Actually with the test I have done on 4 different brand Dyno's a Hole in the middle of the Brush will raise RPM a lil and lower torque due to less surface area on the brush.
The BEST thing about the hole in the middle of the brush though is heat.It reduces heat.
On Reedy MVP's,Orion Core's,Peak Hellfire's and Integy Atlas Stock motor's it's nice to Narrow the brush on each side the way BIG JIM states.This reduces the overlap or "HANGOVER" the brush has on these motor's reducing heat and increasing performance.
Reedy 767 and 766 brushes work well.In offroad or sedan on smaller tracks we tend to run a 766 on the -.
Putnam Greens and Blue's offer great performance in sedan or offroad on bigger tracks.They do seem to eat the comm a lil fast and dont last long.Thats the trade out though for higher performance.The Red Shunts I have used on smaller tracks in sedan or offroad on the -.
Trinity's e Brush or 4499 or the new 99 Plus brushes are also pretty good.
Hurricane has a gold brush that works well in trinity motor's.
Trinity,Putnam,Hurricane brushes do NOT work very well in motor's made by Reedy,Orion/Peak,Integy(Atlas).I guess it's due to content or makeup of the comm itself.Reedy's work best in those motor's.
Make sure the brush hoods are aligned,keep it clean and keep the comm fresh with some decent brushes and your good to go.
Goodluck and have fun.
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Just wanted to say Thanks to RcTech for all these years of Great Information.