So you think the stock rods are up to the task? MO15 seemed to throw a rod judging from his video. I've always had bad luck when it comes to built motors and longevity. I've actually started considering a hybrid turbo to avoid the temptation of pushing the stock motor too far after seeing his vid. Lol.
I believe a number of things taken into consideration can increase the longevity of the rods at high horsepower. One big way the rods would be much less stressed out if these motors ran a high flow ported head, since the 1.6L head starts to choke for air near the 375whp mark. High flow head would allow the engine to run 400whp with way less boost, and as everyone knows less boost means less cylinder pressure, which is of course less stress on the rods. To my understanding, it seems like after 30psi that the lifespan of the 1.6L dramatically decreases in high horsepower applications.
While the head is out , I think ductile liners would need to be installed, to prevent the block from warping form the extreme amount of torque produced in high hp applications.
Fueling can help alleviate stress on the rods too. Ideally the car would have to run on both ethanol alongside 50/50 WMI (ideally 100% meth tho) at the same time. So on top of a high flow head, an E40-E50 ethanol tune + a 100% meth tune would then further allow for the same horsepower for less psi
Also cams can dramatically shift the high torque curve in favor of a high hp curve.
The type of turbo matters too. For 400whp the S290 and the G25 produce considerably less torque than the s280 down low, and low end torque from the turbos do NOT make our eco boost happy. In fact a lot of the torque in the s280 is usually tuned out in high horsepower builds because it is very dangerous for our soft, economy car aluminum blocks to deal with such extreme forces. Otherwise if it not tuned out at some point , your cylinders will oval out and the block will warp pushing all that boost without ductile liners.
TL DR version: High torque and high cylinder pressures kill the rods in our car.
Therefore a motor with a ported head running aftermarket cams with a mixture of ethanol and WMI , and a turbo that isn’t torquey (like the s280 or hybrid ) but produces high horsepower like the G25 or S290 would be the most effective way to dramatically lower cylinder pressure, while at the same time maintaining high horsepower, effectively increasing the life span of the rotating assembly in the block. Basically you’d need a unicorn of a fiesta st to run 400whp without too much worry.