My old Excalibur 40 has the original 3" log style manifolds. Im running 454/420 mercs. What kind of speed/sound would I gain by going to a 4" gil or stainless marine system?

If you have 420's with thos manifolds I would expect a good increase in performance. I always liked the sound out of those (I have them on my Donzi as the center riser style will not fit). But I would guess you would gain a couple MPH with it. Probably 1-200 RMP

I did that change and THE issue you need to watch is the "fall" from the exhaust riser to the transom. My replacement center risers required the 3" insert between the manifold and the riser. We learned that after we had a water reversion problem. Alos--you'll run into clearance problem between hatch cover and riser.

Can't comment on sound changes but I can tell you measure eveyrthing at least ten times.

Hey mild, i had the Gils on mine when i had 454's. I still have one set with the tailpipes for a 40 Excalibur. Im not sure if my parts will help but i can tell you that it will fit.