THE ALL-NEW LT1 SMALL-BLOCK, COMING SOON TO A C7 NEAR YOU
By Chris Werner
Photography by Justin Cesler and courtesy of General Motors
New generations of small-blocks are generally accompanied by fanfare, ticker-tape parades, and claims of the greatest thing since Nutella hit sliced bread. But will some ignore the Gen V release as a simple marketing ploy for an LS engine that just has its fuel injectors in a different place? That would be a poor choice on their part, because not only does the new LT1 differ significantly from all small-blocks before it (and is easily the biggest thing since the LS1), its advancements work together to simultaneously increase fuel efficiency, lower emissions, and bump power potential. And while the Gen 5 is sure to bring new challenges to the aftermarket, so did the LS1... then as now, opportunity knocks for talented folks who are brave enough to blaze new trails.
CORVETTE LEADS THE WAY
In the time leading to any Corvette engine’s debut, the automotive world wonders aloud whether it will “finally” be an overhead-cam design. Of course, it never is (C4 ZR-1 excepted). What generally follows is a time period of critics and skeptics wondering aloud why it isn’t.
Wake up and smell the coffee, people: the pushrod design architecture continues because it’s the only one GM designers feel meets the needs of the Corvette, the car that defines every generation of small-block before it trickles down to everything else in the lineup. These needs comprise an engine that is not just powerful, efficient, and durable, but also compact and lightweight. The pushrod architecture is a shoe-in for these final two goals as it allows for an externally smaller, lower-profile engine, yielding a drop in the center of gravity and a lower hoodline – hello badass handling and top-notch aerodynamics.
In terms of output, the LT1 will make somewhere in the neighborhood of 450 horsepower (the final figure will likely be released about the time you read this). Peak power numbers never tell the whole story of course. Case in point: we’ve been told to expect a 50 lb-ft. torque increase in the low to mid range versus the LS3, and even more torque than the LS7 below 3000rpm.
Like the original 1955 Turbo-Fire, the Gen 5 is still a 16-valve, cam-in-block 90-degree V-8 with 4.4-inch bore spacing. It’s unlike the original small-block Chevrolet in pretty much every other way. (It’s also unlike its namesakes, the solid-lifter 1970 LT-1 and reverse-cooled 1992 LT1, in all ways except pronunciation.)
Cues from the beloved LS-series are more apparent. Like the 1997 LS1, the Gen 5 LT1 features aluminum-block construction, 6-bolt main caps, and a composite intake manifold. The LT1 even shares the same 4.065-inch bore, 3.622-inch stroke, and 376 cubic inches (6.2L) with the latest base Corvette engine. Yet the only physical parts that carry over from the latest LS motors will fit in the front pockets of your greasy garage jeans (some miscellaneous bolts, piston pins, valve spring retainers and locks, and a crank key).
The LT1 really starts upping the ante when it comes to new technology. Active Fuel Management (AFM) and Variable Valve Timing (VVT) are new to the Corvette, and possibly the biggest news of all is the LT1’s Direct Injection (DI) - a first not only for a GM V-8, but for any overhead-valve engine in the world. This new fuel delivery system pays dividends in efficiency, emissions, and in helping to enable an aggressive compression ratio of 11.5:1.
ADVANCED EFI, ADVANCED COMBUSTION SYSTEM
Direction Injection (DI) is also known as SIDI (Spark Ignition Direct Injection) or GDI (Gasoline Direct Injection) to distinguish it from systems used on diesels. GM has been in the business of DI for several years now, with its first application for the U.S. market being the 2.0L turbocharged LNF that saw use in the Solstice/Sky and Cobalt a few years back. (Owners of 3.6L fifth-gen Camaros also have DI under the hood.)
The LT1 brings this latest and greatest form of EFI to the small-block V-8 for 2014. Port Fuel Injection (PFI) systems, used on small-blocks ever since the Tuned Port Injection days, work by squirting fuel into the intake ports upstream of the intake valves. DI opts instead to deliver that fuel directly into the cylinder during the intake stroke - not at the usual 50 or so psi, but at over 2,000 psi depending on engine load - resulting in superior fuel atomization and a more efficient burn of the air/fuel charge. (For more details on DI and its differences from PFI, see “Get to Know Your EFI,” November 2012.) DI also enables some “tricks” that were impossible to implement with older-style EFI systems, including a really neat cold-start routine, explains John Rydzewski, Assistant Chief Engineer for Small-Block Engines. “At startup, there are two injection events, a short pulse early and a second pulse late [both during the intake stroke]. The combustion gases from the later injection of fuel will still be hot when they exit into the exhaust, which helps heat up the catalytic converter more quickly. This gets the engine controller into closed loop quicker.” Another DI system dividend is the ability to run a leaner air/fuel mixture.
The LT1’s compression ratio is a full half-point higher than the highest-compression Gen IV, the LS7. Thank advanced technology and engineering for enabling such a high static compression number, while even allowing for use of 87-octane gasoline on days you are feeling stingy (hopefully those days are not track days - premium fuel will be needed to extract all 450-ish horses). The first major enabler is the aforementioned DI, which is great at cooling the air-fuel charge since the energy to evaporate the fuel comes from the gas phase in the cylinder; with PFI, this heat energy came in part from the port walls and valves. Taking the heat out of the gas rather than out of metal helps cool the charge, so you end up with cooler in-cylinder temperatures heading into the compression stroke. A lower-temperature starting point means the charge can be squeezed a bit more before the heat of compression sends pre-spark in-cylinder temperatures too high.
The second major enabler is the Gen 5’s exceptionally well-developed combustion system, which engineers said they had to design “from scratch.” This is no exaggeration, because while GM (and virtually every other major manufacturer) has experience with DI on overhead-cam, multi-valve engines, to date nobody had incorporated DI into an OHV engine. Perhaps contrary to what one might assume, the motion of the air/fuel mixture within the cylinder is actually more complex in a 2-valve-per-cylinder engine, and as an additional hurdle, more mixture swirl is needed for DI to yield optimal combustion. To deal with all of this, main combustion chamber variables that needed to be tweaked included chamber size, injector nozzle and spark plug placement, valve sizes and angles, and the intricate facets of the piston crown. According to Mr. Rydzewski, all of these parameters have influence and getting them just right was critical “to ensure that the air comes in in just the right way, that its motion is just right inside the cylinder, and that it exits in the right way.” Also crucial was ensuring that during the actual combustion process, “the ignition event is in the center of the chamber and you have a nice outward flame propagation where you can get as much out of what you burn as possible.” GM says computational analysis for the Gen 5 required more than 10 million hours of CPU time, with nearly two-thirds of that devoted to the combustion system, so a whole lot of iterations were evaluated before a near-finalized cylinder head design was even prototyped.
The resulting head features hollow valves spec’ing out at 1.59 inches on the exhaust and a large 2.13 inches on the intake, the former sodium filled and the latter nitrided in the interest of durability (this process was used on some LS engines, but this is its first performance application). The orientation of the valves is different too: each new generation of small-block has featured shallower valve angles than the previous, moving from 23 degrees on the original to 15 degrees on the Gen III and IV (the LS7 was shallowest at 12 degrees). The Gen 5 continues this tradition, with its intake valves being rolled to just a 12.5-degree angle from vertical and an even shallower 12 degrees on the exhaust. Hot rodders have long known that shallower valve angles allow for smaller-volume combustion chambers and help minimize valve shrouding as the valves near maximum opening, hence increasing breathing potential.
Speaking of increased breathing potential, the use of DI also facilitated a higher-flowing, much straighter airflow path to the intake valve. “This is a huge benefit from DI systems,” says John Rydzewski. “With the new small-block we didn’t have to worry about a PFI injector and the correct angle for it, or a fuel rail and where that was going to be, and the need to often put a bump into the intake port.” The benefits of not having a fuel injector in the intake port don’t end there. “With DI, what goes through that port and past the valve is just air. With a PFI system, the fuel displaced some of the air, so the volumetric efficiency was worse by having the fuel run past the valve.”
There are more firsts for small-block history in the LT1 head, as not only did previous-generation engines have equivalent valve angles on the intake and exhaust, the valve stems always occupied parallel planes in line with piston travel. Not so on the Gen 5, as its intake and exhaust valves are splayed (angled apart 2.5 degrees from vertical for each). While this is a huge enabler for the aforementioned in-cylinder mixture motion, a side effect is its moving the valve stem tips further apart from one another. John Rydzewski acknowledged this allowing for even more room between the pushrods (recall the cathedral-style port being required for the narrow pushrod spacing of the LS1, with the offset intake rocker of LS7 and L92/LS3 heads allowing widening of ports), though said it was not exploited since they already “had enough [intake port] airflow for our combustion system.” Our money is on further exploitation of this larger expanse between the pushrods for even larger port sizes on future high-output Gen 5 engines.
VALVETRAIN INCORPORATING AFM & VVT
With fuel economy standards looming larger by the year, the 6.2L displacement of the new LT1 comes as a surprise to many - this author included. Yet, GM says the LT1 is “more powerful, more responsive, and more efficient” than a smaller V-8. While the first two of these assertions are easy enough to understand, the third seems a bit counterintuitive, since more cubes usually result in more frequent trips to the pump.
Enter the LT1’s VVT and AFM systems. In a reversal of what typically happens with powertrain innovations at GM, the Corvette is not the first in the lineup to receive these technologies. Originally known as Displacement On Demand (DOD), AFM started appearing on truck engines for 2005, and is also familiar to owners of automatic-equipped Pontiac G8s, fifth-gen Camaro SSs, and LS4-powered FWDs. VVT debuted two years later on the engine that also introduced the LS scene to affordable rectangular-port heads, the L92.
The only advanced feature incorporated into the LT1 aimed solely at improving fuel economy, AFM uses special deactivating lifters to stop valve movement on half of the cylinders, resulting in the C7 being powered by a 3.1L V-4 engine under light-load conditions. (No word on whether the LT1 can be locked in 4-cylinder mode for when your 17-year-old wants to take the Vette out for ice cream.) Engineers say that 6.2L isn’t just a carryover engine size from the latest C6 Corvettes, but rather was found to be an optimal value “for sustaining fuel-saving cylinder deactivation with Active Fuel Management, based on the car’s weight and the engine’s torque output.” In other words, the 5.5L displacement that had been long rumored for the Gen 5 would not have cut it for powering the car in four-cylinder mode for any appreciable length of time.
Aside from working in concert to allow AFM to be more effective, VVT also improves both low-end efficiency and top-end power on its own by eliminating the inevitable compromises necessary to valve timing events when cam phasing is not adjustable on the fly. Although the intake and exhaust are not adjustable independently (a feature that has been present on the cam-in-block Viper for a few years), GM says this would have added to engine length and mass, so would not have been worth it at this point.
Speaking of cams, the LT1’s is said to be based on the LS3 and specs out at 200/207 duration at 0.050 and 0.551/0.524-inch lift on a 116.5 LSA, with lift events having been optimized based on the new higher rocker ratio and splayed valve geometry. The cam opening and closing ramps were tailored to accommodate AFM, and as with prior AFM-equipped engines, the cam lobe profiles are slightly different on cylinders that have the deactivating lifters (i.e., cylinders 1, 7, 6, and 4). According to John Rydzewski, “This is always the case with AFM because you have to accommodate some additional lash with the AFM hardware. In addition to that, on both the deactivating and non-deactivating cylinders, we did some work on opening and closing ramps for noise refinement.” He emphasized the slight difference in the cam lobes does not affect airflow cylinder to cylinder.
Also impressive: not only did Gen IV engines equipped with VVT or AFM never get anywhere near 450hp, they also never revved past 6,200rpm. “We definitely were concerned because while we had AFM on engines that spin a lot less RPM, this is the first time we’ve gone to 6600,” says Rydzewski. “But when we started running our valvetrain test fixtures, we were able to verify that the engine was capable of incorporating AFM given certain valvetrain parameters. While we didn’t need to change any parts of the AFM system, the main thing we focused on was making sure the valvetrain was of minimal compliance [high stiffness] and of the lowest mass. We went after this with components like the stiffer pushrods and hollow valves.”
Some questions remained unanswered as of press time. One of the most interesting was the applicability of AFM and VVT on manual-equipped cars; the C7 will have been revealed by the time you read this, and we expect both features to be present with stick shift, the first application for either.
BOTTOM END & LUBRICATION
The lower part of the LT1 builds on the success of LS engines by being mainly evolutionary in design, using the lessons learned over the entire portfolio of Gen III/IV engines to tweak improvements in all areas – we mention some of these highlights in the photo captions.
Things are similar with the oil system, which incorporates a variable-displacement oil pump akin to those on AFM-equipped Gen IVs (still driven off the snout of the crank). Completely new on the LT1 is the oil pressure sensor’s location, which now reads from the front of the lifter galleries instead of the rear: readings are lower here by several psi due to it being further downstream in the oil’s circulation path through the engine. Getting the best possible reading on system pressure is crucial not only for AFM operation, but since the main and cam bearings are fed by the lifter galleries and so are at the final points in the oil’s circulation (recall some aftermarket LS blocks’ use of so-called priority main oiling to get around this perceived weakness).
What gets lubricated by that oil has also evolved a good deal, as (taking cues from the high-performance community) both main and rod bearings are a polymer coated bimetal eccentric, delivering reduced friction and enhanced durability. The undersides of the pistons also get a refreshing blast of cool (relative to the piston temperature) oil from oil jets, a feature not seen in Gen III/IV engines excepting the blown LS9 and LSA. Lower piston temperatures translate to long-term robustness, with the extra bit of oil benefitting the piston pins and cylinder walls as well.
Engineers also spent a lot of time perfecting a new, patent-pending PCV system, which is said to both improve oil life and reduce consumption - the latter having been a continuing battle of revisions especially on early LS engines. Incorporating new, integral fresh air separators under the valve covers (note the bulges atop them between where the coils sit) and an integral foul air system in the LOMA, it’s claimed to have triple the ability to separate air from oil. This is good news, because if there’s a shortcoming to DI, it’s in dealing with oil-related deposits on the backs of intake valves: no amount of gasoline detergents can clear this up, since gasoline does not flow past them. Improved valve stem seals also work at minimizing the amount of oil vapor that needs to pass by the intake valves.
We’ve hopefully convinced you the Gen 5 truly deserves all its attendant fanfare, with GM making the lofty claim that this is the “most significant redesign of the Small Block in its nearly 60-year history.” Yet, an unanswered inquiry remains: significant to whom?
The Gen III LS1’s launch having so rocked our worlds, only time will tell if the Gen 5 can be as important to GMHTP readers. The limiting factor in our getting started finding out will be cracking the LT1’s new E92 ECM; how long this will take tuners remains to be seen. Encouraging words were spoken at the release of the Gen 5 at SEMA, where GM’s U.S. VP of Performance Vehicle & Motorsports stated his certainty that “it won’t be long after it debuts that higher-performance LT1s will be hitting the streets and tracks everywhere.” We hope it’s not too long, because we at GMHTP look forward to being at the front lines of it all. Here’s to 1997 all over again.