 is an all-new engine that is not an offshoot of the Gen III/IV/V. It will most likely be considered a Gen VII series engine. It dangles technology like a flat plane crankshaft and dual overhead cams (DOHC) like an early Christmas presents under the tree. Yes, it is still based on the traditional small-block Chevy / LS bore spacing of 4.400 yet with a moderate displacement of 5.5L – 333ci. But that’s the end of any kind of similarity to the original small-block Chevrolet family.
The 2023 Chevrolet Corvette Z06, the future home of the LT6. It's kind of hard to concentrate on the engineering behind the engine when looking at the car it'll power, isn't it?
Titanium connecting rods are tied to a set of forged aluminum pistons that are spun by a forged steel flat plane crankshaft. The flat plane crank is a radical departure from the cross-plane norm. It’s not new technology by any means, but risky from a drivability position due to vibration issues that we will touch on shortly. The reason it is worth the risk are the multiple advantages that a flat plane crankshaft delivers...not the least of which is a lighter rotating mass that is much quicker to respond to changes in RPM along with a screaming exhaust note. And trust us, this baby screams.
All of these attributes are force multipliers contributing to amazing power numbers for an engine this size. With 670 normally aspirated horsepower at 8,400 RPM and a peak torque of 450 lb-ft at an elevated 6,300 RPM, this engine right away puffs its chest out with an amazing 2.01 horsepower per cubic inch. This is the highest revving small-block Chevy (if we can still call it that) in its 66-year history.
One of the most adventuresome contributing factors in this orchestrated power punch is the choice of the flat plane crankshaft. This design feature creates an even firing pulse that alternates combustion from left to right bank that cannot be achieved with the more traditional cross- or two-plane crankshaft. The benefit of this even firing sequence is it makes it far easier to achieve outstanding intake and exhaust tuning efforts. Tuning efforts can be laser-beam focused to pack additional airflow into the cylinders at higher engine speeds to create pump up cylinder filling – otherwise known as volumetric efficiency (VE).
This GM image reveals the flat plane crankshaft orientation with two pistons up and two down. The opposite bank is halfway through its movement. The forged pistons offer very short skirt design working with a set of titanium connecting rods. We expect that the top and second ring thicknesses to measure under 1 mm. To minimize the spinning mass, the harmonic balancer a viscous damped, aluminum unit.
How much is this worth? All internal combustion engines achieve peak VE at peak torque. This is where all engines achieve maximum cylinder filling. The LT6 is capable of well over 100%. Most production engines do well to achieve numbers in the mid-90% range, so this is a major achievement. At 100% VE, the engine has been able to fill the cylinder (in a short period of time) to the full capacity of the entire cylinder.
What is difficult to achieve is approaching 100% VE at peak horsepower, where time is the enemy of cylinder filling. While we have not been able to verify it, there are numbers out there in excess of 100 percent at peak horsepower. The actual horsepower and torque numbers tend to reinforce these estimations.
The LT6 pushes flow through the paired intake valves to push the peak horsepower point to a stratospheric 8,400 RPM and peak torque to 6,300 RPM. This defines a wide 2,200 RPM power band between peak torque and peak horsepower, which will make this engine very tractable and predictable for spirited driving. Traditional V8 engines usually generate a much narrower power band of 1,500 RPM or less.
That small inboard gear on the crankshaft snout employs a traditional chain drive that is used to drive a small camshaft that drives the high-pressure fuel pump. The pump is mounted deep inside the middle of the engine to dampen the noise generated by the pump.
While there’s no denying the LT6’s impressive horsepower achievement, its emphasis on high RPM power is a radical departure from the traditional, low-speed torque style American V8. The LT6 is a small displacement, normally aspirated engine that delivers power in a considerably different fashion. As a point of reference, let’s use the standard Corvette's normally aspirated LT2 as a gauge point. The traditional, pushrod 2-valve 6.2L (376ci) engine creates just under 400 lb-ft of torque at 2,000 RPM, yet delivers only 1.31 horsepower/ci. This is a result of a combination of a 3.62-inch stroke and generous displacement. The 5.5L DOHC LT6 engine enjoys neither of these traits.
Generally speaking, four-valve engines don't have the strong low-RPM torque figures that two-valve engines would normally offer. Historically, Americans are very accustomed to the feel of big displacement engines making torque at low engine speeds. So it should be clear that the LT6 will feel softer at lower engine speeds.
This is not a condemnation of the engine - merely a statement of fact. To compensate the overall vehicle package, Chevrolet added a stiffer 5.56:1 final drive gear ratio to the ZO6 Corvette as added leverage to assist the low torque numbers inherent in this small displacement engine. Add to this the fact that peak torque occurs at 6,300 RPM and many drivers may need some time getting used to the difference.
The dual overhead cams are chain driven through hydraulic actuators for cam phasing. The cams push against finger followers to actuate 1.654-inch titanium intake and 1.378-inch stainless, hollow-stem exhaust valves. This engine’s high rpm intent demanded dual valve springs. Chevrolet says the followers are treated to a diamond-like coating (DLC) for enhanced durability and claim this engine is “lashed for life” and will not need adjustment.
Let’s now dive a little deeper into this amazing powerplant. Generally, a DOHC configuration adds substantially to the engine’s girth, but it appears these new heads do not overtly expand the engine’s waistline, although we do not have specs to confirm nor deny that observation. Forged pistons from CP are important for durability at these escalated engine speeds and consistent with the mantra of lighter is mightier, the titanium rods are sourced from Pankl Racing Systems in Austria. Chevrolet has yet to reveal connecting rod length.
The flat crank, DOHC arrangement, and massive induction system all are intended to enhance high RPM performance. But this comes with a price tag beyond the dollar investment and there is a certain disadvantage to this high-speed effort that is masked by the enthusiasm surrounding a high-revving street engine intent on living in the 8,000 RPM zone.
In order to understand both sides of the flat plane crankshaft approach, it’s important to appreciate certain physical traits surrounding V8 engine operation. This will drive an appreciation for both the benefits and the shortcomings. A flat plane crankshaft positions all four crank pins in one effective plane. All four cylinder crankshafts are built this way so you could think of a flat plane V8 as a four-cylinder crank with double width connecting rod journals.
In the left image, you can see that the small-block Chevy layout reveals how a cross plane crankshaft creates unequal bank-to-bank firing pulses. The curved lines reveal that twice during 720 degrees of crank rotation the engine fires cylinder back-to-back on the same bank. This is based on the small-block’s firing order of 1-8-4-3-6-5-7-2. Note how 5 and 7 fire sequentially and are also adjacent cylinders. Compare that to the right image, which reveals how a flat plane crankshaft fires sequential cylinders bank-to-bank, as in Left-Right-Left-Right. This advantage also simplifies the effect of exhaust pulse tuning to assist cylinder filling. This illustration uses the LT6 firing order of 1-4-3-8-7-6-5-2.
A two- or cross-plane V8 crankshaft is far more common in production engines and places the four crank pins in cross planes exactly 90-degrees apart. There are fundamental benefits for building a crank for a 90-degree, V8 engine but one main detraction is the cross plane crankshaft will be much heavier and require very large counterweights.
The flat plane crankshaft benefits from not needing large counterweights to counteract primary forces that we’ll define in a moment. The elimination of these counterweights makes the flat plane crank inherently lighter and as a result, will allow the engine's RPMs to climb quickly because of the reduced spinning mass.
Let’s jump into some simple geometry that affects all V-configured piston-driven engines. As the crankshaft moves the pistons through a full rotation, there are primary and secondary forces exerted in a V8 engine.
Primary forces are those created by the vertical movement of the pistons and rods. As a piston moves through its travel from top dead center (TDC) to bottom dead center (BDC), this creates a vertical force. For a flat plane crankshaft in a V-8 engine with four pistons up and four down, the force from one set of four is counter-balanced by the other four pistons. This primary force occurs only once in each revolution. A dual or cross-plane crankshaft requires large counterweights on the ends in order to offset the effect of this primary imbalance.
Because the engine is located behind the driver in the C8, both the intake and exhaust point toward the rear of the engine for packaging reasons. The throttle bodies measure 87mm. Of further note is the starter motor location beneath the expansive intake manifold.
There is also a secondary force created by the vertical movement of the pistons that requires a bit more explanation. As the crankshaft rod pin moves from TDC to its 90-degree mid-travel point, the connecting rod creates a circle that changes the effective length of the connecting rod and increases piston acceleration away from TDC. The result is the piston moves further than halfway through its total stroke distance when the crankpin reaches 90 degrees.
Since the piston has moved a greater distance for the first half of crank travel, the piston will travel a shorter distance from the 90-degree crank pin mid-point position to BDC. This difference in piston travel from the top half of the travel to the bottom half is reflected in a difference in acceleration forces that creates a vibration exerted laterally or perpendicular to vertical.
Adding to this lateral vibration is the fact that secondary imbalance forces are created twice per revolution – once when the piston moves toward BDC and also on the way back up to TDC. Adding to this situation is that engine speed amplifies the vibration so that the higher the engine RPM, the greater the force.
Variables that affect the force of this lateral vibration include the weight of the piston and rod assembly, the rod/stroke ratio, and RPM. The more common cross-plane crankshaft inherently cancels these secondary vibrations with a second plane vibrating in the opposite direction. This is the advantage of the cross plane crankshaft.
Keep in mind that this discussion is not about the static balance of the engine that is addressed by adding or removing small amounts of metal from the counterweights. Primary and secondary forces are a separate set of forces.
GM’s selection of a single plane crankshaft for the LT6 is clearly aimed at making more peak power and to give the engine a distinctive exhaust note. The standard equation for horsepower is the following:
This formula indicates that the higher you can spin the engine, the more horsepower you can make – assuming you can achieve the torque at those higher speeds. The combination of DOHC heads with smaller, lighter valves creates a valvetrain capable of sustained high engine speeds.
The challenge in this approach is that this is a very large displacement engine for a flat plane crankshaft. Ford’s Voodoo effort stopped at 5.2L and the largest European V8 is even smaller at 4.5L. Corvette chief engineer Tadge Juechter says that the engine’s secondary vibration was so severe during initial prototype testing that the spin-on oil filters literally unscrewed themselves from the engine. This resulted in a bolt-on cartridge filter system that is far more robust. But his comment is a testament to the effects of secondary imbalance.
It’s hard to ignore the Lt6’s large intake manifold footprint. The enclosure accounts for 11 liters of volume – or twice the engine’s displacement. This dampens errant pulsations in the manifold and allows for unrestricted breathing with its two 78mm throttle bodies. Breathing efficiency is critical when you’re spinning the engine to 8,400 rpm.
As we alluded to earlier, an important way to minimize the effect of the flat plane crankshaft’s inherent imbalance is to minimize the stroke. The LT6 employs a much shorter 3.15-inch stroke, which is even shorter than the smallest Gen III 4.8L engine at 3.26-inches. This shorter stroke also enhances the engine’s ability to rev by reducing piston speed. Conversely, a shorter stroke reduces displacement. To compensate, engineers chose a larger 4.104-inch bore.
The physical advantages of a lighter, single plane crankshaft combined with lighter reciprocating components like titanium connecting rods and short-skirt forged aluminum pistons enhances the LT6’s ability to accelerate quickly. This, of course, is reinforced by the even firing pulses of a single plane crankshaft engine that makes exhaust tuning much easier to manage.
The larger bore further enhances breathing by moving the cylinder wall away from the intake valves, which improves the volume of air delivered to the cylinder. Of course valve flow area for a four-valve cylinder head is always superior to that of a two-valve cylinder head. Chevrolet has yet to divulge the cam specs and valve lift for the LT6. But rest assured that the flow curtain area will be outstanding.
Stacking on other advantages, it appears that the dual overhead cams will also be electro-mechanically controlled. Data indicates the system can exert as much as 55 degrees of movement over the intake cam while the exhaust cam offers 25 degrees. This allows advancing the cams for low-speed torque and gradually retarding cam timing as RPM increases. And because the intake and exhaust lobes can be individually adjusted, this means the lobe separation angle (LSA) has a large degree of freedom as well.
The hollow camshafts are also designed for use with mechanical lifters using what are called finger followers. While most mechanical cams do require occasional maintenance, Chevrolet claims that the precise nature of the shims used on the followers should not require adjustment over the life of the engine. Also, the illustration reveals the dual springs are of a more traditional spring design and not a beehive or tapered spring.
Of course to maximize power, it makes sense to integrate valve action with the intake manifold. The LT6 uses an active manifold with separate plumbing for the left and right banks each fed with an electronically controlled 78mm throttle body. There are also three separate computer governed valves that allow communication between these separate bank runners to offer tuning advantages based on RPM and load. This active control enhances both low-speed and high-speed cylinder filling.
So far we’ve focused mainly on power advantages and the vibratory vagaries of a flat plane crankshaft engine, but durability is another huge factor that includes the lubrication system. Previous pushrod Corvette engines have employed a kind of poor-man’s dry sump system that has proven its worth but was at the same time somewhat limited. The LT6 stepped that game up with what GM calls a six-stage system.
This passenger side view of the LT6 reveals the dry sump scavenge side plumbing into the oil pan along with the large individual primary header pipes paired together with that pair merging into a single pipe for one side of the engine. GM engineers intentionally pair the inboard cylinders and outboard cylinders. A quick check based on firing order reveals each pipe is paired with a cylinder firing 360 degrees later which makes exhaust scavenge tuning very consistent.
The heart of the system is a full-length dry sump pump that scavenges oil from each individual paired cylinder bay in the crankcase while also pulling oil from each of the cylinder heads. The oil is then pumped into the holding tank that contains the 10 quarts of 5w50 synthetic oil that is fed directly to the inlet of the pressure pump to be delivered back into the engine. Chevrolet engineers say that at any point, 80 percent of the oil is available in the tank. This minimizes the amount of oil in the engine which reduces windage issues – especially at engine speeds over 6,000 RPM. This dry sump is similar to a full competition style system and is indicative of the lengths GM went to in order to build an engine that could reliably run in competition arenas.
One advantage to this style of dry sump is that not only does it remove oil from the crankcase, but it is also capable of pulling vacuum in the crankcase to reduce windage and improve power. According to Chevrolet, at full song this is worth nearly 80 kilo-pascals (kPa) which converts to well over 20 inches of mercury (“Hg). That is serious pan vacuum and also points out why the pistons are cooled with oil jets since very little oil will be left residing around the pistons to keep them cool.
This is just a first blush overview of the LT6 and you can expect there will be much closer inspections once the car and engine are in full production. The carbon footprint naysayers may eventually have their day, but internal combustion fans can revel in at least one more flag-waving, gasoline-fed charge into the future. Let the games begin!