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I'm using a 02 civic lx to learn to track I'm eventually gonna put a new engine in it but to learn for now it's not terrible I don't think, I want to know how I can get big brakes in the rear, I only found front brakes for the lx can I use rsx or si disc brakes

 
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most of the braking force is going to be done by the fronts, especially if you're tracking the car. but, if you want big brakes on the back, you'll have to first do a rear disc swap, then upgrade from there.

@sdaidoji a good pad/rotor combo should be plenty for the EM2/ES1, no?

 
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Yes.
It is better to first run at a track and see what the car's weaknesses are, before starting modding the car.
Need to do at the least normal maintenance on the car, but modding, no.
I did not have issues with braking.