Now i am not positive so if someone comes and contradicts my info than believe them, but as far as i know, 421 headers will be better for daily drivers where power across the rpm range is ideal, u want low end power for stop light driving, and top end for entering the freeway or merging to the lane u want to be in, 4-1 headers will give u a better top end power but at the loss of low end power and will decrease the amount of torque ur engine has as the low end, because it decreased vacume pressure by decreasing back pressure, now all this makes perfect sense to me but in explaining it i may have lost a few thigns. for a daily driver ur better off with a 421 and ur better off with ur vtec, as with an all out drag car (which our lil honda's are not really made for) a vtec killer camshaft/s and a 4-1 may be more ideal at a loss of fuel economy and a decreased torque, now i may be wrong so dont quote me on this. anyone who knows more about this subject please let me know also