Merc 350 mpi not producing full power

[QC]

Re: Merc 350 mpi not producing full power

If the horsepower is eaten up by driving the bow down, then it is simply fuel use that you gained . . . the wrong way. Why wouldn't you try EVERYTHING suggested. I think you said the plugs looked perfect before. This may very well be an engine problem, but please recall you said it was right before. That's why I have concentrated on setup

I still want more info on the sister ship. You said GPS and gauge. Who saw them?

FWIW, look back, I agree she should be faster, but every single piece of info is important

 

[aerobat]

Re: Merc 350 mpi not producing full power

You said GPS and gauge. Who saw them?

thats a good question. did you saw with your own eyes that a 260 hp pushes the sister boat past 60 mph measured with gps or did you heared it from dockside talk ?

boat tests published on mercuiser website does not confirm the even a 350 mag can push a 20 footer past real 60 mph .

when you sgueeze now 54 out of it it seems very good to me, but odd you say a 4.3 did the same.

here i found a video from a mariah that should be pretty close to your boat on a testride with a 4.3

SOLD 1996 Mariah Lake Test - YouTube

- he is able to reach 44 mph due to gps which sounds correct...

you should try a stainless prop for your 300 hp v8 and look if you can pick up a little more speed, beyond this i guess you are confused with fairy tales from other owners and some fancy readouts by speedometers that go far beyond reality . 60 mph by gps is a hell of speed.

 

[jdubslopro]

Re: Merc 350 mpi not producing full power

I know for a fact that a 350mpi will push a boat well over 60. A buddy has a rinker captiva 232 open bow that is much heavier and sits very tall comparatively, with dual props is able to get 62mph. I know some of that is due to the utilization of two props, but at the end of the day same hp bigger boat, not something designed for speed either. Mine gets off quicker due to weight and size advantage, but once I hit 40 he's passing me. I will try everything recommended and post results.
I will comment that it it is faster with my 19p 4 blade than my buddy's 18' crownline 4.3 with 21p. I think they are very similar boats.

 

[QC]

Re: Merc 350 mpi not producing full power

I have a 23 footer with a 6.2 MPI and Bravo1 and she will only hit 57. Tested as new, by magazine at 62 Bravo 1s are usually faster than Bravo 3's in top speed. Not usually an issue either way below 60 though.

 

[aerobat]

Re: Merc 350 mpi not producing full power

I know for a fact that a 350mpi will push a boat well over 60. A buddy has a rinker captiva 232 open bow that is much heavier and sits very tall comparatively, with dual props is able to get 62mph.

the rinker captiva 232 bowrider was tested several times officially -

with 350 mag bravo III : 51.4 mph mph

Playing - Rinker 232 Captiva BR (Not in Production ) - | Partner Magazine | Powered By BoatTest.com

with 6.2L MX and bravo III : 57.3 mph

Rinker 232 Captiva Bowrider | Boating Magazine

this tests may vary a little due to conditions of course, but you say your buddy is going +10 mph faster than boattest was able to do with the same boat/engine and even outperforms the also tested 6.2 L MX by 5 mph ????

like mentioned in my first reply : i do not think you have any problem with the engine but REAL 60 mph or more are simply not in the cards .

 

[alldodge]

Re: Merc 350 mpi not producing full power

I have a 95 rinker 232 cuddy with a 7.4 and bravo 1, the speedo says 60 but the GPS says 58 mph

 

[QC]

Re: Merc 350 mpi not producing full power

Important to note, that the 7.4 above ^^^^^ was rated at 300 bhp, so not a bad comparison.

 

[500dollar744ti]

Re: Merc 350 mpi not producing full power

The extra 6mph that you want may be available in a different prop. What exact prop are you running that nets you 54mph?

 

[jdubslopro]

Re: Merc 350 mpi not producing full power

54 was with a 3 blade aluminum brand new prop the local marine store let me demo. I know a stainless will net slightly higher top end due to less flex. It was too sluggish for my liking to get out of the hole with that prop though. I put a 21p 3blade al fresh from the prop shop on it and had 6 ppl (2 in the captain/first mate chairs, 4 in the rear. It wheelied for a few seconds but once it leveled out hit about 46-48 at 5000 rpms. The rpms were higher than I expected, speed was a little lower than I expected but not much. Probably the extra weight, not balanced in the boat (had an inflated tube in the bow). I did notice something kind of strange though. The water spray pattern behind the drive was almost kind of indicating something dragging, there was a lot of spray coming from the side of the drive. I wonder if there is drag from the hydraulic lines hampering performance. I replaced the trim cylinders, possibly when I put them in the hoses weren't routed correctly? Ill take a video next time I'm out. Ill also have to observe some other boats while under way to see if they emit the same spray pattern.

 

[jdubslopro]

Re: Merc 350 mpi not producing full power

Just got home and took a look at the boat...I think I have the trim cylinders on the wrong sides. Instead of the hoses coming out of the top and toward the transom, they come out the bottom. So looks like I'll be swapping those and bleeding the air out...

 

[500dollar744ti]

Re: Merc 350 mpi not producing full power

It's obvious top speed is important to you but you keep dicking around with aluminum props. Why? Does your friend with his 60mph speed use an aluminum prop? 4-blade props take away from your top end, I would recommend a Solas HR-Titan 3-blade for your application. Not the regular Titan, but the HR-Titan.

I would pick the 14.25x22p, Solas HR Titan Stainless Steel Propeller (4551-143-22) - iboats.

This prop (in a 19p for my application), out of all the props I've tried has netted me by far the best top speed performance. I can get 50mph out of this design HR-Titan 3-blade where everything else won't budge over 46mph. Personally, while I'm not guaranteeing your results, this is the prop I would choose to run if I had your boat w/ 350mpi. I feel like it really could put you over 60mph. Your hole shot may suffer slightly but you want top speed right?

If you don't like it you can turn around and sell it for a slight loss, that's what I do when I'm trying out props, you gotta pay to play.

 

[jdubslopro]

Re: Merc 350 mpi not producing full power

because i don't want to spend $300 on the wrong prop. they didn't have any stainless props to demo at the local shop. I have a few stainless props but they are all wrong and I'm having trouble getting rid of them. While top speed is somewhat important, I want to make sure that everything is performing correctly. I don't need it to go 60+ if it means putting in a high performance cam and porting and polishing the heads (I don't want to sacrifice any reliability-high perf cams are known for inducing reversion) I also don't want to overpower the outdrive. Alpha is rated for 300 and that should be where I'm at now. I don't mind sacrificing holeshot a LITTLE for a great deal more top end, but if it's as insignificant as 2-3mph I'd rather keep the holeshot and 50ish top speed. I'm not running this thing at WOT all the time, but it's nice to have the option.