Hey yall
I was just upgrading a gen8 gearbox that I just bought, and I was wondering what spring would give the most power but won’t break in the first few shots. Its upgraded with standart metal gears, 480 motor, full nylon gearbox shell, nylon spring retainer, and metal piston rack. I’ts supposed to be more of a power build than a speed build, so I hope to get at least 280fps. Thanks!
The magic formula back in the day was a 1.18 unequal spring and alloy upgrades. Usually produced about 280FPS all day every day with a green o ring and everything else pretty much stock.
Those springs are considered very forgiving on nylon gearboxes, but Gen 8s are pretty robust so anything from M85 to M95 would be fine. Anything higher, like M100 and the nylon gears will be at risk.
There are plenty of Gen 8 tutorials on Youtube as well to help you out. 
1.18MM was my go to get 340FPS out of them if you get everything else right. Gentler on the boxes too Good advice