Just dynoed my 496( 4.25 stroke)here is what I got. about 2000 rpm torque is 500 goes to 550 about 2500 then goes up gradualy to 600 at 5100rpm very good flat torque curve. Max hp is 515. This is with stock exhaust dyno guys says add 50 60 hp with after market manifolds .My question is this I had stock merc 420 hp motors before what are there specs compared to these? And will I be able to tell the difference for all the trouble and expense. I pushing a big V bottom 40stinger.Also will a prop change be in order?

Those are some nice numbers. If they made that kind of power thru the stock merc manifolds, I would say YES, you would see a gain from some good manifolds or headers. Chances are you will need to go up a couple inches in pitch.

You gained about a 100HP. You should see about a 400rpm increase with the same props.
I don't know about 50-60 hp with aftermarket exhaust. Probably about half that though.

the torgue is whats going to help you in that boat....just have to prop to keep that range

I forgot to take pictures but this is also running the water pump, sea water pump and Alternator.

An aftermarktet exhaust such as Stainless Marine or Dana would really give your engines some kick in torque in the midrange especially,....but even so, a 40' Stinger still needs a whole lot of power to get that hull moving.

600ft/lbs at 5,000rpm with max hp of 515hp at some rpm?

Somethings with that data is off. (tq=hp at 5250rpm)

well, as noted above, the numbers don't make sense.

500 lb-ft of torque at 2500 revs is 261 hp

600 lb ft of torque at 5100 is 582 hp

which seems to be about 160 hp more than you started with...
which is going to now need some prop work i would have to believe

is this really a 40 ft boat with one motor ?

It has two motors!.. I do not have the dyno sheet in front of me but as soon as I do will I will post

well... if this thing makes 1160 hp on 40 ft deep vee, propped correctly, i would expect it to be a low to mid 70 's mph boat.