Ok,

This just in.......

Had my 340 dyno'd this week, I don't have the dyno sheets yet but I do have a report on Extrude Honed Manifolds VS stock manifolds. With ZERO changes other than removing the EH manifolds and replacing with stock unmodified originals (both sets were nice originals of 1971 A-body 340 manifolds, one set EH, the other stone stock, bead blasted clean, no repairs, heat risers removed and blocked off).

A grand total of 4-5 HP difference basically accross the board. Pretty much what I expected.

Additional notation, removing the stock air cleaner netted an additional 8-9 HP gain using either set of manifolds.

This was on a 426 Cube stroked 340 running a hydraulic cam and pump gas friendly 9.7:1 compression, VERY stock 71 TQ carb, intake, etc. (FAST style build 340 but compromised big time by hyd cam choice and lower compression for streetability).