Good day gents,

New here, first post.

I'm kicking around the idea of repowering my cruiser. I currently have an 03 350 Mag MPI with a couple cylinders that have low compression. I was thinking i'd buy a long block 383, install my intake, exhaust and assessors. My question is will my PCM555 ECU have to be reprogrammed? I think it will because i cant see how the 350 program can squirt enough fuel to keep the fuel to air ration right.

That be the case, who does that and what's a ballpark price for having that done?

thanks for your time
Kevin

Let me address a few items.


It might be cheaper to simply use your existing block and add a stroker kit (crank etc) to get a 383 rather than buy a long block. But yes you could bolt on all the externals to create a 383 with your existing hardware.


The rub comes with the PCM555. It can be reprogrammed but only by a few people. This in my opinion is the sticky point. MEFI ECMs have a broader supply of tuning possibilities along with some of the Holley Marine EFI stuff. Unfortunately this adds considerable one time expense.


Let me dispel some folklore. The stock PCM555 tune will run a 383 with everything else kept the same (cam, injectors etc). Yes it is more cubic inches but it won't be extremely lean as some might suggest. Would it be ideal? Probably not. Am I suggesting to run it as is (post mod stock PCM 555 tune)?...not necessarily. How do I know it will work? I've done it with a 350 Mag MEFI 1. Stroker kit, bigger cam, new heads etc. Fired up first try on stock injectors and idled on its own with a bone stock Merc 350 tune. Idle AFRs were in the mid 12's. It actually ran surprisingly well! I then installed much larger injectors and THEN started the tuning process.


What I would suggest is to find a PCM555 tuner first and see if they will work with you. If you find one, build your motor and install all your original stuff. Run the motor on a test stand and see where you sit operationally. You would need wide band O2 to see how the engine is performing in the AFR department.


As an aside, and possible alternative route, since you are only changing the size (cubic inches) of the motor, you could install slightly, and I mean slightly larger injectors to compensate for the increased motor size. This may not be ideal either but is potentially simpler than a full scale tune with the 555. Keep this in mind, if your stock motor is running 12.5 AFR at say cruise rpm and your build a 383 and it runs at say 12.8 AFR with the stock tune you will not notice the difference other than what the gauge displays. Your motor will be happy all day long at that AFR. If the AFR swells up to the high 13.5 or higher at cruise it would likely be time for some tuning. This would apply to all other operating regimes as well. 13.8 AFR at idle ain't hurting the motor. 13.8 AFR at WOT is getting risky.


Good luck and let us know how it goes.

For the money, I’d buy the merc 383 from the reman program complete. Save any and all hassle of tuning or changing any part of the combo. Plug and play

Another great option, but perhaps not as fun....

its about 3 times as much to do it that way, I can save a ton with some elbow grease... and of course ,it wont be as fun...lol

Great post, I'm following this. What type of take off and top speed will you will see ?

lol no idea, I'd like to hear from someone that has done this, I'm really interested in torque, get up on plane faster and maybe go up a couple inches on the prop so I can cruise at lower RPM's.

You might be surprised that you actually don't go up in pitch that much or at all, but rather you are able to run the same prop at a higher rpm due to where the peak power is now being made. It will also depend on your particular hull/boat your are pushing around.

In my experience with carbed superbike race engines, more displacement and/or higher compression actually resulted in needing to jet leaner. It’s counterintuitive, but I guess the extra force pulled more fuel as well. My point being that you really don’t know until you put it to test. However, your injector size may be a limiting factor that possibly needs to be addressed.

I agree that most of the benefit would be in the torque with a touch more on top. It probably won’t turn the boat into a rocket but it should help get up to speed a lot easier.

Injector sizing is important. I would at least find out what injector he has and that would give him a max target hp he could achieve with those injectors. He could then size up if required.