Stock rockers were 1.6 on the 5.0 HO's and 1.7 on the Cobras as stated before. My understanding is the higher ratio rocker allows more lift on the valve, allowing more flow on the intake and exhaust, and more flow = more hp. Roller rockers also eliminate most of the friction that is generated with the stock mechanical rockers allowing for less resistance in the mechanical process of the pushrod lifting the rocker and the rocker pushing open the valve. Less resistance = more hp. I've heard that roller rockers on an otherwise stock motor can be good for up to 25hp.
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