"the HP of a 75 GPX 433?"
est. around the mid 60's area.
Some very fast stock 74 Merc S/T 400 are right at 70hp.
"pipes on a phazer motor ?"
They make good hp piped BUT they can have a midrange lean conditon. Dial-jet helps but you still can (I did) burn it down.
AAEN quiet can single pipe made the most hp at 71, stock was 58.
Fan motors do not like the extra heat also, it's up to you
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