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350 Mag MPI


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We had to get remanufactured core for a 2003 Mercruser 350 Mag MPI on a 2003 Chapperel Sunesta 243. I believe the original motor produced 300hp or 320hp depending on where it was measured from. Before we had the work done we would see 50+ on gps and no issue with planing off with several people in the boat and also towing two tubes/skiers/kneeboarders. With the "new" engine the boat struggles to plane and damn near won't do it with two tows. The top speed is also off by 10mph with just one person in it. 

Any ideas?

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17 minutes ago, Kivalo said:

We had to get remanufactured core for a 2003 Mercruser 350 Mag MPI on a 2003 Chapperel Sunesta 243. I believe the original motor produced 300hp or 320hp depending on where it was measured from. Before we had the work done we would see 50+ on gps and no issue with planing off with several people in the boat and also towing two tubes/skiers/kneeboarders. With the "new" engine the boat struggles to plane and damn near won't do it with two tows. The top speed is also off by 10mph with just one person in it. 

Any ideas?

Sound like a mismatch on the prop. Not getting the rpm up.

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You say core, do you mean short block or did you go with a long block? There are so many possibilities here.  I’m guessing you guys knew what you were doing with the swap, but engine alignment alone could throw you off that much. 
The possibilities are endless🤷‍♂️

Edited by old indy
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8 minutes ago, Big Crappie said:

Sound like a mismatch on the prop. Not getting the rpm up.

The prop is dinged and I'm sure is contributing but I have wrecked plenty of props in my time and I know for certain it's not the prop.

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6 minutes ago, old indy said:

You say core, do you mean short block or did you go with a long block? There are so many possibilities here.  I’m guessing you guys knew what you were doing with the swap, but engine alignment alone could throw you off that much. 
The possibilities are endless🤷‍♂️

This is a good question. I didn't make the arrangements so I will check with my brother.

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2 minutes ago, Kivalo said:

The prop is dinged and I'm sure is contributing but I have wrecked plenty of props in my time and I know for certain it's not the prop.

What’s the max rpm you are running on a light load? Is it in the upper range of max rpm for that 350? If I had to guess that engine is low 5s for max upper rpm range (4800-5200?) 

always shoot for upper rpm with a light load which will bring you to the low end of the max rpm range with some weight in or behind the boat otherwise you will be lugging and losing performance. It sounds like it’s overpropped to me. What prop are you running currently? Diameter and pitch. 
 

does the prop have exhaust venting holes on drilled in the outside diameter of the prop just above the labyrinth seal diameter? 

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Another option is to have multiple props for various uses. One can be for water sports and pulling power that will give you a good hole shot and midrange power, but slower top speed. This will be a lower pitch prop. The other can be a more performance oriented blade if that’s what you want, that sacrifices hole shot for speed. You can’t have both, there’s always a sacrifice in propping, one way or another. You have to be mindful of selection, dropping 1 pitch increases max rpm 200 or so is the general rule of thumb. One last question, are you running stainless or aluminum? 

Edited by thewayout
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1 minute ago, thewayout said:

What’s the max rpm you are running on a light load? Is it in the upper range of max rpm for that 350? If I had to guess that engine is low 5s for max upper rpm range (4800-5200?) 

always shoot for upper rpm with a light load which will bring you to the low end of the max rpm range with some weight in or behind the boat otherwise you will be lugging and losing performance. It sounds like it’s overpropped to me. What prop are you running currently? Diameter and pitch. 
 

does the prop have exhaust venting holes on drilled in the outside diameter of the prop just above the labyrinth seal diameter? 

Im not sure of the pitch on the prop, its an aluminum 4 blade. Same one we ran all last year. I'd have to check to get the pitch and the boat is at my mom's. We thought about the prop being the issue but we assumed the new engine was making the same or very similar power to the old one and therefore the prop should be effective. Perhaps we are wrong?

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1 minute ago, thewayout said:

Another option is to have multiple props for various uses. One can be for water sports and pulling power that will give you a good hole shot and midrange power, but slower top speed. The other can be a more performance oriented blade if that’s what you want, that sacrifices hole shot for speed. You can’t have both, there’s always a sacrifice in propping, one way or another. 

Thought of that also but frankly that's too much effort. This isn't a high performance ski boat, it's a deckboat that a bunch of teenagers like to goof off on and a bunch of adults like to get drunk on.

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1 minute ago, Kivalo said:

Im not sure of the pitch on the prop, its an aluminum 4 blade. Same one we ran all last year. I'd have to check to get the pitch and the boat is at my mom's. We thought about the prop being the issue but we assumed the new engine was making the same or very similar power to the old one and therefore the prop should be effective. Perhaps we are wrong?

Same original outdrive with the same gear ratio lower unit? Both the original and reman are Mag versions? 

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1 minute ago, thewayout said:

Same original outdrive with the same gear ratio lower unit? Both the original and reman are Mag versions? 

All identical.

Edit: they better be, we paid for it to be the same.

Edited by Kivalo
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2 minutes ago, Kivalo said:

Thought of that also but frankly that's too much effort. This isn't a high performance ski boat, it's a deckboat that a bunch of teenagers like to goof off on and a bunch of adults like to get drunk on.

Shit it takes, what 5 minutes to swap a prop out? 1 cotter pin, slide it off, put the new one on, and go. If you are looking for general performance then you will sacrifice one metric for the other. It sounds like you want hole shot and midrange pulling power, so I would be looking to prop down a bit. Stainless has its advantages with a boat that big and heavy with that much torque. 

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2 minutes ago, Kivalo said:

All identical.

Edit: they better be, we paid for it to be the same.

Something sound off. If the engine is running strong and is considered to be in good running condition and not making less then advertised power, I’d suspect something else is going on. If they replaced the gearcase with say an alpha or a bravo, and it was different then the original lower, the gear ratios could be different and you would definitely have to reprop. 

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1 minute ago, thewayout said:

Shit it takes, what 5 minutes to swap a prop out? 1 cotter pin, slide it off, put the new one on, and go. If you are looking for general performance then you will sacrifice one metric for the other. It sounds like you want hole shot and midrange pulling power, so I would be looking to prop down a bit. Stainless has its advantages with a boat that big and heavy with that much torque. 

It's takes 30 seconds to lower the boat, vent the engine compartment, start it and then crack a beer.  :bc:

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1 minute ago, thewayout said:

Something sound off. If the engine is running strong and is considered to be in good running condition and not making less then advertised power, I’d suspect something else is going on. If they replaced the gearcase with say an alpha or a bravo, and it was different then the original lower, the gear ratios could be different and you would definitely have to reprop. 

Yeah something is off. I wondered about gearing but that should be the same exact gears? Honestly the engine seems to run perfectly except it's got less power. 

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That is a valid point magnum is slightly different than the standard 350. That’s why I wondered if it was a short block or a long block and the proper cam as the previous magnum.  If the outdrive is the same one you ran along with the prop then all this  talk about swapping props is Not really part of the issue.  

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Just now, Kivalo said:

Yeah something is off. I wondered about gearing but that should be the same exact gears? Honestly the engine seems to run perfectly except it's got less power. 

No different then taking a pickup for instance and putting grocery gears in it vs say 4.10s. Gear ratios make a world of difference and if you prop for one and it happens to be different then what you actually have, it’s no different then putting 35s on a jeep with stock fuel economy gearing. Performance sucks and it will eat gas

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3 minutes ago, thewayout said:

No different then taking a pickup for instance and putting grocery gears in it vs say 4.10s. Gear ratios make a world of difference and if you prop for one and it happens to be different then what you actually have, it’s no different then putting 35s on a jeep with stock fuel economy gearing. Performance sucks and it will eat gas

Right but this should have the same gears. I assumed they used the old ones but I suppose they could have changed them as well. 

It goes in tomorrow so I guess we'll see what they say.

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Just now, Kivalo said:

Right but this should have the same gears. I assumed they used the old ones but I suppose they could have changed them as well. 

It goes in tomorrow so I guess we'll see what they say.

Assuming everything expect the motor is the same, I wonder if you didn’t get the mag version of the 350.

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Just now, thewayout said:

Assuming everything expect the motor is the same, I wonder if you didn’t get the mag version of the 350.

I really hope they didn't try and do that sort of thing. That seems like a huge mistake for an engine shop to make and the dealer not to catch.

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2 minutes ago, Kivalo said:

I really hope they didn't try and do that sort of thing. That seems like a huge mistake for an engine shop to make and the dealer not to catch.

Well, you don’t just lose 10 mph and a bunch of performance if everything is the same and new, unless something is seriously wrong with the reman and it has low compression already.

Edited by thewayout
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Just now, thewayout said:

Well, you don’t just lose 10 mph and a bunch of performance if everything is the same and new. 

I know man, that's the point of the thread!  :bc:

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First thing I’d check is spark plug wire off a plug or even a broken/cracked porcelin plug. Happens alot when installing or removing the exhaust manifolds snd risers. Very heavy !. I use studs (temporarily) to slide them off and on. 

2nd is fuel psi. Or 1st. :)

3rd is timing. Well, not really timing, but rotor phasing. Setting the timing is different on these than other fuel injected sbc’s. I’ll see if i can find merc info on this tomorrow.

A 350mpi is 300hp. Thete is no non magnum version of it, that I’m aware of. The 6.2lmpi is 320hp.

The 350mpi has bravo drives behind them. Many times with a Bravo III. The B3’s 2 props.

The Merc 350mpi is almost identical to the 96+ 5.7 truck motors.Intake, tbody and injectors are different. Cam is same 196/204 at .050” 109lsa hyd roller cam, same 9.4:1 conpression, same pistons, vortec heads, block, Same distributor, cap, etc.

In fact, just worked on one last week. :)

 

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Edited by CFM
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Kinda funny, finished working on a Chaparral 222 Sonesta yesterday and took it on a sunset shakedown run tonight. Older one, 305 4bbl, Alpha.

 

 

 

D4F841F5-5D90-4AA9-A489-2B83F3340C18.jpeg

Edited by CFM
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I edited above posts above, just noting this in case they where read before I did the edits.

Edited by CFM
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