Power from the one-make 4.2-litre normally-aspirated Gibson V8 has been reduced by 20kW or 25bhp in addition to the 30kW or 40bhp originally planned.
That means the Gibson engine will push about approximately 535bhp (400kW) in 2021 compared with more than 600bhp (450kW) since the introduction of the current P2 formula in 2017.
The minimum weight has been increased by 20kg to 950kg, while the cars will now only be able to run in low-downforce Le Mans aerodynamic configuration at all tracks.
The rule makers, WEC promoter the Automobile Club de l’Ouest and the FIA, had hoped to slow the cars with the original 40bhp reduction in engine power and the move to control tyres supplied by Goodyear.
The P2 grid will now race on the same range of tyres supplied by Goodyear across the 2019-20 WEC season.
The need to reduce the speed of LMP2 cars follows the introduction of the new breed of Le Mans Hypercar for 2021 in place of LMP1.
Their target race lap time around the 8.47-mile Circuit de la Sarthe at Le Mans is 3m30s, which represents a significant decrease in performance from P1.
Photo by: JEP / Motorsport Images
The 3m30s target compares with the fastest P2 race lap at Le Mans last year of 3m27.508s.
A statement from the ACO read: “Following the initial testing phase and consultation with both Goodyear and the teams, the FIA WEC Committee has decided that retaining the previously used compounds is the most suitable option, given their proven and consistent performance in all conditions as well as their drivability.”
A new range of tyre compounds tried in pre-season testing are known to have raised question marks with the teams and drivers.
The statement from the ACO added that the further reduction in engine power “will contribute to lower running costs by extending lifespan of different components”.
The new level of performance in P2 will also be enforced across the European Le Mans Series, as well as in the sister Asian Le Mans Series next winter.
The changes have been announced just two weeks ahead of the scheduled ELMS series opener at Barcelona on 18 April and four weeks before the first round of the WEC set for Spa on 1 May.