Here's why they upsized the ICE from my understanding:
The goal of the LMH rule is reducing cost by make the hybrid system simpler. In the 2016 and 2017 season the hybrid development war became unsustainable. The reason is under LMP1-H rule, hybrid makes up 50% of the total power (500PS out of 1000PS). Because chassis and ICE development was limited, almost all performance gains are from the hybrid system. That's how we got to the ridiculously complex 3-motor hybrid systems on the 919/TS050.
Under LMH there is only one MGU-K on the front axle. In a straight line it's better to shift power to the rear axle where there's more downforce and traction. But since there's no MGU-K in the rear, the ICE has to do a lot more work. The new 3.5L unit is 36% more powerful than the 2.4L. If they stuck with the 2.4L, they either lose performance or risk degrading reliability because now the ICE is more stressed. They were following the golden rule: there's no replacement for displacement.
One more thing to consider is that total hybrid deployment is also reduced, along with the weight increase. So the LMH cars will run out of energy allowance much quicker than LMP1-H cars. If you look at LMP1-H onboards, the cars lose so much power when they run out of allowance that they actually lose speed on straights. This will be significantly worse in LMH. Therefore they went for a much more powerful ICE to make up for the difference.
What's interesting is that LMH cars will likely be faster than LMP1-H on straights because they have a 160hp advantage. The LMP1-H will be a lot faster in corners due to more downforce/less weight/less restrictive hybrid deployment. I like the direction they are going with LMH but I think the power limit is too punishing. The originally planned 750PS should be better.
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