Mercedes-AMG has reportedly delayed deliveries of the F1-powered One hypercar until 2021 due to ongoing difficulties making the engine suitable for street use.

Auto Motor und Sport pinpoints a number of elements of the One’s powertrain that need to be refined and claims that engineers underestimated the difficulties of posed by this very challenging task.

One of the most significant challenges facing engineers is just how to make the F1-derived 1.6-liter turbocharged V6 meet WLTP emissions regulations. In addition, AMG is struggling to ensure the engine doesn’t require electric pre-heating before it turns over, like the race engine does. A V6 engine with such small displacement and high output is also reportedly quite fragile.

Also Read: New Mercedes-AMG One Build Slot Pops Up, This Time For $4 Million

Mercedes-AMG presented the One in concept form two years ago at the 2017 Frankfurt Auto Show, with the promise of commencing deliveries in 2019. That date was clearly overly ambitious, and with the Aston Martin Valkyrie still on target to reach the hands of customers this year, the delay with the Mercedes-AMG One is certainly embarrassing for the company which has dominated Formula 1’s hybrid era since 2014.

According to the original release, the One’s ICE would be combined with four electric motors to pump out a combined 1,035 HP. This would allegedly be enough to send the hybrid hypercar to 124 mph (200 km/h) in less than six seconds and through to a top speed exceeding 217 mph (350 km/h). It wouldn’t come as a surprise, though, if those numbers change for the production car, especially since compliance with the new, stricter emissions standards is a prerequisite for it to get type approval.