We Toast the C5 Corvette's Final Year with a Test of the 2004 Corvette Z06 - Page 4 of 6

by Hib Halverson
text and images ©2004 Hib Halverson
No use without permission

A Bit Underhood

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In so many cars, today, engine compartments look so neat and understated. Other than red coil covers and an "LS6" plate on the throttle body, the engine looks like that in any other C5. It puts out 405 SAE het hp. Rated with the old gross power system of the bygone Muscle Car era, it would make about 450hp. Understated, indeed. No wonder the car runs mid-12s.
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The LS6 was introduced in 2001 with 385hp. The following year, it got bumped 20hp with a more aggresive camshaft, stiffer valve springs and lighter valves. It's remained at 405hp ever since. Some Corvetters compare LS6 to the LT5, a 405hp DOHC 350 in '93-'95 ZR-1s. The comparison proves the march of technology is always beneficial. The '02-'04 LS6 produces the same horsepower as the LT5 but does so with a wider torque band, two-valve heads, pushrod valve gear, slightly less displacement, less weight, better fuel economy and lower cost.

LS6's image was tarnished a bit by an oil consumption scandal in its first year. GM addressed the problem with a technical service bulletin fix in late '01 for all LS1s and LS6es consisting of second compression rings having a redesigned scraper face and a higher tension, oil ring expander. A new, LS6 valley cover with revised sealing was released early in '02. Both these fixes were put in production, the rings at the start of '02 and the valley cover during that model year.

One neat thing about driving an LS6 is its fat torque curve and that it likes to rpm. Whereas the LT5 before it, fell-off past 6000 rpm, the LS6 peaks at 6000 but stays fairly flat to its 6600 rpm rev limit. Another endearment is its exhaust sound, especially with the throttle wide open and the engine pulling to peak power. More sporting and a little louder than that of the base model, it's the first Corvette exhaust I can honestly admit that I could leave stock. The Z06 system is also unique in that its back half is made of titanium which makes it 18 lbs lighter than the standard part.

The LS6 has remained unchanged since '02 and the big story we wanted to focus on with the '04 was handling.

The Z06 uses a specific version of the rear-mounted, Tremec T56, six-speed transmission having special gear ratios. The first three gears are lower and spread wider apart and the upper two gears are lower but closer together. Bill Zabritski, GM's Design Release Engineer for manual transmissions in performance cars, told the Corvette Action Center, "During development, we found the Z06 was quicker with higher ratios in the first three gears."

Click on picture for enlarged image.Of all the C5s we've driven, this '04 had the best shift feel. The only tiny wish we have would be a little shorter shift throw. That says a lot for a linkage that is half as long as the car and operates an internal rail shift mechanism less sophisticated than that in previous Corvette six-speeds.

The rear axle assembly used in the Z06 is generally the same unit manufactured by Getrag and used by all C5 manuals since 1997. It has one difference: a shotpeened ring and pinion which adds an extra margin of reliability to a powertrain having increased torque output.

An annoyance with our test car was an intermittent axle lubricant leak. Several times during our test, upon exiting the vehicle, we smelt that sulfur stink of gear oil on the exhaust. The second time this happened, we rolled the car into the shop, got it up on jackstands and inspected the rear axle. There were signs of leakage on the left side of the case, the rear spring and the suspension crossmember, but we couldn't find the source. Later, in talking with a Corvette service technician, we learned the problem is oil blown out the vent and is common with cars which get driven aggressively such that the lube gets hot. The easly solution, according to our source, is to run the fluid level about an 1/8-in. below the filler. A better solution might be a remoted-mounted vent (typical of some GM four-wheel drive trucks) at the end of a hose, a little distance away from and a bit higher than the trans. It's unfortunate GM couldn't address this during the eight years of C5 production.

How hard will this powertrain make a Z06 run? We installed our Vericom VC-2000 Vehicle Performance Computer then banged-off an astonishing best of, 3.85-second, 0-60 time. Performance in the quarter was equally substantial at 12.45 sec./118.0 mph. That performance was tested using timing with no roll-out, on a typical road surface and at the hands of a driver who can launch the car in a manner that won't run afoul of the "antipowerhop" algorithms in the LS6's engine controls software. We were also blessed with ideal atmospheric conditions: sea level, cool temperature and high humidity.

We think under more average conditions with average drivers, four-flat, 0-60 and high 12.5s in the quarter can be expected of these cars. Pretty damn incredible for a car that's totally stock.

Inside and Out

Some in the automotive press say the C5 Corvette interior looks "cheap". Yet, after a week in this car, I can't say the "old" interior is as nice as that in the new C6 or a Caddilac XLR but it's certainly not cheap.

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The C5 cluster and IP are the bomb. The instruments are placed and accented visually to create a sort of 3D effect we've always thought was pretty cool-even cooler at night with the cluster's ultraviolet-lighting. Right then, coolant temp was shown, but the IP display can read all kinds of interesting and important stuff, some of which we don't even know.
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The Z06's interior appearance is attractive, efficient and focused on providing information to the driver. Standouts are an easy-to-read instrument cluster and an IP display capable of reading the condition of engine parameters along with tire pressures. It even can display engine controls diagnostic information though we were unable to sample that ability because our test car had no Owner's Manual for us to find the instructions on how to use it and other features such as programmable exterior lighting, security system and keyless entry.

All Z06es have a "head-up display" (HUD) with which sporting drivers can read real-time, corner exit speeds. The HUD also reads speed, rpm and other data, and what information is displayed, is user-selectable. No other sports car in the world has this feature.

What other information should this car give the driver? One of our photo assistants, also a Corvette owner, suggested a compass and an altimeter-not a bad idea for a car that flies.

If there's a weakness of this interior, it's the seats. They have side support which is average, at best for a car with such incredible lateral grip and the seat back adjustment mechanism is just junky. Three different times, I got in this car, arched back in the seat to fish keys out of my pocket then had one side of the seat back suddenly lurch into a partially reclined position. I've heard occasional owner complaints of similar trouble. GM should have fixed that early in C5 production. Hopefully that improves in C6.

The end of the wiper stalk is too close to the 2-o'clock position on the steering wheel. When you're driving hard, zipping your hand from the wheel to the shifter and back, if you're not careful; you get the wiper lever, then look like a dork as the wipers dry-hump away while you downshift into a turn. That's another item that should be fixed on the 2005 Vette.