Motor Trend recently put out an article claiming to test how much power the 2020 Chevrolet Corvette Stingray really makes based on a single dyno test. It cannot possibly be accurate, and we can prove it with simple math. We still have more confidence in the figures delivered by GM and certified by the Society of Automotive Engineers than by this one C8 Corvette dyno test. And we’re going to do our best to explain why you should as well.
A chassis dynamometer is something we think of as something that measures “power at the wheels.” When you break it down, what the dyno really measures directly is the force applied by the tires. After that, it’s usually up to an engineer to correctly quantify the gear reductions in place, and the rolling radius of the tire. This is where things tend to go wrong.
Motor Trend, the magazine that brought us the Apple Car, sought a 1:1 transmission gear ratio for this C8 Corvette dyno test presumably to subscribe to an old rule of thumb, minimizing losses through inefficient gear reductions while simplifying their calculation by only requiring that the final drive ratio is quantified.
In any gear however, one can calculate the total gear ratio from input to output with engine speed, output speed, and tire size and correctly calculate torque and power (which is correctly noted by their technical editor Frank Marcus).
If these things are correctly taken into account, then testing in 5th gear vs 6th gear should only yield a small change due to differences in gear efficiency and differences in acceleration of the system during the test. In short, power and torque differences should be small if calculated correctly in both 5th and 6th gears.
Therefore, figures of 478 hp in 6th gear and 558 hp in 5th gear should draw skepticism from trained eyes.
The figure Motor Trend chose to stick with was 558 whp, which was determined in 5th gear since it’s close to a 1:1 ratio. This result simply does not pass the sniff test.
Acceleration is a function of power delivered, weight, and aerodynamic drag. And you can see based on several different results that the acceleration of the 2020 Corvette Stingray agrees more with the figures from SAE-certified numbers from General Motors than the ones determined by Motor Trend.
One glaring example is actually in a Car and Driver comparison test between the last-generation C7 and the new C8 Corvettes. The C8 Stingray goes just 2 mph faster than the base C7 on Grattan Raceway’s straightaway at 142 mph… and even this small advantage for the C8 is aided by its greater corner exit traction, since it entered the straight with a 3 mph advantage based on the magazine’s telematics. From this, it’s safe to assume the C8’s power/weight ratio is pretty close to that of the C7.
In other words, 490-495 horsepower out of the 6.2L LT2 V8 engine, as advertised. Unless of course MT got a special C8 Corvette to test, which we doubt.
It’s not clear what went wrong in the Motor Trend testing. The dyno used is claimed to be SAE J1349 certified – the very testing method that measures horsepower to which automakers base their figures on. Perhaps to save face, the outlet did note that the SAE testing method utilizes a slower ramp than what they employed in their own testing. If heat isn’t an issue, the SAE method should actually yield higher power numbers as more of the engine power goes to the dyno and less goes into accelerating the engine and driveline.