Recommendation for rebuilt cylinder heads

370Dancer said:
So, is it ok to pull a head with rectangle ports and replace with ovals?
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Ovals are better for lower RPMs. You don't want one oval and one rectangle though. They need to match.
 
Just to confirm to all on this thread, I do have oval ports, my castings are 279's. I did order two pairs of these EQ heads and should get them next week. I will try and remember to report back after they have been fitted and engines have a few hours on them, but that will not be until April when the boat will be put back in the water.
 
scoflaw said:
I would believe you have oval ports. No?
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My engine may be different from his. I have 380 HP MAG MPI Horizon engines, 1999. I'll know for sure when I pop the intake. But, service manual 23 says:
MAG MPI Cylinder Heads.PNG
 
I always question why they would use a mag motor in a cruiser, In my opinion those are for go fast boats , doesn’t utilize the power until the upper RPM range
 
joeyleggz said:
I always question why they would use a mag motor in a cruiser, In my opinion those are for go fast boats , doesn’t utilize the power until the upper RPM range
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On my dock, we have 2-340’s, one has the 380 hp, the other 310 hp. Big difference, the owner with 310hp wishes he has the horizon motors in his boat
 
Chris-380 said:
On my dock, we have 2-340’s, one has the 380 hp, the other 310 hp. Big difference, the owner with 310hp wishes he has the horizon motors in his boat
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I think the 310hp is the no frills motor isn't it? I think that's what my boat came with. At least what was in it when I got it. It wasn't factory installed. I'm sure it would have been a slug by itself. With the 279 heads bumping up compression and the bigger cam, I'm sure it will be a big difference.
I guess my point is, comparing the 310 to any other big block is probably not a fair comparison. I could be off, but that's what it seems like.
 
Bigger cam? Like increased duration to bleed off that extra point you just picked up?
 
Chris-380 said:
On my dock, we have 2-340’s, one has the 380 hp, the other 310 hp. Big difference, the owner with 310hp wishes he has the horizon motors in his boat
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Sure who wouldn’t want more horsepower LOL . I go out of my buddies 2003 340 with the Mag motors all the time his boat seems to run best at 3800 RPM for me that’s a little high on a big block cruiser . everything is up to snuff on his boat it just seems that the mag motors are meant to explore higher RPM band. I had both of my 7.4 EFI motors rebuilt to by a reputable race shop I had them fully dyno tested making 340 horse at the crank and almost 490 pounds of torque at 2900 RPM with the peanut port heads . I cruise at 3250 at 21kt and tops out of the 30kt at 4500. Regarding the mag motors I was just trying to follow the logic of their RPM power curve just seems that it was meant for boats exploring the upper RPM range
 
Merc claims the Mag makes more torque as well as HP than the L29 but I've not seen the power curve posted to see where the Mag is making that torque.
 
From a logical standpoint the square port heads free up torque in the higher RPM bands the peanut port an oval port lose torque in the high rpm bands but hold it longer at the low RPM band.
 
scoflaw said:
Bigger cam? Like increased duration to bleed off that extra point you just picked up?
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I don't think a whopping .224 w/ .483 lift stock cam is bleeding all my compression away...
 
Agree with that but certain hulls need to go at minimum a particular SOG just to get on step and maintain it.
 
scoflaw said:
How was the performance on the original setup? Sounds like a heavy boat for that power.
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No idea. I bought it needing the transom assembly and decided to rebuild the motor too. In the mean time I found some 279 heads to go from 118ccs to 99ccs. With that I figured it would be good to bump the cam a little too so I went from 204/209 to 224/224. Nothing drastic.
 
I guess because you said it was a waste in a cruiser, and I don’t think it is, that’s why I posted real world experience
joeyleggz said:
Sure who wouldn’t want more horsepower LOL . I go out of my buddies 2003 340 with the Mag motors all the time his boat seems to run best at 3800 RPM for me that’s a little high on a big block cruiser . everything is up to snuff on his boat it just seems that the mag motors are meant to explore higher RPM band. I had both of my 7.4 EFI motors rebuilt to by a reputable race shop I had them fully dyno tested making 340 horse at the crank and almost 490 pounds of torque at 2900 RPM with the peanut port heads . I cruise at 3250 at 21kt and tops out of the 30kt at 4500. Regarding the mag motors I was just trying to follow the logic of their RPM power curve just seems that it was meant for boats exploring the upper RPM range
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Never did I say it was a waste....but the way it was built was meant to run those motor in an RPM range that you wouldn’t normally do in a cruiser . those magnum and Horizon motors were usually found in speedboats , super sports etc. just because they started offering the package as an up charge across their boat lines does it mean there was any more logic put into it . And at a normal cruising RPM there’s not gonna be a huge difference between any of them, pushing closer to 4000 RPM the magnum and Horizon motors gonna smoke the standard 7.4 . And yes what man wouldn’t want more “horsepower”
 
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