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Ruthless Pursuit of Power: 2008 Edition - Page 6 of 6

Our In-Depth Look at the New 2008 Corvette LS3 Engine

© 2007 by Hib Halverson
No use without permission, All Rights Reserved

Cam and Valvetrain

The Gen 3/4 valvetrain continues to evolve with previous Z06 features appearing in base engines. The LS3 camshaft is based on the one used in the C5Z's groundbreaking, LS6. Its intake lobe comes from the '02-'04, 405-horse cam. The exhaust profile is from the 385-horse LS6 of  2001 and the LS2 of '05-'07. For LS3, in order to reduce overlap, lobe separation was increased a degree over the LS2 cam. The net sum of all this was more intake airflow and a smoother idle.

Camshaft Profile, Intake Comparison

(All lift figures are valve lift)

year RPO	int. lift	int. dur. at .004	int. dur. at .050	int. open at .004	in. close at .004	in. open at .050	in. close at .050	int. CL	int. area	int. area increase
MY01 LS6	13.34 mm .525 in	270°	204°	9° BTDC	81° ABDC	18 ATDC	42 ABDC	118° ATDC	1862.9 mm/deg.	n/a
MY02 LS6	14.01 mm .551 in	267°	204°	7° BTDC	80° ABDC	19° ATDC	43 ABDC	120° ATDC	1936.9 mm/deg.	4%
MY05 LS2	13.34 mm .525 in	270°	204°	9° BTDC	81° ABDC	18° ATDC	42 ABDC	118° ATDC	1862.9 mm/deg.	0
MY06 LS7	15.06 mm .593 in	276°	210°	8° BTDC	88° ABDC	18° ATDC	48° ABDC	122° ATDC	2166.4 mm/deg.	16%
MY08 LS3	14.01 mm .551 in	267°	204°	7° BTDC	80° ABDC	19° ATDC	43 ABDC	120° ATDC	1936.9 mm/deg.	4%

Camshaft Profile, Exhaust Comparison

(All lift figures are valve lift)

year RPO	exh. lift	exh. dur. at. .004	exh. dur. .050	ex. open .004	ex. close .004	ex. open .050	ex. close .050	exh. CL	exh. area	ex. area change
MY01 LS6	13.33 mm .525 in	275°	211°	65° BBDC	30° ATDC	37 BBDC	6 BTDC	114° BTDC	1914.6 mm/deg.	n/a
MY02 LS6	13.91 mm .547 in	282°	218°	69° BBDC	33° ATDC	42 BBDC	4 BTDC	115° BTDC	2046.6 mm/deg.	8%
MY05 LS2	13.33 mm .525 in	275°	211°	65° BBDC	30° ATDC	37 BBDC	6 BTDC	114° BTDC	1914.6 mm/deg.	0
MY06 LS7	14.95 mm .589 in	296°	230°	81° BBDC	35° ATDC	53 BBDC	3 BTDC	119° BTDC	2359.4 mm/deg.	23%
MY08 LS3	13.33 mm .525 in	275°	211°	66° BBDC	29° ATDC	38 BBDC	7 BTDC	115° BTDC	1914.6 mm/deg.	0

The mix of lobe profiles, also, means there are two different base circles, 19-mm for the intake and 19.3-mm for the exhaust. Because of the different base circles, to keep valve train geometry optimized, two different valve lengths are used in the LS3 with intakes 0.6-mm. longer than exhausts.

We interviewed valvetrain Design Release Engineer, Jim Hicks several times in recent years for stories on LS1, 2 and 6 and he told us, "All our cams (prior to LS6) had the same base circle radius. We had a problem, if we wanted higher lifts: the nose of the cam would approach the same diameter as the cam bearing journals or even exceed them.

"Obviously, that means you can't install the cam in the engine-little bit of a problem. Your only alternatives are to increase rocker arm ratio, which we weren't going to do (for this engine), or reduce the base circle radius."

The first base circle reduction, for the '01 LS6, did not require a change in any other valvetrain part, however, the 405-horse cam for '02 was a different story as was the LS3's intake lobe, which uses the same profile.

"I wasn't comfortable reducing base circle that much," Hicks told us, "without compensating for it somehow, because the position of the plunger within the hydraulic lifter is not optimal any more-you're too high in the lifter.

"There are different ways to correct the geometry. The one we selected to minimize the impact on our manufacturing operations was to increase the length of the valve. The valves in the 02 LS6 and intake in the LS3 are 0.6-mm. longer than the valves in other engines."

Another new feature of the LS3 valve gear is offset intake rocker arms. Similar in concept to those used for the current Z06's LS7, they have a 1.7:1 ratio. The offset is .246-in. and exists so the pushrod could be moved sideways about a quarter of an inch, allowing those wider intake ports.

Click image for larger view Bird's eye view of the L92/LS3 rocker arm assembly. The offset intake rocker is obvious. What is not be so obvious is the more robust rocker cover seal surface. Image:  Author.

Click image for larger view The LS3 intake rocker's offset is nearly a quarter of an inch. The material remains investment-cast steel. The rockers are typical of O.E.s in that they have roller trunnions but not roller tips. Roller tips are really not necessary and are used in aftermarket aluminum rockers because its easier to add a roller rather than some other method of hardening the valve tip. Image:  Author.

Taking another page from the Z06 engine book, the LS3 intake valve, because its larger diameter increases mass, now has a hollow stem to get weight back down to where the valvetrain is stable to the same, 6600 rpm rev limiter used on the LS2. The  LS3 valve spring used is the "double-shotpeened," Gen 3/4 high-performance unit, originally developed for the LS6 and used in LS2.

Intake and Exhaust

The Corvette intake manifold has, once again, been changed. One reason is the intake port floor in the head is much higher and the port is wider. That, alone, drove  development of a new intake.

Gen 3s all used a glass-reinforced Nylon-6,6 intake which was manufactured using the lost core process. In 2005, for the Vette's first Gen 4, the LS2, GM switched to a different material, Nylon-6, and went to a vibration-welding manufacturing process. For the LS3, Powertrain decided to go back to a one-piece, lost-core, intake.

"Since the ports moved up and their shape changed," John Rydzewski stated, "we needed a new intake. It's still made of Nylon-6 but it's now done with the lost core process. The manifold is specific to passenger car applications that use this cylinder head.

"A vibration-welded intake has different shells. One port can be a combination of an upper portion and a bottom portion with the runners welded together on the side. It's a pretty good seam, but there might be a little crosstalk (port-to-port leakage) which can rob you of some power. We had seen that in some applications, so we went to a lost-core intake.

"We also reshaped it for better flow. Yoon Lee, who's been on the program for a few years, did the development. Compared to LS2, he reduced restriction by 2-3% at 13.5mm lift. It's just a smoother path, right down to the head. There's some extra structure-some different type of webbing on the bottom of the manifold-to stiffen it up. We, also, went to metallic compression limiters (inside the manifold bolt bosses) vs. the previous composite compression limiters. You've got the long columns and, with time, composites-if you do over-tighten-can creep."

"Creep" is a bit of a misnomer because it implies that the manifold, as a unit, moves. Better terms might be "extrude" or "deform". If you tighten the intake manifold bolts on a plastic intake, over time and especially if the bolts are over-tightened, the plastic may extrude, radially, away from where the fastener load is applied.

For long-term durability," Rydzewski added, "we don't want any creep in our manifold, so we're applying 'Bill-of-Design'-more of what we learned (that metallic compression limiters in the bolt bosses eliminate creep) from other applications."

LS3's injectors come from LS7. With 30-36 more horsepower, LS7's 5-gram/sec. injectors were necessary. The throttle body carries-over from the LS2. There are no functional changes to the positive crankcase ventilation (PCV) system. The induction system ahead of the throttle body is from the Z06 but with some quarter-wave tuners added to attenuate certain frequencies of intake noise.

The exhaust manifolds are similar to the units introduced in 2005 on LS2 but have a slight change in each exhaust runner where it bolts to the head to match the revised exhaust port exits.

These manifolds were revolutionary in '05, as GM employed a new material, cast iron with higher silicon and molybdenum content, which made a stronger casting. As a result, the wall thickness of the part could be reduced by about 25% and that took 10.5 lbs out of the car. A new MLS exhaust gasket was, also, released.

Engine Controls and Bin 4 Emissions

The engine control module (ECM) or "controller" as the guys who develop this stuff say, was new for 2006. Known internally as "E38, this new controller is part of GMPT's "Strategic Engine Management Complexity Reduction Initiative"-yeah, GM is like the Federal Government when it comes to thinking-up names. This program will result in only three engine management systems which share as many parts as possible.

The E38 has enhanced electronic throttle control (ETC) ability and it supports the 58x crankshaft position signals and 4x camshaft position signals which will be more common in coming years. It is a more thermally-robust design which can be mounted under the hood and close to the engine which reduces wiring harness length and complexity. It also uses connectors which meet standards set by the Electrical Wiring Component Applications Partnership (EWCAP) of the United States Council for Automotive Research (USCAR), a organization set-up by GM, Ford and Chrysler to foster the technology base of the domestic automotive industry.  

As for the deep-geek aspects of the E38: its CPU is Motorola's PowerPC. It's a 32-bit, RISC processor running at 40Mhz with 64Kb or RAM and 2 Mb of flash ROM.  The RAM nonvolatile and ignition independent and the flash EPROM, which contains the E38's calibration is nonvolatile and battery independent. E38 the carries over to the LS3 and is used on all Corvettes.

One capability enabled by the combination of the E38, LS3's improved combustion dynamics and enhanced emissions control devices, is compliance with the more stringent, Tier 2, Bin 4 exhaust emissions standard. These "bins" get tougher in a downward progression so, Bin 4 is cleaner than Bin 5. "With Bin4 emissions," John Rydzewski told us, "useful life hydrocarbon emissions are reduced by over 12% and useful life NOx emissions are reduced by over 40% compared to the LS2."

We should add that, Bin 4 emissions compliance comes with 36 more horsepower and no change in fuel economy. Plus if you order the LS3 with NPP exhaust, the darn car sounds better.

More performance, more green, less gas, better sound? Works for us!

Looks like the LS3 Team at GM Powertrain did a damn fine job.

Click image for larger view The power and torque curves for both LS3s compared to the LS2. It's expected that the LS3 would be stronger above 3500 rpm but, the extra displacement, adds torque from idle to 3000 rpm. Drawing:  GMPT Communications.	Click image for larger view No Corvette engine story would be complete without a Kimble cutaway drawing. This art is beautiful. Drawing:  David Kimble/GMPT Communications.

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The Corvette Action Center would like to thank Sam Winegarden, John Rydzewski, Lou Oniga, Jim Hicks, Tom Read and Susan Garavaglia of GM Powertrain Division for their assistance with this article.

For space reasons, this story could not cover many details of the LS3 which carried-over from past engines. For more information on the LS1/2/6 engines see the following web resources:

LS2:  https://www.corvetteactioncenter.com/specs/c6/2005/sequel5.html