4.3 MPI 220hp turning 6000 RPM at 48 mph???

[Racklefrack]

Just purchased a 2012 Glastron MX185 with a 4.3 MPI 220 hp motor / Alpha One with 1.81 gearing -- 72 total hours on the whole boat. Dinged prop was replaced by dealer prior to delivery a week ago with the same 23P prop that originally came with the boat.

Today was the first chance we'd had to take her out (since the test drive). 30 minutes into the ride we decide to open her up to see what she'll do... most other forums report this boat will do approximately 52-ish mph at 4800 rpm, depending on setup, conditions, wind, etc. Mine went 48 mph at 6000 rpm... and I didn't even have the throttle arm pushed all the way forward, I had maybe another \inch or so to go.

Clearly something is not right. Even with a 23P prop on a 1.81 stern drive, this engine shouldn't turn 6000 rpm, right? Aren't they rev limited at 4950 or something like that?

I'm thinking that the tach is not set correctly - maybe it's set up for reading an 8 cyl instead of a 6? Doesn't it make more sense that the engine was actually turning closer to 4600-ish rpm at 48 mph with a 23P prop on that 1.81 drive than it does that the engine was turning over a thousand RPM past it's rev limiter?

I'm really new to boats so I could really use your help getting pointed in the right direction. Is there something I can look at on the tach itself to see if it's set right? Is there there something I can check on the engine to see if the rev limiter is hooked up / functioning properly?

Please remember to use small words and simple ideas... I'm really new to all this Thanks so much!

 

[Rick Stephens]

You have pretty good instincts. A 4.3 should be kept under 5k RPM. 4800 is the targeted max RPM. I would suggest the first thing to do is get a quality shop tach and verify the dash mounted readings. Dashboard tachs have a switch on them to configure cylinders, if yours is set to 4 cylinder it would read high. If set to 8 cylinder it would read low on a V6.

Before you get on the water again, install a GPS app on your phone and use it to verify speedometer readings. If boat tachs are notorious for being flaky, pitot driven speedometers are far worse. They can be so inaccurate as to qualify as guestimators. GPS is the only way to go.

I suspect both your numbers are likely inaccurate. And until you have good numbers for wide open throttle (WOT) performance, there is no way to be certain you are propped correctly nor performing up to standards. Max RPM is set by virtue of what prop is installed. Being propped incorrectly can either allow over-revving or lugging of your engine, greatly reducing engine life. So it is important you get baseline established and correct the prop if needed.

Welcome to the forum, BTW!

Rick

 

[bman440440]

also to ad to this... the computer on that engine ( I believe... and don't know for fact) will limit it to 5500 RPM.. the 2009 Mag 350 MPI I have is limited to that... I found it out when I blew out a hub on a prop when was on a fishing trip.... I could get to 3100 RPM but anything over that it would let loose and hit the limiter at 5500 instantly

 

[Fun Times]

Yes you're correct as an 4.3L MPI engine model has a built in REV limiter within the Electronic Control Module (ECM) that won't let the engine rev past 5150ish RPM... < Depended on what engine models the limiter is set to activate at as things varies per engine model...Plus you might even hear a warning horn sound as the engine RPM hits the rev limiter of the ECM.

So that mentioned your issue sounds like possibly your tachometer settings may be just out of adjustment. Generally there is a dial selector switch on the back side of the tachometer gauge that you'd set the number of engine cylinders to.

Below are some guides if needed for setting tachometers....The video you could skip up to the 2:30 minute mark if desired as he talks a little bit about the selector switch.
http://faria-instruments.com/site_manuals/IS0013f.pdf
https://www.youtube.com/watch?v=QSPWOyzuBMQ

 

[Racklefrack]

You have pretty good instincts. A 4.3 should be kept under 5k RPM. 4800 is the targeted max RPM. I would suggest the first thing to do is get a quality shop tach and verify the dash mounted readings. Dashboard tachs have a switch on them to configure cylinders, if yours is set to 4 cylinder it would read high. If set to 8 cylinder it would read low on a V6.

Before you get on the water again, install a GPS app on your phone and use it to verify speedometer readings. If boat tachs are notorious for being flaky, pitot driven speedometers are far worse. They can be so inaccurate as to qualify as guestimators. GPS is the only way to go.

I suspect both your numbers are likely inaccurate. And until you have good numbers for wide open throttle (WOT) performance, there is no way to be certain you are propped correctly nor performing up to standards. Max RPM is set by virtue of what prop is installed. Being propped incorrectly can either allow over-revving or lugging of your engine, greatly reducing engine life. So it is important you get baseline established and correct the prop if needed.

Welcome to the forum, BTW!

Rick

Thanks, Rick. I spoke to Mercury this morning and they confirmed that there's virtually no way that engine will spin up to 6000 rpm, so through some trial and error we narrowed the problem down the the tach -- as suspected. I verified the switch on the back of the tach is in the right selector -- #2 for 6 cyl -- but I twisted it back and forth a couple times just to "clean" the contacts and it sounded pretty gritty. It's likely got at least a little corrosion in there, probably messing up the readings. Plus, it's a Faria tach which I don't hear all that good about anyway.

I have a Garmin GPS I'm gonna use today for a speed test. Hopefully my playing with the tach helped it read the right numbers -- I'll know as soon as I start it up at idle, which was also reading way too high. But if nothing else I'll get the top speed numbers today... that's halkf the info I'll need to prop her out correctly.

Will keep the forum posted on progress. Thanks again!

 

[Racklefrack]

also to ad to this... the computer on that engine ( I believe... and don't know for fact) will limit it to 5500 RPM.. the 2009 Mag 350 MPI I have is limited to that... I found it out when I blew out a hub on a prop when was on a fishing trip.... I could get to 3100 RPM but anything over that it would let loose and hit the limiter at 5500 instantly

Mercury confirmed this morning that my 2011 4.3 MPI engine is programmed to rev limit at 4950 rpm, and it's virtually foolproof. (I asked because someone in another forum suggested a "wire from the governor might have been kicked off" but it can't happen with this engine. Gotta love the internet!) That's one way we know the problem is the tach because I wasn't even bumping into the rev limiter at all so I couldn't have been at WOT.

I'm just happy to know it's an instrumentation issue... that's easy enough to fix.

Thanks for the info! Every little bits helps someone

 

[Rick Stephens]

Borrow a tach, buy one, anything. There is no purpose in getting GPS speeds unless you know RPM.

 

[Maclin]

Mercury confirmed this morning that my 2011 4.3 MPI engine is programmed to rev limit at 4950 rpm, and it's virtually foolproof. (I asked because someone in another forum suggested a "wire from the governor might have been kicked off" but it can't happen with this engine. Gotta love the internet!) That's one way we know the problem is the tach because I wasn't even bumping into the rev limiter at all so I couldn't have been at WOT.

I'm just happy to know it's an instrumentation issue... that's easy enough to fix.

Thanks for the info! Every little bits helps someone

I was gonna say, mine starts beepin' and limits revs at about 5k as best as I could tell when I had the 19p on it.

 

[QBhoy]

Got to be the tach...
no way in the world it can get to 6000 rpm on an mpi.
My v8 mpi has a rev range of 4600-5000 and will limit at about 5150rpm
i think from memory your 4.3mpi is lower at 4400-4800 and will likely limit below 5000rpm.
If you can get her plugged into diagnostics, it can be confirmed.

 

[QBhoy]

Must admit, I also thought that set up might have a 1.62 gearing. Not based on any facts, but you’d think it might have been better suited with that power and boat.

 

[Racklefrack]

Got to be the tach...
no way in the world it can get to 6000 rpm on an mpi.
My v8 mpi has a rev range of 4600-5000 and will limit at about 5150rpm
i think from memory your 4.3mpi is lower at 4400-4800 and will likely limit below 5000rpm.
If you can get her plugged into diagnostics, it can be confirmed.

Yeah, according to Mercury it's limited at 4950.

 

[Racklefrack]

Must admit, I also thought that set up might have a 1.62 gearing. Not based on any facts, but you’d think it might have been better suited with that power and boat.

I gotta admit, the 1.81 drive is the thing throwing me off the most. I don't quite get how it impacts the prop pitch; there's far more available info on the 1.62 but it's really hard to find any info on propping a 1.82. Guess I'll be the first

 

[porscheguy]

What kind of prop did they put on? Does it have a name or model # stamped on it? 52ish is reasonable for that engine/ratio/boat size. But 48mph @ 4950rpm is 19% prop slip which means you're leaving performance on the table.

I've got an older carbed 4.3 and the rev limiter kicks in at 4950 quite reliably.

 

[QBhoy]

I would think this would be a 50mph gps boat all day long. I know of boats similar (Monterey with a lesser powered carb 4.3) with presumably that ratio...and they easily turn a 23” prop almost to the limiter.
I would imagine your set up would would be close to or on the limiter with the mpi, 1.81 and 23”.
Jeez, 18ft bayliner open bows with a 3.0 mated to a 1.81 (sometimes 2.0) ratio will turn a 21” easily.
Forgive me for asking, but are you trimmed out and running to optimum at those figures posted ? Lots of weight and full of fuel perhaps ?

 

[Racklefrack]

What kind of prop did they put on? Does it have a name or model # stamped on it? 52ish is reasonable for that engine/ratio/boat size. But 48mph @ 4950rpm is 19% prop slip which means you're leaving performance on the table.

I've got an older carbed 4.3 and the rev limiter kicks in at 4950 quite reliably.

No name, no model # that I've seen, just 23P.

To be clear, since my tach was malfunctioning I only went to what it said was 6000 rpm (which we know is wrong) which was 48 mph on the speedo. I wasn't WOT at the time -- I had probably 1/2" - 1" left to push the throttle forward but chose not to. I believe I could have pushed it forward and made it to 52-54 mph but I can't say for sure. Even if I had, I wouldn't have been able to tell what the rpm's were.

 

[Racklefrack]

I would think this would be a 50mph gps boat all day long. I know of boats similar (Monterey with a lesser powered carb 4.3) with presumably that ratio...and they easily turn a 23” prop almost to the limiter.
I would imagine your set up would would be close to or on the limiter with the mpi, 1.81 and 23”.
Jeez, 18ft bayliner open bows with a 3.0 mated to a 1.81 (sometimes 2.0) ratio will turn a 21” easily.
Forgive me for asking, but are you trimmed out and running to optimum at those figures posted ? Lots of weight and full of fuel perhaps ?

Trimmed yes, optimal yes, full fuel yes, lots of weight no. BUT... keep in mind that 48 mph was not WOT. I stopped throttling forward when my malfunctioning tach reported 6000 rpm, which we know is wrong. I probably could have pushed the lever forward another 1/2" - 1" or so and gotten to 52-54... just guessing, but it seems likely. But because of my tach I wouldn't know what the rpm was.

I'm guessing 52-54 because that's what's commonly reported for this boat, and because of how much more throttle I had left. But those other boats probably aren't turning a 1.81 Alpha and I don't know what pitch their props were.

Thanks for the help!

 

[QBhoy]

Makes more sense.

 

[Rick Stephens]

4.3 V6 uses 1.81:1.

V8's use 1.62 or lower ratios. You have the correct ratio drive for any V6.

 

[thumpar]

Keep in mind that MPH gauge could be off more than that current tach so the numbers all need to be checked with a reliable way.

 

[Racklefrack]

4.3 V6 uses 1.81:1. V8's use 1.62 or lower ratios. You have the correct ratio drive for any V6.

Well now I'm even more confused. I have found a dozen different references to 4.3 V6's using Alpha 1 1.62's, and even Glastron's original build sheet specs out using the 1.62 in the original configuration:
-

Boat Model:	MX185BR-2012 MX 185 BR I/O 2012
Engine Model:	GC-12-4.3MPIC-A 4.3MPIC/A 220HP 1.62:1

-
According to Glastron, it was the dealer who swapped out the drive to a 1.81 and a 23P prop to compensate for the altitude in Colorado where it was originally sold. Otherwise, it would have been sold with the 1.62.

The only reason this is even important is because IF the change to a 1.81 / 23P is not a normal thing, then I don't want to repeat it by installing a new 23P... I'd rather put on a prop that was more "normal" for this boat's operation at closer to sea level, like a 19P or 21P. If, however, the 1.81 / 23P IS a normal thing, then putting on a new 23P wouldn't be that bad an idea.

Ugh... I've only owned the boat a couple of weeks and I already wanna shoot myself in the head