Sorry I guess what I said is confusing, I already have the long bolt logs and snails. The snails are stepped so u can run 3" or 3.5" exhaust. Switching to 3.5" means new exhaust hoses, new tips and cutting new holes in the transom. Just want to know if it's worth it.
I don't consider myself any kind of expert on log manifolds. We have always considered them trash can ballast and have looked closer than to throw them on the scrap pile.
Knowing what I know about airflow as it pertains to intake and exhaust...
I believe that I have read that, to install the larger exhaust hose, the smaller boss is cut off. If not, the minimum cross-section area is the same either way. This would create a huge area of gaseous expansion where the velocity of the exhaust gasses leaving the log would slow considerably and become turbulent.
What does the inside of the outlet look like? What will it look like if it is cut?
If you cut it and the opening remains small, I would say, there is no point in cutting it and it could do more harm than good.
Looking at the pictures, It looks like there is an abrupt step at the boss for the smaller hose. I think having that bottleneck would be more detrimental to performance than anything else. The shape of the inside of that bottleneck could make the cross-sectional area appear 30% smaller than it actually is, depending on how abrupt it is.
From an airflow standpoint, not taking into consideration design inefficiencies of the logs, you will need to make some pretty decent power to flow 3.5" exhaust. For reference purposes, Flowmaster recommends dual 3.0" for a horsepower range of 350 to 450 and 3.5" for the 450 to 650 range, but I think that assumes headers.
To start, I would base any decision to modify your exhaust on the shape of the transition from the log outlet into the hose.
Good luck,
Joe
P.S. I bet there is more performance to be gained by port matching. Don't grind the outlet of the head to match the gasket. Grind the logs to match the heads. The opening of the logs can be intentionally larger than the head ports, however a smooth transition at the port roof is most beneficial, followed by a smooth transition along the sides. On some engines, an international mismatch along the sides of the ports can get a little screwy around the bolt holes. The floor should have an intentional mismatch of around an 1/8". This restricts flow back into the port along the floor during conditions of reversion.
