Mercruiser Questions

[bjh517]

Here is what I have learned:
Mercruiser 5.7 Carb is rated @ 250 HP
Mercruiser 350 Magnum MPI is rated @ 300 HP

Every other manufacturer I’ve looked at (Sea Maxx, Marine Power, Indmar, etc) is rating their carbureted 5.7 @ 300 – 315 HP and their MPI or TBI models @ 325 – 335 HP.

My Question:
If I purchase a new Mercruiser 5.7 carbureted engine, can I simply put on a new manifold and a Holley 600 and be up to 300HP, or do these other companies do something more to their carbureted 5.7s? I would wait out my warranty before doing such a thing. I am assuming Mercruiser is using Vortec heads on their new 5.7s, am I correct?

In other words, what would I have to do to a new Mercruiser 5.7 to get it up to 300HP?

As usual, thanks in advance for the advice,

Brian

[ashjosh]

The old 5.7 liter were rated @ 260HP, the new ones @ 300hp or 325hp have more than meet the eyes, the actually have a different cam to be able to handle the flow of fuel and also the heads have somewhat larger valve (202 heads), I will be experimenting with a tuneport fuel injections system out of an Iroc camaro, the only problem is the computer not being water prof I have to mounted on a modified radio box, but hopefully if this works I will be able to get around 375hp on a 5.7 liter without many changes, just the cam. If you have thru hull pipes that's the best for the exhaust other that that you will loose some hp if using the regular exhaust system.
If you plan to used a holly 600cfm with a aluminium manifold then I suggest look at the cam specs and your valve size specially the intake one you will need something around 194 to 202 valve for better flow the cam will depend on the boat size, and remember that the bigger the cam the more radicle the engine could get and the more spark you will need that means that your electronic ignition will need to be update as well. Just be carefull because once you start low you will want to keep on going high and the money keeps going out.

[ashjosh]

The old 5.7 liter were rated @ 260HP, the new ones @ 300hp or 325hp have more than meet the eyes, the actually have a different cam to be able to handle the flow of fuel and also the heads have somewhat larger valve (202 heads), I will be experimenting with a tuneport fuel injections system out of an Iroc camaro, the only problem is the computer not being water prof I have to mounted on a modified radio box, but hopefully if this works I will be able to get around 375hp on a 5.7 liter without many changes, just the cam. If you have thru hull pipes that's the best for the exhaust other that that you will loose some hp if using the regular exhaust system.
If you plan to used a holly 600cfm with a aluminium manifold then I suggest look at the cam specs and your valve size specially the intake one you will need something around 194 to 202 valve for better flow the cam will depend on the boat size, and remember that the bigger the cam the more radicle the engine could get and the more spark you will need that means that your electronic ignition will need to be update as well. Just be carefull because once you start low you will want to keep on going high and the money keeps going out.

[Ranman]

You may also want to note that the Merc specs are PSHP or Prop Shaft Horse Power. The Others may be rated at the crank. This means that my MAG 350 @ 300PSHP really makes like 325 at the crank. Just another thing to consider.

[Rootsy]

AS ranman stated, mercruiser rates horsepower at the prop.. which is a 10% less than at crankshaft.. so about 30 hp loss through the alpha on a sbc.

the 250 HP version is 2V carbureted and has a different ignition module. the 300 mpi is injected.

ROTATING assemble IS IDENTICAL.. 2 bolt main, cast crank, PM rods, Dished Hypereutectic pistons yielding 9.1:1 static compression ratio. the camshafts are the same grind. hydraulic roller.

each motor has vortec cylinder heads, 64 CC chambers, 170 CC intakes volumes, 1.94 intake / 1.50 exhaust valves... .480 max lift without machining guides. straight plugs

the 350 mag MPI motor has SS smooth flow risers in the exhaust, the 250 2V 350 does not.

the old 260 and 270 (mag) motors were different than the new motors.. flat tappet camshafts, old style cylinder heads, etc. apples and oranges cept for CID...

i;ve had my 1997 (originally) 210 hp 2V 350 from the 16 apart 4 times and i;ve blueprinted piston crowns and bore and stroke as well as inspected the bottom end.. my motor did not have vortecs either but instead 217 castings. i've seen 250's and 300 mpi motors apart... same rotating assy and same camshaft... i think someone slipped with the pen when they rated the 350 at 210 - i was told this motor came about because of a shortage of 220 hp 305's for a couple of years in the mid to late 90's.