Back in October, Motor Trend wrote that, during its dyno testing, a 2020 Chevrolet Corvette had made horsepower and torque numbers well above the rated 495 horsepower and 470 lb-ft. The publication concluded then that the car “produces more power than what Chevy claims.” But now in a new follow-up, the team admits that it conducted the testing improperly. Here’s why the figures were so high.
There are far too many factors involved in chassis dyno testing to take results at face value, which is why I hedged in my story about Motor Trend’s Corvette C8 dyno results, which ranged from 478 HP and 536 lb-ft of torque at the wheels to 561 HP and 515 lb-ft at the wheels. Motor Trend assumed a 15 percent drivetrain loss, and thus wrote that those numbers corresponded to roughly 562 HP, 630 lb-ft at the crank, and 660 HP and 606 lb-ft at the crank, respectively.
Clearly, that’s quite a big gap between runs, even if they were run in different gears. Plus, the output is well above the 6.2-liter LT2's official 495 horsepower, 470 lb-ft rating from the Society of Automotive Engineers, whose certification rules require very limited variance in horsepower and torque among production engines.
So, what went wrong? Well, for one, the dyno was set to all-wheel drive mode, which means it assumed the engine had to overcome more inertia than it really did. Motor Trend’s technical director Frank Markus breaks it down in the follow-up story:
Now we’ve learned that our dyno operator selected two dyno settings that were incorrect. One is a key parameter that had yet to be released for the C8 and so was estimated from C7 data. That is road-load horsepower at 50 mph. This factor incorporates friction and aero drag, and it can be measured empirically by conducting coast-down tests, but because dyno-testing the C8 was a last-minute fill-in to our schedule when Real MPG testing proved impossible on our last day with the C8, we had no opportunity to measure it. The dyno operators used 12.6, when Chevrolet has informed us that the correct factor for our Z51 should have been 15.4. But the bigger boo-boo was that the dynamometer was also set to assume all-wheel drive.
These two factors conspired make the dyno believe the Corvette’s powertrain was overcoming way more inertia than it really was, which led to the inflated results. Sadly, there’s apparently no way to virtually “rerun” the test in the computer with corrected parameters, so we simply must get another C8 back and run the test again.
Note that this aligns with what a representative from Mustang Dynamometer—the company that built the dyno that Motor Trend used—told me over the phone about what he thought may have contributed to the inflated numbers.
Markus then discusses in the follow-up story how his team analyzed the acceleration of two Corvettes that had undergone track testing, and used the figures to back out some “road dyno” figures. Assuming 15 percent drivetrain losses between the crank and wheels, Motor Trend determined based on the data that one Corvette seems to make roughly 500 horsepower and the other makes about 465 horsepower.
Markus suggests that there’s quite a bit of uncertainty in these “road dyno” estimates, and that really getting to the bottom of this will require more testing on a proper dyno. Ultimately, his conclusion hedges quite a bit more than the previous article’s confident claim that the red Corvette tested produces more power than Chevy claims. From Motor Trend:
For now, let’s just say we’re convinced our red test car was certainly making every bit of its rated output of 495 hp at 6,450 rpm and 470 lb-ft at 5,150 if not slightly more. Stay tuned as this saga continues.
Something seemed wrong about the previous story’s numbers, and indeed, now we know that it was wrong. I’m glad we’ve got a bit more context behind why.