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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 intent with expected suspension and tire improvements, on tracks with long, sweeping turns, engineers noticed excessive oil consumption at high-rpm and high, lateral acceleration.
Further testing during the winter of ’98/’99 proved oil was being trapped in the valve covers then sucked into the engine through the PCV system. The solution, which took the first half of ’99 to perfect, was new PCV hardware. Taking a page out of the 13-year-old, 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 (engineers sometimes call it "cal") for the LS6 differs mainly in larger capacity injectors and in fuel and spark software. The LS6 injectors flow 28.5 lbs/hr., whereas the LS1 units flowed 25 lb/hr. We asked John Juriga about the cal. "Obviously, with higher compression ratio and higher output, the fuel curves changed as well as the spark curves–quite a bit different calibration to handle the higher air flow rates, higher fuel flow rates and higher compression. We started with the LS1 spark and fuel, then modified that until we gained the power we wanted. We also did sweeps looking at emissions–NOx, hydrocarbons and CO–at part throttle conditions making sure that is all optimized."
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What will the leader of GM’s ruthless pursuit of power spring on us next? Well, he’s obviously not saying at this point, but we bet it’s going to be 405hp in ’02. Photo: author.
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Juriga also indicated that since 1997, there’s been a change in how engine controls work in the "power enrichment" mode which is enabled at high engine loads. Previously, though an engine might be operating in a manner that met the requirements for closed loop control of the fuel flow, beyond a certain throttle opening, the engine controls would ignore the oxygen sensor inputs and set fuel delivery straight from the lookup tables.
"We run closed loop even at wide-open-throttle,"
Juriga states, "but what we do is we just run rich of ‘stoich’ (engineering slang for "stoichometric," a scientific term for ideal combustion)
but it’s all a closed loop system. The only open-loop portion we have is just the very, very beginning at start-up. It’s that way until our oxygen sensors ‘wake-up’–they don’t function until they reach a certain temperature. As soon as the O2 sensors start sending a signal, we stay in closed-loop all the way to wide-open-throttle.
We asked Juriga if the task of calibrating such a powerful engine and still meeting LEV was a major challenge. We got the short answer,"No. It really wasn’t all that bad.""
The LS6’s fuel cut-off is at 6600 rpm. Of course, the first question C5 hot rodders are going to ask is, "What happens if I change the rev limit so I can run harder on the drag strip?" We asked Dr. John what keeps the LS6 from rev’ing to 6800 or so. "Fuel cut-off," he replied laughing. On a more serious note, he added,
"With the current hardware, my recollection is the valves float about 6800-7000 rpm. The concern is all the other parts in the engine, not just the valve train. We can work on the valve train to get it to go that higher speed. Then we start being concerned about oil film-thickness in the rod bearings, rod bolt strength and piston temperatures. Even though we upgraded our piston, if we were to go with higher speed, we’d have to go with another improved piston as well as upgraded piston pins and I’m talking about forged pistons and floating pins. We’d also have to go with an improved strength rod.
A lot of the hot rodders reading this might trivialize GM Powertrain’s position and say, "Heck, what a bunch of wimps. Just put some bigger valves springs on, bigger injectors and change the rev limit. Considering that viewpoint, we asked Dr. John why the conservative approach. "You could run an LS1 or an LS6 to seven grand," Juriga answered,
"and not immediately put a rod through the side of the block. What is a concern is how often and how long your run the engine like that. We have our durability goals that say we gotta be able to run these engines for 125,000 miles at a certain confidence level. If we had the engine running at those higher speeds, we’d have to validate the engine at the higher performance level for that period of time and that’s something we’re not ready to do right now."
We like the tail end of that sentence: "...not ready to do--right now." Reading between the lines our guess is Dr. John and his bunch continue in the ruthless pursuit of power. No doubt, the basic Gen III architecture will support increased power output.
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 the arrival, but I was close on the power. My next fearless forecast? An even more powerful Gen III is just around the corner. Think 405hp for 2002
Let’s see if I’m right.
The author and The C5 Registry would like to thank John Juriga, Jim Hicks and Dave Roman of the GM Powertrain Division for special assistance in the preparation of this article.
The author offers a special word of thanks to Tom Hoxie for his support in making this article possible. Until the end of 1998, Mr. Hoxie was Assistant Director of Chevrolet Communications. After that, he worked for
Marcom, the company ChevyComm retained to organize the Corvette Z06 media preview in May of 2000, the event at which much research was done for this article.
Tom Hoxie is now retired and playing golf in North Carolina, but remains a steadfast supporter of the Corvette. The author is indebted to him for special assistance at the Z06 preview and many Corvette media events in the past.
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