Why is Chevrolet sandbagging the Corvette?

The people at Motor Trend strapped down the new mid-engine Corvette to a chassis dyno and got some interesting results. Specifically, the calculated crank power they achieved was 656 hp and 606 lb-ft of torque. That is significantly more than the 495 hp and 470 lb-ft of torque that Chevy claims for the Corvette. Motor Trend did several runs, in different gears and at different engine temperatures. They even strapped down a different vehicle to ensure that the dyno wasn’t erroneous. It wasn’t.

It’s goes deeper than just the dyno numbers. The 0-60 mph acceleration takes 2.8 seconds. The quarter mile comes in at 11.1 seconds at 123.2 mph. Those are very fast numbers for a vehicle that weights between 3,535 and 3,637 pounds. Plugging into all into an old trusty-ish trap-speed-horsepower calculator, at 3,535 pounds we get 513 horsepower. At 3,637 pounds we get 528 horsepower. But let’s add a conservative 200 pounds for a suited/helmeted driver and fuel and we get 557 horsepower. This calculation takes only the quarter mile trap speed into consideration, so initial traction issues don’t really impact it. It’s highly unscientific but done years, the results yield higher numbers than Chevrolet’s but lower than MT’s dyno.

Chevrolet says that the numbers are lower due to the SAE horsepower certification process during which a ton of heat soak occurs. That should still raise some eyebrows as the difference between 656 and 495 horsepower is still very substantial.

So what the hell is going on here? Why is Chevy claiming much lower horsepower than it actually is? Are perhaps the first few new cars ringers? Was the SAE tested car just underpowered?

I am scratching my head because I don’t know. While there are environmental and physical testing differences, it should be to less than than five percent. GM would not make media ringer cars this day and age as many customer vehicles will undoubtedly be dynoed soon enough. The SAE certification has historically shown lower power numbers, but not by this factor. Could the MT dyno testing be off that much?

What ever the case, the thing to take away from all this is that the new mid-engine Corvette is damn fast and there is clearly room for more performance to be gained from this initial setup.

Photo: Motor Trend

UPDATE: Jason Cammisa has an interesting perspective on this:

 

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The C8 Corvette Stingray does NOT make 650 hp! ⁣ ⁣ A certain media outlet published a story today that they dyno’ed one and got more than 500 hp at the wheels… calculating back to 650 hp at the crank.⁣ ⁣ This isn’t possible. 650 hp from 6.2 liters of displacement can’t happen with at only 6500 rpm without forced induction. ⁣ ⁣ Horsepower is a function of torque x rpm… and there’s a maximum torque-per-displacement (otherwise known as BMEP, or Brake Mean Effective Pressure) that’s vastly exceeded by Motor Trend’s dyno runs. ⁣ ⁣ MT’s 630 lb-ft number suggests the LT2 makes 17.2 bar BMEP. The most efficient naturally aspirated engines are around 13. The LT2 is SAE rated at 12.9.⁣ ⁣ Did it have turbos on it that no one saw? I suspect not… but they sure didn’t show up in acceleration testing…⁣ ⁣ Here’s the real reason I know the dyno results are invalid. See the screen-shot on image 2. ⁣ This is my acceleration simulator. My nerdiness knows no boundaries, but I’ve been using this calculator since the early 1990s. ⁣ ⁣ I inputted a DynoJet plot from a 7-speed manual Z51 C7 I found on the Internet… 417 hp and 415 lb-ft at the wheels. Then I plugged in the C8’s actual weight, gearing, tire size, and I estimated drag coefficient and frontal area.⁣ ⁣ The numbers (in black, on the right) line up almost exactly with the testing results @roadandtrack got (in red) – within ~0.3s all the way to 150 mph.⁣ ⁣ The most damning is top speed, which calculates to 182 mph. Chevrolet claims 184 mph for the Z51 C8. I’d expect a couple mph higher with the C8’s LT2 engine (495 hp instead of the C7’s 460 that I used here.)⁣ ⁣ If the thing really made 650 hp, top speed would be vastly higher. Like, 200+ mph.⁣ ⁣ MT’s dyno piece doesn’t pass science muster. I’m disappointed that they published it — something was clearly broken on that dyno. They should have brought it elsewhere and re-tested. ⁣ ⁣ Or at least realized that the numbers didn’t line up with the reality of their tests. ⁣ ⁣ #C8Dyno #CorvetteDyno #C8Power #C8 #C8Corvette #Corvette ⁣#Dyno #DynoJet #Dynamometer #Science

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8 Comments

    1. It seems Toyota did the same with the Supra though, And, at least in GM’s case, the Corvette is still offers some of the best hp/$ at the underrated figures, so why risk getting in hot water lest someone eventually has reason to think they’re overreporting hp?

      More cynically though, there’ll be the eventual Z06 and ZR1’s and such who’d benefit from the regular Corvette being a little sandbagged.

      1. The Toyota might be issues of heat-soaked power rating versus what is seen on the first couple of pulls, or boost levels dependent on intake air temp or similar.

  1. Cammisa is spot on that >600 lb-ft is ridiculous for a 6.2L n/a engine. Should have put it on another dyno to check.

    Or perhaps MT applied roll-out like they do for 0-60 runs?

    Edit: having had a read of the article, they’ve also listed the gearbox ratios (2.905, 1.759, 1.22, 0.878, 0.653, 0.508, 0.397, 0.329), then said after you mulitply by the transfer ratio 1.459 that makes 5th gear closest to 1:1, not 4th. Well, not really; otherwise if you follow that logic why not include the diff ratio to say 8th gear is closest to 1:1? Outside the gearbox is irrelevant for losses within the gearbox.

    I think the error comes from not having the right parameters in the dyno software – if you divide the power and torque by the transfer ratio 1.459 it comes to 449 hp and 415 lb-ft which is much more like it. That makes sense given they didn’t initially know about the transfer ratio.

    1. I agree. Cammisa’s evaluation calls out apparent issues with the dyno. There’s no sandbagging, just shoddy journalism.

      1. Derived power calculations can be problematic too. I remember a story from the 1970s where one of the local touring car racers was making 400hp* from his engine having thrown the kitchen sink at it development-wise. He was tempted to buy an engine from the US where they were making 450hp*, however upon getting the engine and putting it on the dyno he was using it actually made less power! I can’t remember the exact power levels, nor the exact reason for the difference in testing procedure or base-line calculation that was the reason for the discrepancy – one thing is for sure, it wasn’t the air spinning the other way in the carburetor intake due to being in the southern hemisphere!

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