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The intent was to gain a few MPH from my 447 low-deck in Super Gas (throttle-stop) racing...
I swapped out my 4:30 gears for a set of 4:56s, which netted 100-200 more RPM in the lights, and no difference in MPH.

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What compression ratio are you running? What is the shift point and finish line rpm you are seeing?
In a "small" 447 CID and that much duration (280 at 0.050"), it should like 7500+ rpm. But the tight LSA (106) will favor mid range rather than peak hp (hp corresponds to mph).

In addition, the unported Victor heads may not flow much better than your well ported 915's. If that is the case, it is a shame to have heads with potential that is not used since the porting was not done.

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This is more on par of what I was thinking. I`ve seen smaller motors w/more cam BUT good comp. and high flowing heads and they screamed...............it`s all about the combo and obviously bigger cams/heads need some r`s to make power plus maybe not as critical w/your combo but maybe a bigger carb.