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    Motor Trend admits error in Corvette C8 dyno testing

    https://www.autoblog.com/2019/11/12/...rsepower-dyno/

    Cliffs: Mustang dynos require a proper calibration before a run. The operator selected two settings that were incorrect: road-load horsepower and that it was an all wheel drive car. The dyno believed the powertrain was overcoming way more inertia than it was, leading to inflated numbers.

    In October, Motor Trend published the vexing, perplexing results of a series of dyno runs with the 2020 Chevrolet Corvette. The first of six runs returned 558 horsepower and 515 pound-feet of torque at the wheels, which would have meant more than 620 hp and 570 lb-ft at the crank. It only got stranger from there, and the magazine never got close to the official, SAE-certified figures of 495 hp and 470 lb-ft. The story was so wild it did three loops of the Internet. Even folks who bleed Bowling Green said it couldn't be right. When journalist, one-time MT editor and occasional Autoblog contributor Jason Cammisa wrote about the results and explained why such numbers were impossible for the Corvette's LT2 engine, he said, "My best guess is that the dyno operator made a mistake." Well guess what: Motor Trend has finished its investigation into the matter and it turns out its former editor was correct.

    "We've learned that our dyno operator selected two dyno settings that were incorrect," wrote Technical Director Frank Markus. "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 ... 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 (to) 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."

    In the meantime, MT got its digital pencil and paper out to calculate a "road-dyno" pull from its best drag strip acceleration run, tallying 425 hp at the wheels at just above 6,300 rpm. Chevy engineers told MT that parasitic driveline losses are less than 15 percent, but wouldn't divulge a number. MT, using a 15-percent driveline power loss, said its 425-hp number turns into 500 hp at the crank. Then MT got the chance to do a road-load test at a track with a different 2020 Corvette. Running the numbers on that yielded 395 hp at 6,250 rpm and 350 lb-ft at 4,300 rpm. Backing out parasitic losses at 15 percent, this second Corvette rang up 465 hp and 415 lb-ft.

    As MT itself notes, these are not dyno runs, but if accurate, the two Corvettes produced significantly different results. The second number is also less than the SAE-certified horsepower rating of 495 hp. Jalopnik spoke to Gary Pollak, a program manager at the Society of Automotive Engineers, after the first MT story. Pollak told the site that once the SAE certifies a figure, a manufacturer can claim a number within 1% of the certified figure, and, "All production engines must be within 2% of the certification claim." A 2020 Corvette shouldn't throw less than 485 hp nor more than 505 hp at the crank, else the SAE will come knocking. So far, out of everything MT's done, only Markus' math effort of 500 hp meets the SAE criteria.

    Head to MT to read the latest in its own words. The least confusing line to come out of any of this is, "Stay tuned as this saga continues."
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