Key parameters were botched, but the C8’s still plenty powerful
During its last day in our care, we put the C8 Corvette on a dynamometer and got some eye-opening results. It was a Mustang dyno we’ve used in the past that has generally provided reasonable results and which provided thoroughly believable numbers on a 2020 Ram 2500 with a Cummins later the same day. We had lots of trouble believing the numbers produced during this test, however, and so our bottom line was: “One thing we know for certain: The 2020 Chevrolet Corvette C8 provided for all our testing produces more power than what Chevy claims.” Many of you have also written in to express your own “reservations.” We’ve remained silent while conducting a thorough investigation, which is now complete.
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 (80 km/h). 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.
But everyone’s test numbers have been so impressive with this red Corvette that we still longed to find some proof of its actual horsepower, so we tried to extract some “road-dyno” pulls out of our best drag strip run by examining the acceleration from the upshift point to redline in second, third, and fourth gears. We computed the corrected time to each 1-mph (1.6-km/h) increment from 46 to 55 mph (74 to 88 km/h) in second, from 70 to 80 mph (113 to 129 km/h) in third, and from 102 to 112 mph (164 to 180 km/h) in fourth. Then by analyzing the car’s acceleration rate between each of these mph points, given its mass and what we know about its aerodynamics, we backed out a rough estimate of the power at the wheels. These runs included the rpm at which the power peaks, but peak torque could not be estimated from these pulls.
The data point cloud was pretty widely dispersed, but all the pulls include data points above 450 hp, and with a bunch of smoothing we got a polynomial trend line that peaks at 425 hp just past 6,300 rpm (which, assuming 15 percent losses, comes in at 500 hp).
Then an opportunity presented itself to mount our test gear to a different Corvette at a different racetrack, one that included some relatively flat and level areas where we could run a proper “road dyno” pull from 1,500 rpm to redline in second and third gears. This testing occurred on a different (orange) Corvette Z51. Using our fanciest 100-Hz Vbox we measured very fine time-to-speed data that included the longitudinal g measurements. The second- and third-gear pull results were in remarkably close agreement: Horsepower at the wheels came out at roughly 395 hp at 6,250 rpm with torque at just under 350 lb-ft at about 4,300 rpm. Assuming our 15 percent crankshaft-to-wheels parasitic loss figure, this car comes in a shade under the rated output at 465 hp and 412 lb-ft.
Is this simply car-to-car variation? Does road-dyno testing involve too much uncertainty or speculation? We probably can’t know until we test some more C8 Corvettes on the dynamometer and the drag strip. 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.