Im running 400 hp 355 small blocks with alpha drives using the coupler that attaches directly to the crank. These engines will only run 5500 rpm max. What id like to do is get rid of the heavy 153 tooth merc flywheels and go with 168 tooth SFI approved flexplates. The only disadvantage i can see in losing the heavy flywheel is losing the rotating inertia for shifting purposes at low rpm and possibly stalling instead of stumbling on the shift. I plan to idle between 700-800rpm in neutral. Let me know what you guys think. Im very curious as to what critical purpose the heavy wheel has in respect to my setup

flexplates are meant to have a torque convertor to take the shock of power on power off,,
the shock of the prop engaging or throttling thru waves I imagine would not be good for a flexplate..
They do make aluminum flywheels that would accomplish your goal

How would the shock affect the flexplate if the coupler is not attached to it?

good point...dunno then.. someone smarter than me will have to answer!

I have a 375 horse 350. Put a 16 pound flywheel on it, figured the lighter flywheel would reduce rotational inertia for quicker reving, but not bring it as low as a flexplate and cause stalling during shifting. I have no issues. Also since my rpms are in the 5200-5600 range, and I am not slamming a clutch into it at 8000 rpm I was not as concerned with getting a high dollar race approved one. Also my legs aren't wrapped around the bell housing either.

a lot of V-drive and jet boats run a flexplate, usually a JW Wheel or similar, to let the engine rev faster. these are light boats so the inertia of the flywheel is not as critical. I would think part would depend on the size and weight of the boat and the conditions you boat in. a heavy boat will pull the engine down faster or if you boat in rougher water that requires throttling alot the light flywheel may make the engine drop too much in rpm when you get off the throttle.

What are you trying to accomplish with lighter flywheels?

in a heavier boat don't bother without a lot of H/P...

Tried the flex plates and lost the ability to idle in gear. I have a two new flex plates available....cheap. Not a big deal to try as long as R&R the engine twice isn't a concern.

in your application a flexplate could be used bit imo it would be a waste of money as nothing would be gained.the only reason you need a flywheel or a flexplate is as a means to spin the engine for starting.