Originally posted by PA 3L SVT:
Actually, one of the benefits to Extrude Hone is the surface finish when compared to port/polish with a rotary tool.

With EH, you get a uniform finish that IS NOT mirror smooth (when viewed microscopically).

With rotary tools, you get a very smooth finish. And, if done poorly, you get "ridges" (for lack of a better word) where tool passes overlap.

The inherent "roughness" of the EH finish is better for fluid flow when compared to a smooth finish. Something about laminar shear and the fluid (air) along the smooth surfaces moving slowly, and the EH finish "disturbing" the fluid along its surfaces and not allowing it to slow down at the edges. (Sorry, I never took fluid dynamics, so I can't give a better explaination. But I have read this in the past, and it made complete sense at the time.)

All in all, it is my understanding that when comparing two ports that have the same cross-sectional surface area for flow, the EH will flow better than a rotary tool finished surface.



That would be one of the minor benefits of the EH, but hardly a benefit in any street driven engine. If you're trying to get that last few HP on restricted engines in competition, like say, NASCAR, then every last air particle counts and it makes sense to EH. The benefits on a street engine are nil unless the porting can't be done properly by hand.

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