Booster CAPS, are they ROAR legal?
 

Can anyone tell me if these are ok for racing or not?

Not sure. But they for sure work. I have six added to boat esc. New skidoo has not starter or battery to start. All Capps. https://www.snowest.com/2017/02/2018...ummit-freeride

I know in oval racing only stock caps are legal

mod probably is not an issue but 17.5 or stock classes i can see an advantage.

As far as I have been told and in my experience they are legal, you're not adding extra power, the car will still only have as much power as the battery can provide. Now that is for replacement or upgrade ESC caps which allow the storage of more energy (uf) but you negate any advantage if you put in a cap that has a higher resistance than the stock cap irrespective of how much energy it can store. Bottom line is your battery will have more influence on power than a $50 capacitor.
Some 'booster' devices which Turnigy and I think Dynamite produce which actually act as a voltage booster. Those ones ARE illegal.

might want to clarify where the booster caps are used (BEC vs ESC input), I have used some that will temporarily boost the power to 3S using the 3rd channel switch to gain more speed (in one of my bashers) for a couple seconds which can be significantly advantageous, this type of system is definitely not allowed per ROAR rules:

+ YouTube Video

If the cap is on the ESC, it's ROAR legal. Contrary to popular belief they do not add horse power nor do the improve lap times. Some can actually make things worse over the stock cap. At Tekin we've spent days on JUST cap testing. The small TT3520 cap we spec on our 1/10 ESC's was chosen on purpose and optimized for our ESC's. So be careful just swapping stuff out.

i believe hobbywing recommends the same. everyone is so anxious to swap out the single cap they've put on their xr10 stock spec, but i believe hobbywing has said it's unnecessary.

Ofcourse not legal. Those booster caps add some voltage above the limited 2S for a small period. If rules say 2S max with 8.40 or 8.44 volts then thats it.

Booster caps cannot add voltage. They can only handle/deal with the battery voltage.