Same ports, same performance potential, same flow numbers stock. The 47s have a 45 degree intake and the 18s have a 30 degree intake however. Same valve size.
The 47s are a lower compression head by a little but they can be milled and will be a better pump gas head. FWIW
No advantage to the 18 other than the small chamber and the end bolt holes being drilled already on the exhaust side. The 18s are even press in stud.
If you need to have really high compression then they are the only choice however.
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