Well...the only thing I know that could be a performance difference in the PI SOHC engines is the 99 has a 66.5mm throttle body compared to the 65mm throttle body of the 2001 + models. This should make the 99 mustang faster...right.... :winks
The late 01's (and of course 02's) have a different water pump (pulley and belt a little different, too), and the 02's have a mildly re-worked tune (perhaps there's a spot where the 02 power is greater than the 01 because of this, but the overall peaks are 260hp stock).
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