FRANKFURT (Reuters) - Tesla Inc's TSLA.O attempts to humble Porsche with a new lap record on Germany's legendary Nordschleife racetrack have reignited a controversy about the value of lap times, as the circuit steps in to quash claims of cheating.

A driver steers a Tesla Model S at the Nuerburgring race track, in the western German low mountain range of the Eifel near Adenau, Germany October 15, 2019. REUTERS/Stefan Baldauf

Porsche and U.S.-based Tesla are battling to establish supremacy in lap times for four-door electric sportscars, but comparisons are not exact, as conditions, ranging from car modifications to tyre types, vary with each test.

“We want to have circumstances that can be understood and replicated,” said Nuerburgring spokesman Alexander Gerhard, adding that the racing circuit operator had moved to tighten rules by which a car maker can claim a certified lap time.

Setting a new record time for four-door electric cars would give Tesla’s ageing Model S a boost just as German rivals Audi, Mercedes-Benz, BMW and Porsche prepare to launch their own electric cars.

Automakers use the Nordschleife, one of the world’s most treacherous courses with 40 right-hand and 33 left-hand turns, to hone a vehicle’s sporting characteristics and burnish a car’s image for marketing purposes.

Tesla was spotted this week with a variant of its Model S sedan at the circuit, which is 20.8 km (12.9 miles) long, with slanted cambers and a 300-m altitude difference between its highest and lowest points.

German car magazine Auto Motor und Sport said a Tesla was seen recording an unofficial time of 7 minutes and 23 seconds, which would beat a lap time set by Porsche’s Taycan, which Porsche says achieved a lap time of 7 minutes and 42 seconds.

But Tesla’s challenge has prompted questions about whether the Silicon Valley car maker is playing fair.

“The car was heavily modified,” said Stefan Baldauf, who photographs prototype vehicles being tested on the circuit for a living.

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“Aside from a roll cage and the driver’s seat, the interior had been stripped out,” he added. “The windows were blacked out, so it was hard to tell.”

The Tesla also appeared to have semi-slick tyres, used only on racing circuits and unsuited for everyday use, Baldauf said.

A Porsche spokeswoman told Reuters its Taycan was tested using standard tyres.

“A comparison is hardly fair if this is the vehicle used to demonstrate that Tesla is faster,” Baldauf added.

Notary Jens Boehle, who certified lap times by Porsche, said, “Scope for cheating is as big as you can imagine. Is it a prototype vehicle, a standard road legal vehicle, or is it a specially modified racing version of a standard road car?”

“These are just some of the questions that need to be answered.”

Tesla Chief Executive Elon Musk responded on Twitter this week.

“The final configuration used at Nürburgring to set the record will go into production around summer 2020, so this is not merely for the track,” Musk said.

Tesla declined to comment on its lap record effort.

Officials are trying to standardise speed record attempts, Gerhard, the Nuerburgring spokesman, said. In some runs for the record, cars were allowed to make a flying start using a 20.6-km (12.8-mile) stretch of the track.

“We now mandate that the full 20.8 km is used,” Gerhard added. “And we employ a notary to measure the time following rumours that another manufacturer had cheated, by speeding up video footage.”

It also mandates where timing devices should be located.

On Twitter, Musk confirmed that Tesla is using the Nordschleife to market its “Plaid” mode on a 7-seater Tesla Model S.

“We expect these track times to be beaten by the actual production 7 seat Model S Plaid variant that goes into production around Oct/Nov next year,” Musk said.

Porsche says its 919 Evo hybrid racecar now holds the overall record for all vehicle categories, with a lap time of 5 minutes and 19 seconds, set in 2018.

Nuerburgring hopes the new rules, introduced this year, will help revive the circuit’s popularity as a venue for competitive benchmarking.

“We want to be a believable benchmark,” Gerhard said.