MIGUEL CORTINA
I'm looking through a window, hands around my face, staring at a screen with all sorts of graphics and numbers that are too small to read. On the other side of the window, a red 2020 Chevrolet Corvette is strapped to the dyno with its rear wheels spinning very, very fast.
The double-paned windows are not enough to keep the engine note from reaching other parts of the facility. The speedometer on the screen reads 150 mph, but the front wheels are not moving at all. Once the engine noise winds down and the wheels stop spinning, the numbers we're looking for pop up on the screen—558 hp and 515 lb-ft of torque at the wheels. "We've got a hot rod!" says international bureau chief Angus MacKenzie, who has been standing next to me all this time.
We're in shock. A quick math check reveals that's an estimated 656 hp and 606 lb-ft of torque at the crank if we assume a 15-percent drivetrain loss—way over the 495 hp and 470 lb-ft that Chevy claims. (That 15 percent represents the power consumed by everything between the engine crankshaft and the drive wheels, including inertia of all the spinning parts, power to run the hydraulic pump in an automatic or twin-clutch transmission, the drag that occurs when gears spin through lubricating oil, friction between the gear teeth, etc. It's an educated guestimate frequently used across the industry for modern light-duty automatic transmissions—manuals experience slightly smaller losses; older or heavier-duty automatics slightly larger ones.)
But let's rewind. Earlier that morning, Chevrolet dropped the 2020 Corvette with VIN 10 at our headquarters in El Segundo, California, to perform a Real MPG test by our partners at EQUA. The early-built production car was shipped from the plant in Bowling Green, Kentucky, to Milford Proving Ground in Michigan for a check before it made it down to Southern California. The Corvette was tested by MotorTrend the week prior at Fontana and a few days later at the Hyundai-Kia proving ground for our Car of the Year competition. The 'Vette spent every night under the vigilance of the Chevrolet public relations team and was handed back to the editorial team in the mornings. The evening prior to the dyno run, the Corvette was driven by an editor from Tehachapi, California, to our headquarters, where it was taken by the Chevy team for the night.
That sunny September morning, the EQUA technician told us he didn't have the right equipment for the test—he needed a different set of tubes to seal around the exhaust outlets; the ones he had wouldn't resist the higher temperature from a mid-engine sports car. Our Plan B? Take it to the dyno before the Corvette got on a trailer to go back to Michigan that evening.
Which is how Angus and I ended up staring through the glass at those surprising numbers. We asked the dyno technician to run the test in fifth gear after a call Angus and I had with Chris Walton, our road test editor, who estimated that fifth gear could probably be the closest 1:1 ratio. (A call with Chevy would later confirm that, but more on that later.) We decided to run the test again.
The massive fan starts to blow air straight at the Corvette as the rear wheels start moving. Our attention shifts to the screen—558 hp and 512 lb-ft at the wheels, almost identical numbers as the first run. Assuming a 15-percent drivetrain loss, those numbers go up to about 656 hp and 602 lb-ft. Still too high. What is going on?
Runs three and four are also done in fifth gear; the dynamometer reads 561 hp and 515 lb-ft, then 556 hp and 523 lb ft. That translates to 660 hp and 606 lb-ft and 654 hp and 615 lb-ft at the crank, respectively, assuming the same 15-percent drivetrain loss. We get Chris back on the phone.
The numbers are too similar. We don't have the gear ratios, so Chris suggests we try sixth gear to see if the numbers change. The engine starts revving, wheels start spinning, and once again everyone is staring at the screen.
The numbers are different. The dynamometer reads 478 hp and 536 lb-ft at the wheels. Per our calculations, that means the crank is outputting about 562 hp and 630 lb-ft of torque. Both numbers are still high, but why did the torque number rise instead of going down? The next run, our sixth, would be the last.
Our eyes are glued to the screen. The bright green numbers appear on the monitor, reading 478 hp and 544 lb-ft at the wheels. If we continue to assume a 15-percent drivetrain loss, this 'Vette would be producing about 562 hp and 640 lb-ft at the crank. We're all scratching our heads.
We couldn't come up with a conclusion other than the engine makes a lot more power than the advertised 495 hp and 470 lb-ft of torque. We call Chevy and ask them to set up a call with engineers and to provide the 'Vette's gear ratios.
A short time later, Chevy PR provides the following gear ratios:
2.905
1.759
1.22
0.878
0.653
0.508
0.397
0.329
"There has to be another transfer gear ratio between these and the axle ratio, I guarantee," MotorTrend technical director Frank Markus says. Using the rear wheel speed and tire size, Frank was able to estimate the output shaft ratios, but we waited until the call with engineers to confirm.
A couple of days later, engineers provide us two reasons for the discrepancy of power. The first one is that when the Corvette is cold, it actually produces more horsepower than when it's hot. But that doesn't make sense—we saw similar numbers after six runs, and the car was pretty hot by the end of the second run. The other explanation is that Chevrolet certifies most of its engines through the SAE, the Society of Automotive Engineers, which follows a strict set of rules and standards to determine the horsepower and torque ratings. In other words, the SAE acts as an independent party that's present during the engine tests and is the one who determines the final output ratings. Their testing does not involve a simple pull from idle to redline, either. Rather, rpm are slowly ramped up and allowed to stabilize before accelerating further. This process results in significantly more heat generation than any single pull from our six dyno runs. For that reason, the engineers say, it's not uncommon for single chassis dyno pulls to register higher output (and it is extremely unlikely any car will ever generate less than rated output). According to Chevy engineers, this is an expensive process, given that someone from the SAE has to be present, but the automaker has done this for years, and it's a procedure that it continues to follow with most of its engines in the U.S.
A quick check of the SAE database reveals that the certification test of the 6.2-liter V-8 LT2 engine with the optional exhaust system took place at the Pontiac Engineering Center in Michigan on April 9, 2019. Jordan Lee, chief engineer of the Corvette's engine, signed the certification on July 15—just three days before the reveal in Tustin. The engine was rated at 495 hp at 6,450 rpm and 470 lb-ft of torque at 5,150 rpm The vehicle code name 2020 Y2XX underwent three tests, and the power and torque graph looks similar to the one we had at the dyno.
Per the SAE website, only a few manufacturers certify their engines through them. General Motors dominates the list, and a few Ford and FCA engines have also been certified.
Chevy engineers also say that the drivetrain loss of a dual-clutch transmission, like the one in the Corvette, is less than 15 percent—but hesitated to give us an exact number (and even if it was zero loss, we still measured more wheel-horsepower than rated crank horsepower). They did provide the gears with the transfer ratio for the manual limited-slip differential and the electronic LSD. Our Z51 with an electronic LSD had a 3.454:1 axle ratio, and after applying the 1.459:1 transfer ratio the effective gear ratios are:
4.239
2.567
1.780
1.281
0.953
0.742
0.580
0.480
Turns out fifth gear is the closest 1:1. And the fact that the LT2 engine was certified by the SAE means that there could be more power getting to the wheels on our comparatively cool chassis dyno pulls.
Why the huge discrepancy with the numbers? We still don't know. The dyno we used complies with the SAE J1349 procedures, and we've used it multiple times in the past. To prove there wasn't a problem with the dyno, we ran a 2020 Ram 2500 Limited powered by the 6.7-liter turbodiesel Cummins engine, which produces 850 lb-ft of torque but is not SAE-certified. The dyno read 760 lb-ft at the wheels, which means there's about 890 lb-ft at the crank, much closer to the numbers Ram claims.
One thing we know for certain: the 2020 Chevrolet Corvette C8 provided for all our testing produces more power than what Chevy claims. The question is, will all subsequent production Corvettes match this one's output? You can be sure we'll be testing many C8 Corvettes to come, and we'll endeavor to test retail customer cars as well as press cars. Stay tuned.
Source: MotorTrend
Comments