Because like, has GM ever told us the truth?
It was the spring of 2006 when I finally learned the true specs for the LS7 engine that inhabited the engine bay of the then-new Corvette Z06: 7008 cubic centimetres (427 CID), 512 PS (505 HP) at 6300 RPM and 637 Nm (470 lb-ft) at 4800 RPM. These numbers were considerably larger than the LS2's, however upon a second look, they seemed meagre. Let's take the following in account:
- the LS7 revs almost 10% harder than the LS2;
- the LS7 displaces over a litre (17.5%) more than the LS2;
- the intake valves are bigger than the LS2's;
- the exhaust valves are also bigger than the LS2's;
- GM rated the engine as topping out at 7100 RPM, yet it is known to be 8000 RPM-capable;
- the final horsepower count is 25% higher than the LS2's and torque is just 18% taller.
At the time, I was 13 or so years old, so I was obviously more interested in popping zits, pretending I could chase skirts, listening to music at obnoxious volumes and understanding that all-too-vile thing called Maths to care. In fact, back then I didn't even know for sure what torque figures were. But as the years accumulated, zits were replaced with beard, I started to actually chase skirts, I discovered what headphones were, and I had enemies other than Maths, my mind started to ask questions. One of those questions was how the LS7, which supposedly was an LS2 set for the battlefield, produce so little additional torque - my first question was about torque, not horsepower. After all, it has the combined displacement of a C20 and a Coyote, so why does it produce little more torque than a Gen V LT1? Shouldn't it blow pretty much everything off the road, apart from turbocharged and supercharged engines and the Viper's V10? Well, I think it does and that General Motors doesn't want you to know. Let me explain how.
Dyno figures don't add up
When I decided to look for C6 Z06 dyno figures, this was the first dyno chart I found. According to this chart, a stock LS7 engine as mounted in a 2007 Chevrolet Corvette Z06 produces 452 PS (446 HP) and 595 Nm (439 lb-ft) at the rear wheels. Assuming there's a drivetrain loss of fifteen percent, the flywheel output can be calculated as being of 532 PS (525 HP) and 700 Nm (516 lb-ft). That's considerably more than what GM declared. It's also of note that in this dyno chart, maximum power seems to be reached slightly before 6000 RPM, while maximum torque happens only a little after 4000 RPM (compare with GM's official figures of 6300 and 4800 RPM, respectively). Those figures certainly make a lot more sense than the comparatively puny official numbers, at least if we take in account the gargantuan displacement of the LS7.
The Z06 is too close to the ZR1 performancewise
By the official numbers, one would expect the C6 Z06 to stand considerably behind the ZR1 in the straights. For starters, because the LS7's official 512 PS (505 HP) at 6300 RPM and 637 Nm (470 lb-ft) at 4800 RPM are small fish compared to the LS9's 647 PS (638 HP) at 6500 RPM and 819 Nm (604 lb-ft) at 3800 RPM, even though the Z06 is slightly lighter. Then, there's the fact the Z06 came with the Borg-Warner T56, which Hot Rod described as "prone to missed shifts" and Evo compared to a train's lever, while the ZR1 has the far more modern and convenient Tremec TR6060. Going by official figures, the Z06 has a power-to-weight ratio of 361 PS/T, while the ZR1 scales at 418 PS/T. That's a big difference. However, when the two cars are tested, the differences vanish. When Car and Driver road tested the whole Corvette range in December 2008, they weighted the Z06 at 1456 kg (3210 lbs), slightly more than claimed, and managed 60 mph in 3.6 seconds, 100 mph in 8.3 seconds, the quarter mile in 11.7 seconds, and the 5-60 mph run in 4.1 seconds. The ZR1, weighted at 1519 kg (3350 lbs), less than claimed, went up to 60 mph in 3.4 seconds, 100 mph in 7.6 seconds, the quarter mile in 11.5, and did the 5-60 mph run in 4.0 seconds. With even bigger differences in power-to-weight ratio than the ones I calculated, and the ZR1 just stands barely above the Z06 in all acceleration exercices (including a pathetic 0.1 second margin in the 5-60)?
Okay, let's ignore Car and Driver, they're not a serious car magazine. Road and Track did a similar test (plus a run in Willow Springs) in April 2012. The Z06, weighted at 1501 kg (3310 lbs), did 60 mph in 3.6 seconds, 100 mph in 7.9 seconds, the quarter mile in 11.7 seconds, and ran Willow Springs in 1:15'15. The ZR1, weighted at 1544 kg (3405 lbs), closer to claimed, did 60 mph in 3.5 seconds, 100 mph in 7.3 seconds, the quarter mile in 11.5 seconds, and ran Willow Springs in... 1:15'99. A full 7 car lengths behind the Z06. That's already with the almighty Tremec TR6060 in both cars, which puts both engines more-or-less in equal terms.
I couldn't find many numerical tests from magazines this side of the Atlantic, but Auto Motor und Sport did measure 100 km/h in 4.0 seconds, 160 km/h (approx. 100 mph) in 7.9 seconds, and 200 km/h in 11.9 seconds for the Z06 with the T56, while the ZR1 did 100 km/h in 3.8 seconds, 160 km/h in 7.8 seconds, and 200 km/h in 11.2 seconds. (Unfortunately the Germans decided to be lazy this time and did not weight the cars). That's a very insignificative difference between both.
They had possible reasons
When Dodge released the Viper back in 1992, the engine was too noisy for the American EPA to certify it, thanks to the sidepipes. So instead, they decided to downrate the car at 400 HP at 4600 RPM in order not to scare American schoolchildren. In 1995, they decided to dump the sidepipes, so the car's real 415 HP at 5200 RPM could be revealed without Americans, being Americans, going all berserk that "the evil sports car scares our kids!!11" (no offence meant to Americans). When the C6 Z06 came out, Chevrolet advertised it as a "505 HP economy car". The American EPA did not issue a "gas guzzler" tax for it. Did General Motors simply downtune the test engine to pass the requirements, like they did back in the early 1970s for the Super Duty 455?
Other important question is related with range positioning. While the C6 ZR1 only came out in late 2008, one couldn't be silly enough to think that the ZR1 had been entirely developed in response to the 8.4L Viper. It takes a lot more time for such an extensive upgrade over the standard model to be developed, meaning that the plans for a 600+ HP, US$100+K Corvette were already known internally when the Z06 was to be released. And we all know how GM loves to underrate engines to prevent market cannibalism.
The conclusion is that there is no conclusion and my findings were inconclusive. I don't have a fully assembled, stock LS7 near me attached to a functional dyno, so I can't prove the validity of my statement. However, there's a fair amount of evidence and hypothetical causes for the possibility that the LS7 V8 was considerably underrated from the factory, especially in terms of torque. I'm not pretentious enough to claim it a fact without more than dyno sheet analysis and comparison between car magazine road test data. Still, I believe they did. After all, GM always lies.