Do I need to run a external BEC on a Tekin RX8 ? motor is a PRO4 4600 and a Savox 1258 for the servo. They are in a Durango DESC410 4WD Short Course.
Thanks !

I would recommend it. Running an external BEC in 1/8th scale and 4wd SC alleviates so many issues. Besides preventing brown outs, an external BEC (Castle specifically) let's you bump up the voltage going to the RX/servo. A good test is look at your personal transponder light when you turn the wheels on the ground. If it flickers then the build in BEC in the ESC is not up to snuff.

+1 an external BEC is always usefull with Savox servos, the RX8 BEC isn't the strongest on the market and it won't allow you to use servos in 7,4v so I definitely recommend a BEC

On the JQ I run a RX8 + T8 motor + 1258TG
On the Durango I run a MMP + T8 motor + 1258TG

I'm using a 10A CC Bec on both set at 6v which is the default voltage of the RX8

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I run the same motor, esc, sevro setup in my Losi SCTE with a KO receiver with NO problems whatsoever.

doesnt this issue have to do with the receiver? some do up to 8.4v some only 6v the lower v receivers brown out when a hv savox or any hi voltage servo is used, and this could be solved with a capacitor at the receiver?

Exactly, it's also related to the receiver. When in doubt -> use a cap/ bec

I think I need to put one on. Ran without it and sometimes when I nailed the throttle and turned at the same time, like to spin the rear around it would die and go straight. Guess that's what is called a "brownout"