Do not mount 4 point harness to the floor. it could potential collapse your back with enough impact load. Maximum downward angle should be around 15*-20*. Stick with the 3 point installation. If your going to autocross hard enough that you think you may need a 4 point harness, then I would be considering a proper harness bar for it as well.
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Mike Redpath
Musclerodz & Customz
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