I had a gen V MPI stock 502 that ran 68 MPH on a 23P Mirage, this spring I installed harden Headers, external steering, MSD, bigger Air cleaner, matched and flowed injectors, 25P High Five prop, and had the ECU remapped. Top speed now was 74 MPH at 4900 RPM.
There was 156 hours on the old motor. The head gasket failed between cyl #5 & 7. This leaned out #7 and you now the rest. KA POW.
I order a new gen VI (not rebuilt) long block engine from Marine engines INC. in Holliston MA.
Swapped over all the goodies babied it for about 10 hours, and Ive dropped RPM's to 4500, and GPS at 64.1 MPH.

Now I've got 23 hours on it and no change. I looked at my new engine receipt and the sold me a GM 502, not a merc 502. is this the problem??? cam specs different? compression? what between the spring purchase and this new engine I've dropped about $18,000 and my boats 10mph slower.

I'm at a loss, and am very frustrated, I hope you guys can give me some ideas.

Please help
Curt

One more thing if this helps. I never new what the timing was on the old motor and now we were told to set the base timing at 8 Deg Adv. Should this be diff from the GM Vers Merc?

8 deg should be the same, have you checked total advace ???

You need to call the place that built the engine and get the max hp at what rpm. Then you can start optimizing again. Your peak hp is probably in a different spot due to engines not being built the same.

BT

GM 502 and a Merc 502 are two different animals altogther.Boat motors are designed to pull harder at higher and constant RPM's.If you can give us your cam specs I can tell you what ya got.

Ill get in touch with the place i bought it from and see what they can tell me
ill reply with the results

I've had their motors and was happy with them. I've heard there is no difference between Merc and GM, since GM supplies Merc with motors. The difference is in the higher hp motors because Merc hand assembles them to their specs.

The long block you bought is 435 hp, probably measured at the crank. Should be run at 5000 rpm. Make sure timing is correct (I don't know what it should be) and your fuel supply is set up right.

Put the 23p prop on and see what happens.

What about CR? Is there a difference in the CR? That will effect your power too.

There is absolutely, NO difference between a GM & Merc motor 502 in the short block except:
Merc uses a 8QT oil pan (GM Truck)
Merc uses a different cam Mostly HYD Rollers ( Older Flat)
Merc uses Flywheels instead of flex plates.
No other difference.

On the heads:
Same castings
Upgraded Springs for the cams
Very good valves in the blue motors with Crane Rollers
They clip the spring off the exhaust seal

They use GM head gasket & Intake gaskets.

I would say it is a different cam not matching the remapped ECU.

Did you use the cam that came with the new motor?

I have to agree your problem is with the cam and the power curve