Chevrolet Corvette LS6 Small Block Engine - The Ruthless Pursuit Of Power

Hib Halverson Mar 1, 2001 0 Comment(s)
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Drive it like ya stole it, and the LS6 engine in the new Corvette Z06 kicks butt, big-time. Shift an LS6 at 6,500 rpm, just 100 rpm shy of its rev limiter. While the motor peaks at six grand, its power curve from there to the 6,600-rpm fuel cut-off is nearly flat. Better yet, it out-powers the '97-00 LS1, not just at the top end, but everywhere off idle.

This new member of the Gen III small-block engine family presents us with a chance to quantify the rate at which technology marches by comparing it to the LT5, introduced more than a dozen years ago.

The LT5 was the first American production V-8 to pass the one SAE net horsepower-per-cubic-inch mark. This all-aluminum, four-cam, 32-valve, 350-cubic-inch V-8 was in all Corvette ZR-1s and is responsible for the performance that made those cars legends.

For a dozen years, LT5 reigned as the most powerful production engine in any GM car since 1969. It kept the ZR1 King-of-the-Hill Corvette until the Z06 debuted this fall. When it was introduced in the summer of 1988, the LT5 generated 375 SAE net hp at 6,000 rpm and 370 ft-lbs of torque at 4,800 rpm. That kind of performance was cutting-edge...for its day.

LT5 Was Then. LS6 Is Now.
This new Gen III puts out 385 hp at 6,000 rpm and 385 ft-lbs of torque at 4,800 rpm. A dozen years of engine technology advancement gets us 10 horsepower and 15 ft-lbs of torque with identical peaks. If that doesn't seem like much, consider that this improvement comes from an engine with slightly less displacement, smaller physical size, only one cam, only two-valves-per-cylinder, pushrod valve gear, less weight and better fuel mileage.

Development History
GM's Generation-Three small-block engine family debuted in the 1997 Corvette as the LS1, a 346-cubic-inch, 345hp V-8. The following year, LS1 was in the F-car in a slightly different iteration, putting out either 305 hp or with optional cold air induction and low-restriction exhaust, 320hp. The LS1 was a home run right out of the box. The C5 version made one SAE and the F-car version gave the '98 Camaro performance unmatched by anything but the Vette and the Viper.

John Juriga, Assistant Chief Engineer for Gen III Passenger Car Engines at GM's Powertrain Division has been on the Gen III program since its beginning in 1992. He describes GMPT's work on those engines as the "ruthless pursuit of power." Clearly, that philosophy drove Juriga and his team of engineers as they began work on the LS6.

As Juriga's staff of engineers began full-scale development, the first major challenge was the camshaft and cylinder head package. Both pieces were significant evolutions from the LS1 parts and the work took about a year during 1997 and 1998. The other major task was a redesign of the bare block to improve its "bay-to-bay" breathing and its strength. That took place in mid-to-late 1998.

The final major challenge was a sort of "surprise" late in the program. Once prototype engines were available for installation in vehicles, race track testing demonstrated oil control and consumption problems in certain extreme duty situations. The problem, discovered in the fall of 1998, took about nine months to solve. The LS6's final development and validation was during mid-to-late 1999. Pilot engines were built in the first quarter this year and the first production units last spring.

The LS6 Lower End
The LS6 and the LS1 use an all-aluminum block. It's a "deep-skirted" design (block structure extends below the crankshaft centerline) with six-bolt main bearing caps (four vertical bolts and two horizontal bolts per cap) and head bolt threads deep in the main bearing bulkheads (for minimal block distortion and maximum head gasket clamping force). LS1/6 blocks are semi-permanent mold castings of 319-T5 aluminum. The LS6 case is different from LS1 blocks in the design and strength of the main bearing bulkheads.

As the pistons move up and down, they force air in and out of the spaces (or "bays") beneath them. At high rpm, this reciprocating air flow is violent and really whips up the oil. While the LS1 block has some machined openings between bays, the LS6 block, because the engine has about 500 more usable rpm, needed larger windows at the base of each cylinder to better accommodate "bay-to-bay breathing."

Obviously, cutting windows at the bottom of each cylinder reduces the strength of the block's key structural area, the main bearing bulkheads. With 40 more horsepower, 400-500 more rpm and even more powerful derivations of this engine to come, the block needed to be even stronger than it would be without the windows. In their ruthless pursuit of power, Juriga and his engineers had to do more than simply reprogram their CNCs to cut those windows. Finite element design work along with a lot of thrashing engines to death on the dyno eventually resulted in the special LS6 block with bay-to-bay breathing windows and more strength than the LS1 block.

Like the LS1, LS6 uses centrifugally-cast, gray-iron liners cast into it at the foundry. Their bore is 99 millimeters (3.8976 inches). In 1997 and 1998, the LS1 could not be overbored. But, for 1999, liners changed and a service overbore of .010-inch was possible; this carries over to the LS6.

The LS6 crankshaft is the same cast, nodular iron unit with rolled-fillet journals used in the LS1 since 1997. Its stroke of 92mm (3.6620 inches), makes the LS6's displacement 5.665 liters or 345.69 cubic inches. Drilled main bearing journal centers reduce weight and assist in bay-to-bay breathing. For the 2001 model year (MY01), the reluctor wheel, pressed onto the crank to trigger the crankshaft position sensor, was redesigned to enhance sensor signal output. For MY01, all Gen IIIs use a new main bearing having reduced diameter variation. That allowed a slight decrease in main bearing clearance which reduces the potential for bearing knocks during starts in extremely cold weather from engines having bearings on the high-side of the variation. Lastly, LS6 cranks use a lightweight harmonic damper having an aluminum hub which cuts 2.6 pounds.

The 6.1-inch, sintered, forged and shotpeened PF1159M steel connecting rod introduced in 1997 carries over to the LS6. Beginning in MY01, all Gen III rod cap screws are stronger through a change in manufacturing process used to heat-treat and roll the screw's threads.

LS6 puts out more power and runs faster so it has a brake-mean-effective-pressure (bmep) that peaks about 15 percent higher than LS1's. Because of this, LS6 needs a more robust piston. The LS6 unit is cast of a eutectic aluminum/silicon alloy called "Mahle 142." Both M142 and the previous material, M124, also contain small amounts of copper and nickel, but M142 has slightly more of both. Mahle 142, offers increased strength and less expansion at high temperature. That offers better control of piston-to-bore clearance, both at the skirt and the ring lands which prevents piston noise along with enhancing durability and oil control. Because the LS6 piston material has more favorable expansion characteristics, the slight barrel-shaped profile used in the machining of the piston had to be changed, too.

Camshaft And Valvetrain
While the camshaft in the LS6 is more aggressive than the LS1 cams, its basic construction is the same: machined from a steel billet, rifle-drilled for less mass and with a camshaft position reluctor just ahead of the rear journal.

The new LS6 cam has a valve lift of .525-inch intake, and .525-inch exhaust, compared to .500/.500 for the MY00 Y-car LS1 profile. Measured at .050-inch tappet lift, LS6 intake duration is 204 degrees and exhaust is 211 degrees. The LS1 for MY00 was 198/208 degrees. The LS6 lobe centers are 116 degrees apart where as LS1 has them at 115.5 degrees. At .050-inch tappet lift, both cams have no overlap, but at .005 lift, LS6 overlap is 45 degrees and LS1 is 49 degrees. The overlap numbers make the new cam seem less aggressive, but that's clearly not the case.

Even though the LS1 looks like it has more overlap based on degrees, the LS6 actually has more overlap based on lift area: .52 versus .42 inch-degrees. That is why it's better to use lift area to quantify a lot of this cam data rather than just degrees.

We agree. With today's varying lobe configurations, comparing cams by the area under the profile is a better choice. Another issue to consider is, up to now, virtually all Gen III camshaft duration data released by GM's various communications entities has been "altered" to facilitate comparison to Gen I and Gen II small-block V-8 profiles which, except for the LT4, used 1.5 rocker ratios. These specifications are not skewed to a specific rocker ratio. Again, the duration numbers in degrees, when used for comparison purposes, can be deceiving. The lift area, in inch/degree, is a more consistent method of measurement.

Why does the LS6 have more actual overlap when, measured by duration in degrees, it appears not to? The LS6 valve accelerations and open/close ramp configurations are dramatically different from those of the LS1. The accelerations are higher, especially the negative acceleration over the nose, which increased about 10 percent. That's where the increased lift and duration come from. GM's engineers also changed the ramps. In the '97 model year they put constant velocity opening and closing ramps on the cam to limit valvetrain noise. For 2001, because (LS6) is a more aggressive application, they went back to a constant acceleration opening and closing which moves the valve faster. That gives more lift area and allows tighter lobe centers (than with constant velocity ramps having the same lift).

Obviously, with more lift, duration, higher acceleration rates and more usable rpm, the LS6 needs a more aggressive valve spring. The LS6 spring is made of different material (chrome/silicon/vanadium steel wire vs LS1's chrome/silicon steel wire) and has a different wire shape (oval vs. round), as well as being wound more tightly. These three features make a big difference in valve spring pressure: on the seat, 90 pounds for the LS6 vs. 76 pounds for the LS1, and over the nose, 260 pounds for the LS6 vs. 220 pounds for LS1.

The rest of the LS1 valvetrain, including the 1.7:1 investment cast steel, roller-fulcrum rockers carry over to the LS6. Generally, the Gen III valvetrain is more robust and lighter than the Gen II/I pieces. The natural frequency of the Gen III valvetrain is 740 Hz where as the old small-blocks were in the high 600s. This is why the Gen IIIs can rev higher than production Gen Is and IIs.

Cylinder Heads: The Ruthless Pursuit of Power
The same basic, performance characteristics of the LS1 head are used in the LS6 unit: 356-T6 aluminum casting, replicated ports offering the charge air a straight shot down to the intake valve, 15- degree valve angle, 2.00-inch intake valves and 1.55-inch exhausts with three angle faces and seats, head bolt and rocker stud bosses and oil return holes located such that they impact the intake ports as little as possible.

The major change in the LS6 intake port was alteration of the port's "short turn" or "short side" radius, the area of the port just upstream of the valve where the port floor curves down to the valve seat. The LS1 port suffered an air flow stall at high valve lift, induced by the short-turn radius and the goal was to eliminate that.

Ron Sperry, one of GM's top motorsports cylinder head guys during the '80s, lead the team that did the 1997 LS1 head. One of the restrictions put on Sperry was whatever he did with the intake port, injector targeting, which affects idle quality and exhaust emissions, was to be the prime concern. Ideally, port-injected engines should have injectors squirting fuel straight down the port, directly on the back of the hot intake valve. The temperature helps vaporize the fuel and the turbulence of the charge air blowing around the valve does the rest. For least emissions we want really good vaporization.

While Sperry did a lot of cool stuff with the original LS1 intake port, a compromise he was forced into was port walls that didn't interfere with injector targeting. The fear was if fuel contacted walls, it would end up as droplets or pool on the port floor. Anything other than a fine spray burns poorly and causes exhaust emissions to go up. To keep injector spray off port walls, LS1's port floor was flat, low, and had a short-turn radius tighter than was ideal for optimum performance.

For the LS6 intake port development, the port floor was raised to recontour and soften the short-turn radius. That improved flow as it transitioned into the combustion chamber. When air flows though a port of varying cross-section, as was that of the LS1, there are localized fluctuations in flow velocity which reduce efficiency. The mid-section of the LS6 port was widened and the roof was raised in the interest of better consistency in cross-sectional area. Effects of these changes on injector targeting and emissions were addressed with improved fueling software and calibration.

At high-valve lift, flow improved. At .550-inch lift, just slightly more than the LS6's maximum, flow jumped 10 percent. That GM validated the flow increase at .550 lift means the port has additional potential given more camshaft. Is this a clue to the future? Perhaps.

The LS6 combustion chamber is quite different than that in LS1 heads. The compression ratio (CR) gains four-tenths of a point to 10.5:1, but getting there wasn't easy. A CR increase has a hydrocarbon (HC) exhaust emissions penalty, but GM wanted the payoff: more power, increased thermal efficiency and better fuel economy. The addition of the small, auxiliary catalytic converters engineers call "pup cats" required for the Corvette to meet the LEV standard also allowed the engine the small increase in HC from the higher compression.

The engineer's challenge with the chamber was to increase compression by reducing its size but without shrouding the valves which would hinder air flow and reduce performance. Compared to the LS1, the roof of the chamber in the new head was lowered. This not only decreased chamber size, which increased CR, but it improved air flow over the short-turn radius. Additionally, lowering the roof slightly unshrouded the valves, which also improves flow into the chamber. The exhaust port was also revised with the same goals: improve short-turn radius and make cross-sectional area more consistent. The port exits were given the pronounced D-shape that many racing cylinder heads use. The changes netted about a 10 percent improvement in high-lift flow.

There were some other changes to the cylinder heads that were driven by the revised ports and combustion chambers. Obviously, coolant passages had to be revised. Also, the oil drain back holes were altered to clear the different ports.

Additional Changes in the Intake and Exhaust Tracts
A unique item of Gen IIIs is the composite ("plastic") intake manifold. They offer low mass, low cost, and cheap manufacturing, but have expensive tooling costs as compared with aluminum or iron manifolds. Cost is why the performance aftermarket has, to date, not offered intake manifolds for the Gen III. GM knew when the LS1 went to production more performance would come from a revised intake manifold. The biggest change in the LS6 intake was an increase in plenum volume. This was accomplished by dropping the plenum floor down as low as allowed by the engine block valley cover. Additionally, sharp edges at the junction of the intake runners and the plenum were smoothed. Some dead air pockets (areas of no flow) were eliminated. The result was the new manifold being worth 10 horsepower, just by itself.

The other changes for the LS6 are the higher-capacity mass air flow (MAF) sensor from the 366-cubic-inch Gen III truck engine. It's worth 2-3 hp and its integral intake air temperature (IAT) sensor simplifies the engine controls and reduces cost. The LS6 also uses a different air filter assembly capable of slightly better flow.

The exhaust manifolds on the LS6 and 2001 LS1s are new cast-iron items. Previously, the LS1 used a double-wall, fabricated, stainless-steel manifold. Most '97-00 LS1s had cats downstream in the exhaust under the floor. Double-walls, available only with a fabricated manifold, were required to retain exhaust heat in the interest of quicker light-off of the catalytic converters. The addition of pup cats immediately below the exhaust manifolds on '00 California cars and all '01s eliminated the need for the stainless manifolds.

Closing Out The LS6 Story
The last major change made during the development was in the positive crankcase ventilation (PCV) system. In the fall of 1998, LS6 in-vehicle testing was underway at road race facilities in the north-central U.S. Once Z06 prototypes neared production, engineers noticed excessive oil consumption at high-rpm and high, lateral acceleration.

Further testing proved oil was being trapped in the valve covers, then sucked into the engine through the PCV system. The solution was new PCV hardware. Taking a page out of the LT5 book, LS6 uses a valley-mounted oil separator assembly rather than the rocker-cover units of the LS1. This significantly reduces oil aeration and oil consumption and simplifies the system.

The engine controls calibration for the LS6 differ mainly in larger capacity injectors and in fuel and spark schedules. The LS6 injectors flow 28.5 lbs-hr whereas the LS1 units flowed 25 lbs-hr The LS6's fuel cut-off is at 6,600 rpm. Of course, the first question enthusiasts are going to ask is, "What happens if I change the rev limit so I can run harder on the dragstrip?" We asked Juriga what keeps the LS6 from revving to 6,800 or so and he told us it was essentially the fuel cut-off. But he went on to point out that the total combination of components would be stressed if the engine was run any faster than that. Parts like rod bolts, pistons and bearings would be pushed to their maximum limits at those levels.

In 2003, Chevrolet will celebrate the 50th anniversary of the Corvette. It's a marketing guy's dream and a likely time for GM to introduce the next version of the Gen III. This writer's guess back in 1997 was the future "high-performance" iteration of the LS1 would come in a few years and be a "375hp class" engine. I was about a year off on when, but I was close on the power. My next fearless forecast? GM might call an even higher performance Gen III the LS7. I think it will be about the same displacement but will have a supercharger and about 425 hp.

Let's see if I'm right.

The new LS6 small-block, available in the Z06 Corvette, is Chevy's 3rd Generation of small-blocks made specifically for the Ch...
Hib Halverson Mar 1, 2001


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