Systema ENERGY complete gearbox + 7.4v LiPo?
 

I was gonna get the Systema ENERGY gearbox and the Magnum motor, and was just wondering if a 7.4v LiPo (or even a 11.1v if it won't demolish my gears) would work in this setup? By the way, it's for a CA M4.

It will work perfectly fine, though I don't see the "energy saving" light weight parts holding up very long if you use an 11.1v battery. You don't even get a damn MOSFET with the mechbox, which I find to be essential for anyone running lipos. The Energy line uses cheaper plastic parts and tries to pass it off as a efficiency enhancement. The theory behind this is that the lighter parts means that it requires less energy to cycle the mechbox.
I personally think that most Systema parts are overpriced crap. The Energy line is a littleless overpriced but compensate by being crappier. You can get a damn good AEG instead for the $250+ for the mechbox and the $100. The only remotely good thing about this gearbox that since all the parts are Systema, then you should have zero compatibility issues. Other than that, don't waste your money.

http://www.airsoftgi.com/product_inf...oducts_id=6512

It says it has a MOSFET installed, and instead of getting the Magnum motor, I may get the A - Z motor.

I'm pretty sure the low powered sets do NOT come with a MOSFET. Only the M150+ sets come with a MOSFET... There's no reason why the low powered sets need a FET installed.

Very true. I'll look at those M150+ sets.

EDIT: I found the gearbox on Red Wolf, and it doesn't say it does have a MOSFET, but it says it pulls off over 460 FPS, so I don't see why they wouldn't put one in if it shoots that high.

If you're going to get an M150+ set, I can only recommend an 11.1v battery pack. 7.4v will NOT cut it.

But can I still use a 11.1v if I downgrade the spring to less than 400 FPS, because I want to use this at a CQB arena?

Yes. You should easily get 30+ RPS with a decent battery,

I wouldn't recommend it. You will dramatically increase your chances of stripping the piston, unless you short stroke it.