I have a few 35v radial capacitors lying around and am thinking of making a capacitor bank for my ESC. My system is running on 6S lipo and my peak current is less than 120 amp. I want to be able to run 25C batteries. How much uF capacity do I need to be able to smooth out the voltage ripple?

I think you should first ask the manufacturer of the ESC what ripple voltage is acceptable. Only they know what the design constraints were for their product.

You would also need to guarantee that the capacitor ratings were not exceeded, but there's probably no point worrying about that until you get the information above.

What are you running that you peak is any where near that? 6S at 110 amps is over 3 Hp or 2,400 Watts!! I don't think a 25C pack would be ideal. Capacitors won't help at all with the amp draw.

If you tame down ripple currents via additional capacitance, you've just added another part to strain, and more weight. Especially if you're using electrolytic caps, as they undergo electrolysis at the plates under high frequencies.

Better off solving a ripple current issue with lower IR, higher sustained-amperage batteries.

That said, I think the Castle cap bank is a column of 50v 2200uF caps, but don't quote me on that, it's been a while since I last saw one.