I am not arguing that they are low but they are not off by 40. CAI maybe 3 rwhp, bassani headers most likely hurt hp and DEFINITELY hurt torque with a stock motor, cat back 0 rwhp if you have the fix, The canned tune was worth 3 rwhp on my 98 Cobra, flywheel and pulley add up to what 5-8 rwhp? I think most people are using a 19% correction for the Mustang dyno so 257 works out to around 305 hp at the flywheel. Car was rated at 320 but most 99s never got near that. It might be off by 10 from where it's supposed to be but you are most likely using the OEM fuel filter and spark plugs still, IDK? Maybe not but it's not that far off.
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