Cams-- Again, Page 2

Ruthless Pursuit of Power: The Sequel

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LS6 Evolution: Bigger Cam–Again

There are two big changes in LS6 for 2002. The first is evolutionary and the second is revolutionary.

Looks like a steel billet, roller lifter camshaft for any GM small-block, right? Well, not so quick.
A big difference between Gen III cams and all small-block cams that have gone before is the Gen III’s larger base circle radius. That allows both high cam lift and lower valve train loads. Both increase performance and durability. That said, both LS6 cams, have slightly smaller base circles than LS1 and truck cams.
Image: Author.
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The ’02 LS6 cam is an evolution of the .525-inch-lift, 2001 part. It’s more aggressive profile has the highest valve lift, about .550-in., of any cam ever installed in a production small-block V8, Generation I, II or III. We interviewed John Juriga for this article and he said about the ’02 cam, "It required the most engineering because it’s a fine balance to try and gain more airflow without disrupting emissions, loosing low-end torque or creating durability problems. We upped the lift from 13.3 to 14 millimeters on the inlet and went to 13.9 on the exhaust."

We also spoke to the LS6 camshaft engineer, Jim Hicks, and he agreed. "Yeah, it was a fairly large challenge. We were taking the valve lift velocity and acceleration up to a new level. We have never run anything even approaching 14-mm. in the small-block’s history–not in a production application, anyway.

"The most aggressive part of the profile is the intake event. We held the duration constant and increased the acceleration to get the added lift area. That was done to improve engine performance.

"The exhaust event–actually, we carried over the peak accelerations of the ’01 LS6 cam and let the duration grow."

John Juriga explains engine differences - Click for Larger View

"Dr." John Juriga explains differences in LS6 cams to writers and reporters at the ’02 LS6 preview last May. He had a lot to be happy about. The ’01 Corvette Z06 the the LS6 engine was well-received. His Gen III passenger car engine team is at the top of its game with the 2002 LS6 and they’ve got more good things in the works. Image: Author.
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LS6 Camshaft: Intake Comparison
(All lift figures are tappet lift)

year PN	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 12560950	13.34 m .525 in	270°	204°	9° BTDC	81° ABDC	18 ATDC	42 ABDC	118° ATDC	1862.9 mm/deg.
MY02 12565308	14.01 m .551 in	267°	204°	7° BTDC	80° ABDC	19° ATDC	43 ABDC	120° ATDC	1936.9 mm/deg.	4%

LS6 Camshaft: Exhaust Comparison
(All lift figures are tappet lift)

year PN	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 increase
MY01 12560950	13.33 m .525 in	275°	211°	65° BBDC	30° ATDC	37 BBDC	6 BTDC	114° BTDC	1914.6 mm/deg.
MY02 12565308	13.91 m .547 in	282°	218°	69° BBDC	33° ATDC	42 BBDC	4 BTDC	115° BTDC	2046.6 mm/deg.	8%

To preserve idle stability, the MY02 intake duration at .050-in lift was held to the ’01 specification. Idle quality is a pleasability issue with Corvette customers, but more importantly, it impacts exhaust emissions. For LS6 to meet the national low emission vehicle (LEV) standard, it’s got to idle smoothly. With the intake duration frozen, the only way to increase air flow and, thus, performance was to add valve lift.

The exhaust duration at .050 lift was lengthened and valve lift was added. Both increased exhaust port flow.

Another change made to preserve idle quality was to spread the lobe centerlines apart. "We spread them by retarding the intake lobe two degrees and advancing the exhaust one degree," Jim Hicks said. "Typically, when you delay the intake closing point, you give up a little torque at low speeds, but it doesn’t hurt power. In fact, if anything, it might help power a little bit.

"The main reason why they were spread like that is: with the longer exhaust duration, we had to spread the lobes to maintain overlap at the ’01 LS6 level so the idle quality wouldn’t be degraded.

"There was no downside from a power perspective and the torque really wasn’t hurt either. We made up for it in the additional lift area."

The ’01 and ’02 cam profiles graphed against each other. MY01 is in red and MY02 is in black. The extra spike in the center is a 5x blow-up of the overlap area. The line at 1.27-mm.. lift represents the .050-in. lift the aftermarket cam companies use as a benchmark.
Chart: GM Powertrain Division.
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Another major difference between 385-horse and 405-horse cams is the base circle radius. The base circle for the ’01 LS6 is smaller than that of the LS1 and the truck cams and, for MY02, it’s even smaller. Most Gen III cams have a 19.7-mm. base circle but the ’01 LS6’s is 19.3 and the ’02’s is 19-mm.. Both reductions were to accommodate increases in valve lift.
We asked Jim Hicks why the base circle had to get smaller when lift increased? "All of our cams (prior to LS6) had the same base circle radius. We had a problem with that base circle, if we wanted to go to higher lifts: the nose of the cam would approach the same diameter as the cam bearing journals or even above 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, or reduce the base circle radius."

The ’01 base circle reduction did not require a change in dimensions of any other valve train part, however, the 405-horse cam was a different story. "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’s 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 are 0.6-mm. longer than the valves in all other Gen III engines."

With .025-in more intake lift, the same intake duration at .050-in but a little less duration at lash and the same rev limit; something had to be done to the rest of the valve train to keep it in control at high rpm. While the ’02 exhaust lobe doesn’t have quite the aggressive profile as the intake, it’s still got more lift, so something had to be done there, too. The choices GM made were to decrease valve weight and increase valve spring pressure.

"With the higher lift, we needed to reduce the mass of the valvetrain or start getting into a float condition," John Juriga commented. "We went to hollow stem intake and exhaust valves, very similar to what we used in the ’96 LT4. We pushed the edge with a state-of-the-art, 0.8-mm. (valve stem) wall thickness–very thin stuff. The exhaust stems are sodium-potassium filled."

Not only are ’02 LS6 valves taller, but these cutaways, made before the valve goes through the final machining steps, tell a bigger story: the significant mass reduction in hollow stem valves. The one at center right is the exhaust valve and its cavity is filled with the Sodium-Potassium compound. Image: Author
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The ’01 intake weighed 99 grams but the ’02 weighs only 76. The ’01 exhaust weighed 86 grams but the ’02 exhaust weighs 63 grams. The exhaust stems are filled with a 78% potassium/22% sodium mix to help cool the valve. "NaK" is unstable and may spontaneously combust when exposed to air with 50% or higher humidity. Do not cut open or shorten 02 Z06 exhaust valve stems.

As a result of the more aggressive cam, the valve springs were, also, changed. "Even with the lighter valves," Juriga said, "we still needed better control because we open and close them very fast. The closed loads are the same–400 Newtons (90lbs) on the seat–but the open loads increase from 1150N (259lbs) to 1310N (294lbs) for both springs."

A bigger cam, lighter valves, stiffer springs–so goes the ruthless pursuit of power. And–what’s this new camshaft and valve train stuff worth, by itself? Jim Hicks: "You can do a direct a-b and there’s an easy 8-10hp there, whether you change the back-pressure or not. There are other changes in the ’02 package–exhaust system and induction system–which increase the power more. The overall power increased about 20hp and the cam was half of it. Again, that’s because it’s a short-duration design with low overlap so it’s not really affected that much by back-pressure.

"In fact, we ran the ASA (the American Speed Association, an oval track racing spec. series that uses a modified version of the LS1) cam, back-to-back with this ’02 LS6 cam. If you don’t change the exhaust system, you only pick-up 2 hp with the ASA cam, but if you drop the back-pressure to something near zero (ie: a racing exhaust), then it’s more like 20hp. So, that’s the back pressure effect I keep talking about."

What about that ASA cam? It’s got only .525-in. lift but, at .050-in., it’s got 226° intake duration and 5.5° overlap versus the LS6’s 204° and no overlap. Clearly, it’s designed for higher rpm and more power but it needs a very low restriction or open exhaust. Will it work in a hi-po street LS6? Well–kinda sorta. Once you get the cam and the right valve train pieces, the biggest problem comes if you have to remain emissions legal. Getting the engine to run with the ASA cam but without the OBD2 diagnostics blowing codes would be a challenge you have to tackle–but only after you design a cat converter set-up and exhaust system that has low enough back pressure such that the ASA cam’s potential can be realized.

What about updating ’01 LS6es with the ’02 cam? Don’t do it, unless you add the ’02 valves and springs. Why? "The biggest issue is:," Jim Hicks stated, "without the lightweight, hollow-stem valves, you loose about 300-400 rpm in limiting speed. If you continue to run the LS6 calibration, where the fuel cutoff is 6600 rpm; you’re gonna be running into some significant valve train distress at 6200-6300 rpm. I have no idea how durable that combination is going to be over time."

LS6 Revolution:
Pups Out, New Cats In