Merc 350 mpi not producing full power

[QC]

Re: Merc 350 mpi not producing full power

Quick question. Did this originally have closed cooling? I just noticed that Fun Times identified it as a Bravo model, there might be a clue there. A closed cooling flash file won't run well with raw cooling.

 

[jdubslopro]

Re: Merc 350 mpi not producing full power

The motor is a horizon and came from the factory with closed cooling. The thermostat is exactly the same (160 or 170 I don't remember which one) and temp gauge indicates constant ~170* which leads me to believe its a 170 tstat.
I usually have 5-6 ppl on board as well as a full cooler, half tank (15 gals or so), although my top run (52mph) yesterday was solo with no additional weight besides life vests anchors and buoys. Speed only fluctuates about 3-4 mph between loaded/unloaded.
I have always thought it was bad for the drive to porpoise so I don't allow it to do so...
I believe merc has measured HP expressed in terms of PROP HP since the 90s, and my 4.3 v6 was a 2bbl which I believe is rated at 195hp. This motor is rated at 300 prop hp (330 crank) from all the information I've been able to find.
Speed source on sister ship is both gauge and GPS.
Another new discovery today, a friend with a BIG 23' rinker with a 350, dual-prop counter-rotating spinning 26p props with a 2.20 gear ratio hit 62 on GPS earlier this year...my boat is easily a quarter ton lighter...why can't I go this fast lol

 

[jdubslopro]

Re: Merc 350 mpi not producing full power

Sorry I didn't specify, I am running raw-water cooled.

 

[QC]

Re: Merc 350 mpi not producing full power

The motor is a horizon and came from the factory with closed cooling. The thermostat is exactly the same (160 or 170 I don't remember which one) and temp gauge indicates constant ~170* which leads me to believe its a 170 tstat

This is what I was worried about. How do you know they are the same? Hopefully Fun Times will chime in again.

 

[Tail_Gunner]

Re: Merc 350 mpi not producing full power

What was RPM just before she started to rev up? Speed? Did she hit the rev limiter at 5400? This happened below trim limit? Can you make her porpoise? If she blows out before porpoise you need to shift some weight aft and retest. Can make a significant difference.

Please answer each question. All important.

Umm you need to check the tach at 5000 it would have started missing and 5400 it would missed hard and fast.......Ahh that tach has been set to sense a v8 has it not.

As to the closed cooling vs open there is a very slight enrichment in a merc...you would never notice this however...Its just a attention to little detail thing.

 

[QC]

Re: Merc 350 mpi not producing full power

Not positive on when the rev limiter would stat hitting, but should be below 5400. He said tach was switched early on, I'd still like to see it verified.

Porpoising won't hurt the drive, and I am not saying you should porpoise, she should be able to porpoise. That tells me weight is balanced right

 

[jdubslopro]

Re: Merc 350 mpi not producing full power

I know the tstat is the same bc I verified part numbers with Mercruiserparts.com and they were identical. This was an issue brought up early on in the swap in one of my previous threads, and I verified it before moving on.
As far as porpoising I would say yes it is able to do so easily, I will pay closer attention to this next time I'm out. It gets up on plane very quickly, I would say noticeably quicker than the previous motor. I've never noticed speed being any greater while porpoising, but I've never had it doing so very long at all.

 

[jdubslopro]

Re: Merc 350 mpi not producing full power

Finally was able to test it with a 23p. Max rpm was 4600 and max speed was 54. It was a bit hesitant to get up on plane but did so more easily than my v6 with 21p.
It didn't porpoise, with 550lbs of passengers at the helm/back seat.

 

[QC]

Re: Merc 350 mpi not producing full power

It didn't porpoise, with 550lbs of passengers at the helm/back seat.

See that still tells me weight balance. Imagine if there was no one in the back seat. Weight would be biased forward with just you. I seriously think this a lot about balance. If you could get the bow up alone I think you could get to 56 easy. The 54 was GPS?

 

[jdubslopro]

Re: Merc 350 mpi not producing full power

I didn't verify with GPS, but my speedo has always been within 1 or 2 mph when compared to GPS. I've been looking into swapping the upper gear case for a 1.5 or 1.47 gear ratio, I'm really just not sure if there's going to be a noticeable performance difference and I don't want to drop $900 for little/nothing. Is anyone running a similar setup? What would be the correct(ish) prop to use with a 1.47 and 2650lb vessel?

 

[cdiamond]

Re: Merc 350 mpi not producing full power

i read the op's first post

then the second - BINGO !!

you're still running the v/6 outdrive !!

it's been explained here before many times

YOU CAN'T PROP A BOAT TO PERFORM PROPERLY - WITH THE WRONG RATIO OUTDRIVE

 

[500dollar744ti]

Re: Merc 350 mpi not producing full power

Finally was able to test it with a 23p. Max rpm was 4600 and max speed was 54. It was a bit hesitant to get up on plane but did so more easily than my v6 with 21p.
It didn't porpoise, with 550lbs of passengers at the helm/back seat.

That sounds about right, now you probably just need to play with trim and props to get a few more mph.

Do you have any kind of doel-fin or hydrofoil on there? If you do, it's robbing you of 4-5mph top end, I tested mine with and without and that's how much it pulled off my top end.

 

[jdubslopro]

Re: Merc 350 mpi not producing full power

No hydrofoil, never believed in them. Anybody wanna let me try out their drive leg lol? There's a gear case for $300 on eBay with a cracked cavitation plate...I know it'll be MUCH easier to repair that than install new gears into my drive...

 

[500dollar744ti]

Re: Merc 350 mpi not producing full power

Where are you located?

 

[jdubslopro]

Re: Merc 350 mpi not producing full power

Western Massachusetts

 

[jdubslopro]

Re: Merc 350 mpi not producing full power

I've seen this same setup (5.7mpi and 1.62) in heavier boats capable of turning 62-65mph. I've also seen this motor with 1.5 ratio running 60-65. I expected (and would be happy with) 60mph...hopefully you can understand my disappointment.

 

[Tail_Gunner]

Re: Merc 350 mpi not producing full power

i read the op's first post

then the second - BINGO !!

you're still running the v/6 outdrive !!

it's been explained here before many times

YOU CAN'T PROP A BOAT TO PERFORM PROPERLY - WITH THE WRONG RATIO OUTDRIVE

That is not exactly correct while a 1.47 fear ratio would be optimal 1.6 is in no way going to effect overall performance.

 

[QC]

Re: Merc 350 mpi not producing full power

I think this ratio is manageable as well.

I still want the OP to shift weight aft., test alone, trim her until she porpoises, and then bump down trim slightly to control. Move your batteries aft. Tools aft, ice chest aft, anchor aft. Nothing but air, and your azz, forward of the rear bench.

 

[500dollar744ti]

Re: Merc 350 mpi not producing full power

I've seen this same setup (5.7mpi and 1.62) in heavier boats capable of turning 62-65mph. I've also seen this motor with 1.5 ratio running 60-65. I expected (and would be happy with) 60mph...hopefully you can understand my disappointment.

Weight is not the only factor in determining top speed. Just because your boat weighs the same as another boat doesn't mean it will perform the same as that other boat.

For example, here's a 20' Malibu with 400hp that tops out at 49.6mph.
Malibu?Response LXi (2009- )? Reviews,performance,compare,price,warranty, specs,Reports,Specifications Layout, video | BoatTEST.com

Then a 18' Maxum, 2500lbs with 4.3MPI 220hp, 46.0mph top speed.
Maxum?1800 SR3?2003? Reviews,performance,compare,price,warranty, specs,Reports,Specifications Layout, video | BoatTEST.com

Now a 18' Stingray, ~2200lbs, 135hp 3.0 4-cyl, 49.4mph
Stingray?185 LX? Reviews,performance,compare,price,warranty, specs,Reports,Specifications Layout, video | BoatTEST.com

Ok so when we find shorter 17' Bayliner with the same 135hp 3.0, weighing ~2000lbs, top speed 41.4mph.
Bayliner?175 BR (2012-)?2012? Reviews,performance,compare,price,warranty, specs,Reports,Specifications Layout, video | BoatTEST.com

Maybe that will make you feel better about you not getting your 60mph.

 

[jdubslopro]

Re: Merc 350 mpi not producing full power

Nothing at all has changed besides the addition of 110hp, and a little weight from the extra cylinders. The speed has increased a negligible amount. Also there is an identical boat with less HP (4bbl carb) hitting 62mph.
While changing my technique might help squeeze a couple more mph out, I still feel that the speed should be increased by more than a couple of mph without changing anything else due to the large gain in HP.
I know that hull design plays a huge factor in the boats top speed, as well as weight and technique, but if the exact same hull with less hp gets 62...I want it too. Btw the other boat is running a 21p and I can only assume a 1.5 or so drive ratio (maybe 1.47)

I'm going to pull the plugs and see how it's burning, I feel like its not producing its rated output. It doesn't misfire or skip at all anywhere in the rpm range, but if its not burning correctly then that could have some effect on power output. I have run it with 92 and 89 octane and noticed no difference.