2000 mercruiser 350 mpi modifications

[bannister]

Re: 2000 mercruiser 350 mpi modifications

No they don't.......
What Don is suggesting, is go to 20 pitch props.
But when I do that (did it last year) I do get out of the hole faster BUT I need 4000 rpm to cruise at 24 knots and burn much more fuel too......
This is why I am looking for more hp.....

 

[TilliamWe]

Re: 2000 mercruiser 350 mpi modifications

Something is wrong here. With a 2.2 drive and 22p propset, that thing should run much better. You don't need more horsepower, you need all 300 that your engine is supposed to be producing. Either your boat is waterlogged, your antifouling paint isn't working, or your engine is not running right. Good luck.

 

[bannister]

Re: 2000 mercruiser 350 mpi modifications

Boat is fine, antifouling is a week old, bottom is clear and smooth.
Can it be something like wrong timing for example? is it possible at an mpi engine?

 

[TilliamWe]

Re: 2000 mercruiser 350 mpi modifications

It's something, because there is NO logical reason that a 300hp 350 withg a Bravo 3 can't turn over 4200rpms in a 25' cruiser. None.

 

[bannister]

Re: 2000 mercruiser 350 mpi modifications

.....engine hours.....295.....

 

[Tail_Gunner]

Re: 2000 mercruiser 350 mpi modifications

Your turning a 22p prop with a 2.2 gear ratio and getting 34.6 mph at 4200rpm..thats a slip factor of 13%. Pretty good slip for a boat of that size...

So if you drop the pitch to a 20...which will gear it for 4800 rpm and at 13% slip...that yeilds 37mph.

Seems a bit light a good freind has a 26' cuddy sport boat with a stock 350mag and it runs 55... Is the speed your posting GPS and are you sure your tach is accurate..Boat tachs and speedo are more for entertainment that anything else...

 

[bannister]

Re: 2000 mercruiser 350 mpi modifications

My speed reading are knots based on GPS only.

 

[Tail_Gunner]

Re: 2000 mercruiser 350 mpi modifications

Could you post a pic of the boat ....what type of boat is it?...26' cuddy is not descriptive

If thats the boat...a 5.7 is not going to hit 50....nor is a 454....You can build a 5.7 to 400 hp but it would be exspensvie say 4000 ....and that large of a boat would put a heavy strain on a built 350...That is only a opinion

 

[bannister]

Re: 2000 mercruiser 350 mpi modifications

That's my boat. You even got the color right....

I am not looking to hit 50. I am looking to get out of the hole easier and cruise easier.
The 22 pitch that I am now turning are factory originals and are what Mercruiser is suggesting for this engine and boat also.
The engine compression is perfect too.
BUT I can not go over 4200 rpm nor get out of the hole without tabs.

Starting to look at the timing and fueling things this weekend.......

Any thoughts?

 

[TilliamWe]

Re: 2000 mercruiser 350 mpi modifications

You probably aren't supposed to be able to get out of the hole without tabs. Not quickly, anyway.

 

[Tail_Gunner]

Re: 2000 mercruiser 350 mpi modifications

You probably aren't supposed to be able to get out of the hole without tabs. Not quickly, anyway.

exactly The 22p prop your talking about is it a 4 blade and is it alum or ss. Your already over propped maybe you should prop it right and go form there..And tabs are essential in larger boats.

 

[TilliamWe]

Re: 2000 mercruiser 350 mpi modifications

exactly The 22p prop your talking about is it a 4 blade and is it alum or ss. Your already over propped maybe you should prop it right and go form there..And tabs are essential in larger boats.

Tail Gunner, he has a Bravo 3, so his holeshot should be superior to every single prop drive. But still, it's not going to be amazing. However, since he can only turn 4200rpm WOT, his holeshot probably STINKS. 4200 WOT is 800 rpms too low. And changing to a 20 pitch propset is NOT going to raise his rpms to 5000. It just isn't going to happen. Something is not right here. The tach, the "knots", the engine, or the operator. Something.

 

[Don S]

Re: 2000 mercruiser 350 mpi modifications

Tail Gunner, he has a Bravo 3, so his holeshot should be superior to every single prop drive. But still, it's not going to be amazing. However, since he can only turn 4200rpm WOT, his holeshot probably STINKS. 4200 WOT is 800 rpms too low. And changing to a 20 pitch propset is NOT going to raise his rpms to 5000. It just isn't going to happen. Something is not right here. The tach, the "knots", the engine, or the operator. Something.

His problem could be nothing more than improper trim of the boat using engine trim. If it's to low, he is going to loose top end but have a better hole shot.
Problem is, none of us are there to see how the boat is operated, or hear what the engine sounds like, detect misses in the engine or other running problems or how the boat is loaded or anything like that.

Sometimes the best troubleshooting technique is to have an experienced tech that also knows how to handle all types of boats to go for a ride and use all his senses to help determine the cause of a problem. Something that can't be done on a keyboard on a forum.

 

[bannister]

Re: 2000 mercruiser 350 mpi modifications

Ok,
Props are 22 pitch SS 3 blade. 20 pitch props are SS 3 blade too, they do improve the hole a bit but still she turns 4600 max with poor fuel economy (65lts/h).
Boat is tested trimmed out until there is no more change in rpm or speed. Speed is monitored by GPS only.
All gear is at the bow. Gear is a deflated 6 foot dinghy, a toolbox, life jackets, spare props, and a full fridge.
Engine room has the engine, 3 batteries, the water tank and the water heater.

Thought that the problem might be me expecting too much and last week 2 experienced boaters tested my boat for themselves with the 22 props. They both agreed with me that something is wrong. The boat behaves like it is underpowered or something is "holding" the engine back. Trying to get out of the hole without tabs and trimmed all the way down, the engine gets stucked at 3000-3200 rpm for about 12-15 seconds. Smaller props make that a 5 seconds faster.
My mechanic (Mecruiser certified) says that my engine runs perfect and that my only cure is a 6.2 mpi.
Still that does not explain the low max rpm and poor hole even with smaller props.
It should be either one problem and not both at the same time.

 

[Don S]

Re: 2000 mercruiser 350 mpi modifications

No they don't.......
What Don is suggesting, is go to 20 pitch props.
But when I do that (did it last year) I do get out of the hole faster BUT I need 4000 rpm to cruise at 24 knots and burn much more fuel too......
This is why I am looking for more hp.....

So, what is your WOT rpm with the 20's?

 

[bannister]

Re: 2000 mercruiser 350 mpi modifications

Ok,
Props are 22 pitch SS 3 blade. 20 pitch props are SS 3 blade too, they do improve the hole a bit but still she turns 4600 max with poor fuel economy (65lts/h).
Boat is tested trimmed out until there is no more change in rpm or speed. Speed is monitored by GPS only.
All gear is at the bow. Gear is a deflated 6 foot dinghy, a toolbox, life jackets, spare props, and a full fridge.
Engine room has the engine, 3 batteries, the water tank and the water heater.

Thought that the problem might be me expecting too much and last week 2 experienced boaters tested my boat for themselves with the 22 props. They both agreed with me that something is wrong. The boat behaves like it is underpowered or something is "holding" the engine back. Trying to get out of the hole without tabs and trimmed all the way down, the engine gets stucked at 3000-3200 rpm for about 12-15 seconds. Smaller props make that a 5 seconds faster.
My mechanic (Mecruiser certified) says that my engine runs perfect and that my only cure is a 6.2 mpi.
Still that does not explain the low max rpm and poor hole even with smaller props.
It should be either one problem and not both at the same time.

4600 rpm max. cruising at 24 knots requires 4000 rpm burning 52 lts/h.

 

[Tail_Gunner]

Re: 2000 mercruiser 350 mpi modifications

Hmm ok so 20s turn the boat to 4600 what is the speed..I ask to see if its linear aka the same slip as the 22's. I really have no experience with duo props but bravos and duos are not earth shaking stuff..With that said 13% slip on that boat is not shabby duos or not. Since this is a efi motor and you feel underpowered and have poor econ..(plenty of gas) id check your very close for denotation that would retard the timing and loss of power would result...Your merc certified mech should have the software necessary to see the detonation hick ups if they are there.
As Don has said about armchair quarterbacks good luck im out..

 

[bannister]

Re: 2000 mercruiser 350 mpi modifications

Ok,

Thank you all for your time.
From what it looks like, most probable cause of my problems might be timing since denotation is unlikely with 95 or 98 octane gas that I am using (lower rating pump gas in Greece).
I have ordered Mefi scan & tune to do it my self ( I am not going to spend more money and time on mechanics with no results) and if nothing is found or improved I will leave it like that this summer and build me a 383 as a winter project for next year.
That should be enough to move a 25 foot cruiser around with pride.

Have a nice boating season everyone....

 

[Mischief Managed]

Re: 2000 mercruiser 350 mpi modifications

You could have something as simple as a water shutter (exhaust flapper) ot two broken off and partially blocking the exhaust. That would cause every problem you've described.

 

[TilliamWe]

Re: 2000 mercruiser 350 mpi modifications

You DO NOT need an engine change. That boat should perform MUCH better than it is with its current engine.

Try getting some of the gear out of the bow. I know that shouldn't help holeshot, but it might be keeping your bow in the water at WOT. 3 batteries, a full water tank, and a full hot water heater is "a lot" of weight in the stern. But again, nothing that can't be overcome.

You keep mentioning "knots" as you speed measurement. Do GPS units really measure speed in "knots" or is it really MPH?

The reason I say the boat doesn't need a engine is that my father had a Chaparral Signature 27 cruiser that was 9' wide, and definately weighed more than your boat. It had all the same things you mentioned, water system, hot water heater, 3 batteries, and a 350MAG MPI with a Bravo 3. I don't remember his gear ratio, but he ran a 22p propset, and he was very happy with holeshot and top end speed. He got 42mph at 4800rpm WOT in his. And got about 2.2 mpg at cruise. Yes he did have to run 3500rpm to get a nice cruising speed. I spent lots of time on the boat, and while it was not holeshot rocket, it never felt labored. Even LOADED up with the gear that we three people took on a 9 day cruise once.