Smooth when up at 6500 was due to the accuracy of the factory balance job, surly you must understand it had nothing whatsoever to do with cast rods used in the motor.
And just the getting to 6500 cleanly was also in part due to the RA4 motors use of valve springs with a open pressure of 320 PSI .
Back then only solid lifter motors from Chrysler with there Hemi and two Chevys BB motors with solid lifters had more spring pressure.
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I do stuff for reasons.
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