How much better would Merc mag roller cam 12551622 be compared to the stock hyd flat tappet cam that came in old 502 going on a single 30 ft boat has good ex

About the same on top. The roller will be of shorter duration, so will have a little more oomph on the way up the rpm scale. Doubt you would feel much thoug in a boat.

I’ve got two of them that I’ll let go cheap. Gen VI 502 MPI.

No noticeable difference. The early flat tappet 502EFI was 415 hp just like the later 502MPI.

If I recall that cam was short on duration and lift. Do you have the 502/465 hp’s? If so those where what I thought was a decent cam.

Don't know what cam I have early 90 s 502 flat tappet idles like a kitten how about 1.8 rockers building a mild 502 saving $$$ for 540s but using what I got now

1.8 rockers would give 540 lift without changing duration,quick trick???

I run 1.8 rockers because my heads flow the numbers in the 0.680" range I want, and they are a complete combination build. Also putting the lift I desire on a smaller duration lobe isn't something cam companies sign up for. I would not run 1.8's just to run them on a stock build. The springs might go into coil bind if not accounted for with original 1.7's. Minimum gains.

I agree. I highly doubt you’d see any gain whatsoever. Set the money aside and save for your future build. Not sure what your build is gonna be however you can make some pretty decent power with your 502. If plan on going all out on 540’s with some good aftermarket heads then makes sense. If your intent was to keep current rec port heads on a 540 platform then I’d say stick your money into the heads for a well thought out 502 build.