What if you went to work one day and your boss had a sit-down with you. The news: He's going to pay you 5 percent more, you have to do 5 percent less work, the work you do will be 5 percent easier, your commute is going to be 5 percent shorter, and you get 5 percent more vacation time. When you go home that evening, your wife is 5 percent better looking, and that cold beer waiting for you in the fridge is 5 percent taller and 5 percent colder. Suddenly, 5 percent is a number that sounds pretty righteous.
Don’t blow a gasket, the fifth generation of the small-block Chevy is called the LT1. We d
Chevrolet's fifth-generation small-block V-8, called the "LT1," is kind of like the 5 percent scenario spelled out above-at least in automotive terms. It's no secret that the '14 Corvette will be an all-new design, and the Gen V small-block will power it to new levels of performance with an all-new design of its own. One interesting tidbit: We're told unequivocally that the base model C7 will do 0-to-60 in less than four seconds. You'll remember it wasn't long ago that we were popping champagne bottles when the Z06 did 0-to-60 in 3.9. In an era where all of us are getting 5 percent less and paying 5 percent more for it, the LT1 is a very welcome improvement.
Across the board, the 6.2L LT1 is incrementally better in every way than the 6.2L LS3 it replaces; the LT1 makes 450 hp at 6,000 rpm and 450 lb-ft of torque at 4,000 rpm compared to the outgoing LS3 that makes 430 hp at 5,400 rpm and 424 lb-ft at 4,500 rpm. We're promised by Chevy that fuel economy will get a boost too, although how much better than the LS3's 26 mpg highway number we'll have to wait and see. (Five percent would be right in the wheelhouse.) Finally, on the emissions side, the LT1 puts out 25 percent fewer cold-start emissions (the most problematic ones to reduce) than the LS3. As a result, the '14 Corvette will be the quickest, cleanest, and most fuel-efficient base-model Corvette ever produced.
Across the board, the 6.2L LT1 is incrementally better in every way than the 6.2L LS3 it replaces…
The engineers at GM Powertrain attribute these gains in efficiency to a trifecta of new-generation technologies that have been seen elsewhere, but never put together in a domestic V-8 until now. The LT1 is the first engine in the Gen V engine family, which will feature direct injection, continuously variable valve timing, and active fuel management-all three being the result of over 10 million hours of computational analysis, 6 million hours of computational fluid dynamics simulation, and 0.1 quadrillion bytes of disc space. All of this happened in just under two years, and is the effort that will underpin a whole host of Gen V engines that will power all of GM's larger cars and trucks.
Of the next-generation technologies employed in the LT1, direct injection (DI) is the newest, and is the linchpin that everything rides on. As its name implies, DI injects fuel directly into the combustion chamber, a process that if timed and targeted optimally, can yield substantial improvements in efficiency. GM Powertrain engineers broadly refer to it as the advanced combustion system, because in actuality DI's efficiency gains are highly dependent on having the right intake runner, piston, and combustion chamber shape as an integrated system; it is not merely good enough to bolt DI on and call it a day.
Here’s the LT1 head (on the left) compared with the LS3’s head from the outgoing Corvette.
From a hardware standpoint, DI makes use of a high-pressure mechanical fuel pump that is driven off a "tri-lobe" at the end of the camshaft; the result is fuel pressure on the order of 2,175 psi. Why so much? Since the fuel is injected directly into the combustion chamber during periods of intense pressure, the line pressure must be high enough for positive flow into the combustion chamber, and high enough to produce adequate atomization. What you get from this is a really effective way of cooling the combustion chamber-a property that vastly improves octane tolerance. To take advantage, Chevy has increased the compression ratio to 11.5:1. The magical thing about this is if you can raise the compression ratio without detonating or pulling out timing, you get more power without having to add fuel. As a bonus, DI does away with spraying fuel at the intake valve, and cold-start emissions are improved in a big way. Moreover, unlike its Gen IV predecessors, the LT1 can operate safely on any fuel octane and in any high temperature environment with relative impunity. Except for a slight loss of power, the engine simply does not care.
One look at the LT1’s combustion chamber and you can easily spot the direct injection. The
Continuously variable valve timing (CVVT) is something we've seen in Gen IV V-8s, but never in a Corvette. As we reported in our Dec. '12 story on variable valve timing, the aftermarket (COMP Cams and Mast Motorsports) has already caught up and is embracing the technology in its various forms. The advantage of CVVT is that by changing the valve timing-in particular the intake valve closing event-it's possible to maximize cylinder filling and volumetric efficiency over a broad powerband. A simple way to look at CVVT is to imagine that you're doing a series of dyno tests on a camshaft. In the first series of tests, you advance the camshaft until you find the engine's best performance at low rpm. In the second set of tests, you retard the cam until you find the greatest horsepower peak. Now imagine turning a dial and being able to put that cam phasing anywhere you want between those extremes without pulling off the timing cover and changing the timing. The LT1's variable valve timing is like having the ability to retard or advance the camshaft at will, taking into account load and rpm, and eking the most from its 376 ci. In the LT1, CVVT "clocks" the camshaft over a range of 62 degrees. As such, the timing gear is not attached directly to the cam, but to a cylinder with oil-filled chambers that act on the "clocking" of the camshaft. CVVT is the reason why the LT1 makes 395 lb-ft of torque at only 2,000 rpm, and why the torque curve is identical to the much larger LS7's at every point below 4,000 rpm.
Active Fuel Management
The third punch in the LT1's arsenal of advanced technology weaponry is Active Fuel Management (AFM). AFM unobtrusively disengages four of the LT1's eight cylinders when load conditions make it conducive to saving fuel. AFM technology has been in widespread use in the Gen IV LS family for several years, but this is the first time it's shown up in a Corvette engine. AFM is estimated to improve fuel economy by anywhere between 5.5 and 12 percent, so why hasn't it shown up on a Corvette sooner? Maybe it's because GM feared the Corvette customer is more likely to notice it engaging and disengaging, or maybe they merely delayed its introduction long enough to figure out how to make it interact with the new DI technology. We can only guess, but it's about time AFM showed up in the Corvette.
…the ’14 Corvette will be the quickest, cleanest, and most fuel-efficient base-model Corvette ever produced.
At the heart of the LT1's AFM system is the lifter oil manifold assembly (LOMA), which orchestrates the ebb and flow of pressurized lube to lifter sets in four of the LT1's eight cylinders. These special lifters either activate their corresponding valves, or they keep them closed. In GM and Chrysler vehicles we've driven with cylinder deactivation, it operates seamlessly and quietly. It should be noted that the degree to which AFM works without intrusion is largely the result of lots of tedious testing and programming to calibrate the wireless throttle body, exhaust tuning, fuel delivery, ignition timing, and cylinder deactivation. In theory, it works as though nothing is happening, but sometimes aftermarket exhaust systems make the cylinder deactivation a more obvious trait. We look forward to driving the C7 to see how well it works. In theory at least, we should see the Corvette's highway mpg go from 26 mpg, to anywhere between 27 and 29 mpg just on the basis of adding AFM alone.
Although they have roughly the same dimensions, the LT1 piston/rod assembly (left) is sign
Active Fuel Management (AFM) is the deactivation of four of the LT1’s eight cylinders, all
The LT1’s camshaft is different than most in two important ways: the variable valve timing
The LT1 block features cross-bolted nodular iron main bearings to handle the prodigious to
With the LT1's torque in LS7 territory, and with new demands placed on the lube system by active fuel management and variable valve control, it was deemed that the LT1 needed a higher-volume, higher-pressure oiling system with a greater dynamic range. GM Powertrain went through dozens of iterations on the lube system before arriving at a variable-displacement, duel-pressure pump. This crank-driven pump is partially controlled by the engine management system; a sensor reading pressure off the second main journal was deemed the end point of the lube circuit where pressure is typically at its lowest. The engine management reads pressure from the second main bearing and controls a solenoid that varies or dithers the pump's pressure valve orifice between its two extremes. This maintains an ideal lube circuit that has the advantage of low parasitic loss under low load, and a robust protective capacity during severe use.
For those Corvette customers wanting to autocross, drag race, or road race their cars-and here we're talking about way more than what's typical-Chevy will be offering the LT1 with an optional dry-sump lube system. This raises oil capacity from 6.5 quarts to 10.2 quarts, and ups the ante with an air/oil separator, external sump tank, and a two-stage scavenger-style oil pump. It's worth noting here that even without the dry-sump's oil/air separator, the LT1 features new oil/air separator schemes in the valve covers and the LOMA-and that's on top of improved bay-to-bay venting in the block's design and a seriously effective windage tray/crank scraper mounted to the cross-bolted nodular iron main caps.
Before we know it, we’ll be plucking these jewels from junkyards and seeing them shipped to our door in crates.
Like the wet sump system, the dry-sump's pump will have variable-displacement, dual-pressure control, but it adds a gerotor-style scavenge pump stage that is better suited to capturing oil that's heavily foamed or entrained with gas bubbles. We'll also note that this will be the first time in over 20 years that Chevy has not recommended a pure synthetic for the Corvette (typically Mobil 1). The LT1 oil spec is the Dexos spec-a semi-synthetic 5W30.
With 450 hp on tap and a 6,600-rpm redline, the LT1 crank is forged steel. Check out the 5
At a glance, the LT1's cylinder heads and intake don't look all that different from the LS3, but looks are deceiving. The Gen V architecture is all new from the block up, and significant changes were driven by the combustion system. The LT1's cylinder heads are the product of 75 design iterations that aimed at maximizing combustion efficiency. Engineers analyzed mixture motion and pressure dynamics in an effort to convert the least amount of fuel into the greatest amount of energy, and by the time of the design cutoff, the powertrain team had settled on a satisfying array of improvements.
Given the efficiency of direct injection, it's not surprising that engineers settled on large 2.13-inch hollow intake and 1.59-inch hollow sodium-filled exhaust valves. Air is admitted into the chambers at a very shallow 12.5-degree angle versus the LS3's 15 degrees. (The exhaust also gets help at 12 degrees versus the LS3's 15.) Intake ports are rectangular like the LS3, but the location of intake and exhaust valves has been reversed from the Gen IV, providing a slight bump in efficiency and better packaging.
Rather than go to a fabricated stainless steel exhaust manifold like on the LS7, engineers
It is perhaps the combustion chamber and piston face that contribute the most to the high-efficiency DI equation, with their shapes being the culmination of all that computational fluid dynamics. Each combustion event is a well-choreographed dance of high-swirl mixture motion, with atomized fuel entering at the ideal time to cool the chamber. Then on cue, a centrally located plug lights a precisely shaped fuel slug at just the right moment when it produces the greatest force gradient on the sculpted piston top.
All this permits a much higher static compression ratio, which in turn supports a very delayed intake valve closing (remember, fuel isn't coming through the intake valve anymore so there's mostly upside to a late intake closing). All this is great for building torque, lowering emissions, and extending the upper end of the powerband.
The Future ...
The LT1’s intake manifold is a long-runner cross-ram type like the LS3, but is more box sh
The LT1 is just the first salvo in the Gen V engine family. If the past is any indication, Chevy and GM with filter variants of the Gen V into trucks and other cars in the coming years. The next-gen technologies employed in the LT1 will also find themselves in other engine families present and future. Who knows, maybe we'll even see them with other technologies like turbocharging, independent variable valve control, and regenerative braking. All we know is the clock is ticking on tighter fuel efficiency standards, and at the rate of technological improvement that the LT1 has shown us, the future certainly looks bright.
It won't take long for the LT1 and its variants to populate the new vehicle fleet in the coming years. Before we know it, we'll be plucking these jewels from junkyards and seeing them shipped to our door in crates. It might seem scary dealing with direct injection at the moment, but it really poses no more of a problem than fuel injection or computer control did back in their early days. We can be forgiven for entertaining the idea that our hot rodding world might come to an end with tighter CAFE regs, but thankfully the true hot rodders at GM Powertrain dug their heels in and came up with a winner. And for that, we can give each other a high five.
The added demands of the lube system required an all-new variable-displacement, dual-press
The LT1 has been dyno tested to withstand as much as 1.3 lateral g’s, and it wouldn’t be p
The optional dry-sump lube system takes the LT1’s oil pump one step further with an additi
By The Numbers:
Gen V Small-Block LT1
||90-degree V-8 with overhead valves; continuous VVT
||6.2 liters (376 ci)
|Bore x Stroke:
||4.06 x 3.62 inches
||cast aluminum with nodular main caps
|Main bearing fasteners:
||six, including two cross-bolts per cap
||powder metal, 6.125 inches in length
||eutectic cast-aluminum alloy
||319-T7 cast aluminum with 59.02cc combustion chambers
||12.5 degrees intake, 12 degrees exhaust
||2.13 inches, hollow
||1.59 inches, hollow sodium filled
||hydraulic type with tri-lobe for fuel pump drive
||.551-inch intake /.524-inch exhaust
||200 degrees intake / 207 degrees exhaust (at 0.050 inch)
|Lobe separation angle:
||“runners in a box” design; composite construction
||87mm electronically controlled throttle body
||58X with individual coil-on-plug and iridium-tip spark plugs
||450 at 6,000 rpm
||450 lb-ft at 4,000 rpm
|Max engine speed:
||6,600 rpm (fuel cutoff)