Merc 4.3 MPI vs carb

[JoLin]

For me it boils down to what I can understand and fix on my own, versus what I need someone else with specialized diagnostic equipment to do for me. I've had 2 boats with EFI (throttle body, not MPI) and both ran great. But, I had an issue with one of them that I couldn't figure out. Hadda pay a mechanic to fix it.

Current boat (purchased in 2014) has carbs. It's run flawlessly for 4 seasons. I know that when the time comes I can rebuild a carb. Fuel, spark and air are all separated in my engines. Those components are so electronically intertwined in a modern engine that it's tough to even figure out which is causing the issue.

It's true, younger boaters are more likely to want MPI, but reliability and repair considerations don't even enter into it. I'm generalizing, but most wouldn't know the difference between a fuel injector and a carburetor jet anyway. They're gonna hire someone to do it for them. I'm not trashing them, they simply grew up in a different era.

My .02

 

[QBhoy]

With regards to diagnosing faults and perhaps being hard to do on the mpi....
The smart craft will tell you faults and codes when the occur.
If it doesn't have smart craft there is the vessel view mobile device for a small amount of your hard earned. This is a great wee Bluetooth device that send all the smart craft info to your tablet or phone. Actually has a wee screen with all the details on the engine. Rpms, temps, fuel consumption and range service details and countdown etc. Loads of handy things.

 

[Lou C]

Even with the easy diagnose-ability provided by scanners, you still have to interpret the data to make the repairs. And that requires more than just computer tech skills. Still the ability to just bolt on a new Marine Edelbrock or Holley and get like new performance is very attractive to me. I can still get all the parts I need for a Quadrajet that has not been made for 30 years. Not sure that will be true of marine EFI systems in the future. So you have to get use to the quirks of starting a carb boat. Do you use a lawn mower, weed wacker, leaf blower, snow blower or small portable outboard? Its all the same principal.....An EFI boat is not worth more to me than a carb boat, in fact it is more of a concern due to parts availability and price.

 

[JoLin]

With regards to diagnosing faults and perhaps being hard to do on the mpi....
The smart craft will tell you faults and codes when the occur.
If it doesn't have smart craft there is the vessel view mobile device for a small amount of your hard earned. This is a great wee Bluetooth device that send all the smart craft info to your tablet or phone. Actually has a wee screen with all the details on the engine. Rpms, temps, fuel consumption and range service details and countdown etc. Loads of handy things.

I don't doubt that it's entertaining, but how much is it telling me, that I don't already know with my old carbed engines? RPM? Engine temp? I already know them. Range at a specific rpm is useless to me. I'm a saltwater boater in usually choppy stuff. RPM varies constantly. What I DO know (through 500 miles of long-trip cruising) is that I can bank on 1.0 mpg for trip planning. Maintenance requirements? I do all that seasonally and it's a very comprehensive list. Do I need to know my fuel consumption the the tenth of a gallon? Nope. It's a boat. It uses a lot of fuel compared to a car.

I wouldn't pass up a boat I liked because it has MPI, nor would I pass on it because it has carbs. I personally prefer carbs. Easy to understand, easy to fix.

My .02

 

[Maclin]

I echo this one from JoLin
"I wouldn't pass up a boat I liked because it has MPI, nor would I pass on it because it has carbs. I personally prefer carbs. Easy to understand, easy to fix."

The boat I have now is 4.3MPI, but I chose this boat based on overall condition and it was exactly what I was looking for. A carb engine would have been A-ok too. Truth be told the MPI scared me a little bit, almost a detractor factor. I am the penultimate Carb hotrodder from the 1960's, though, just what I was brought up with. New stuff is great also, but I can't afford boating unless I do everything myself and Carbs are my second language.

 

[QBhoy]

I don't doubt that it's entertaining, but how much is it telling me, that I don't already know with my old carbed engines? RPM? Engine temp? I already know them. Range at a specific rpm is useless to me. I'm a saltwater boater in usually choppy stuff. RPM varies constantly. What I DO know (through 500 miles of long-trip cruising) is that I can bank on 1.0 mpg for trip planning. Maintenance requirements? I do all that seasonally and it's a very comprehensive list. Do I need to know my fuel consumption the the tenth of a gallon? Nope. It's a boat. It uses a lot of fuel compared to a car.

I wouldn't pass up a boat I liked because it has MPI, nor would I pass on it because it has carbs. I personally prefer carbs. Easy to understand, easy to fix.

My .02

Each to their own I suppose. It's saved me a fortune on diagnostics by the dealer. Stick with the carb.
I don't suppose you will have a mobile smart phone either....new fangled fancy tech that they are ! Haha.

 

[QBhoy]

It was also very useful for getting the prop just right with next to no slip. I love to strive for perfection and efficiency. It's in my nature I suppose. Like I say, matters to some and not to others.

 

[jimmbo]

I love watching the Part Changers, I mean service techs, when they read the codes from the scanner. Right away, they want to change the sensor. Then when it still runs poor, they want to change more parts. Somewhere in their training they never were taught how to diagnose the symptoms and what the codes are saying.

 

[JoLin]

Each to their own I suppose. It's saved me a fortune on diagnostics by the dealer. Stick with the carb.
I don't suppose you will have a mobile smart phone either....new fangled fancy tech that they are ! Haha.

A shame you spend so much time performing diagnostics. I'd rather check the weather app on my 'droid and go boating.

Saw a funny banner on facebook the other day... "I've invited a few friends over to sit and stare at their phones. Please join us if you're free."

 

[tpenfield]

MPI engines run great until they don't . . . then all the traditional troubleshooting logic goes out the window. I have never had problems with the carb'ed engines, yet my preference is for MPI.

​For me it is a matter of how the fuel gets to the cylinder, and IMO the closer to the cylinder you can do that, the better . . . That is why I am not a big fan of the TBI (aka EFI) engines, because the fuel is introduced at a similar point to a carb'ed engine. The only benefit is that it is more precise. . . but who's to say it will get to where it is going in that same precision if it has to travel a lot further than an MPI setup :noidea:

The challenges with MPI/EFI is that it takes a computer to manage the various engine functions that carb'ed engines operate mechanically. Since it takes a computer to run them, it takes a computer to fix them . . . As for the codes that can be read from the controller (computer) the logic is only so sophisticated . . . If the computer program sees that a sensor is out of range of where it 'thinks' it should be, then it often produces a malfunction code. However, there could be (and often is) another aspect of the engine's operation that is causing the sensor to be out of range.

​that is why we often see that replacing sensors does not fix the problem . . . it just happened to be the computer's 'best guess' at what the problem is. . . and the diagnostics hide and seek goes on.

 

[QBhoy]

Very good points from both above !
The vessel view also checks the weather for you....haha.

 

[QBhoy]

I'd also imagine that most carb fans (I used to and still am one)....once they have had an mpi and see how much it saves them on fuel....wouldn't go back to a carb.
How many v8's in a boat do you know of with a carb that will cruise using only 19 litres per hour ?

 

[bruceb58]

I'd also imagine that most carb fans (I used to and still am one)....once they have had an mpi and see how much it saves them on fuel....wouldn't go back to a carb.
How many v8's in a boat do you know of with a carb that will cruise using only 19 litres per hour ?

If a carb is jetted correctly, it can easily happen if the MPI is a version that doesn't have O2 sensors. The MPI has the advantage of being able to adapt to altitude where the carb isn't. If both are running at the proper stoichiometric ratio, their fuel consumption will be identical.

 

[QBhoy]

If a carb is jetted correctly, it can easily happen if the MPI is a version that doesn't have O2 sensors. The MPI has the advantage of being able to adapt to altitude where the carb isn't. If both are running at the proper stoichiometric ratio, their fuel consumption will be identical.

Surely not Bruce. You are talking about an impossible situation. Have a look at the Merc figures and the economy calculators for these engines. There is no comparison and can't possibly be compared in terms of fuel efficiency.

 

[JoLin]

My boat wouldn't use 19 liters (5 gallons) per hour if it had a matter/anti-matter converter in it.

Just for the sake of argument, let's say MPI would give me a 20% improvement in consumption. Fuel cost is about 20% of my annual operaing budget, so we're talking about an overall cost savings of 4% annually. In my particular case, that's $240. that will disappear the first time something goes south on the MPI.

As I said before, if the boat I want has EFI/MPI (like my previous 2 boats) I'll buy it. If the boat I want has carbs (like the one I own now) I'll buy it. All other things being equal, I'd rather have carbs.

My .02

 

[bruceb58]

Have a look at the Merc figures and the economy calculators for these engines.

Have a link?

I have seen numerous tests and the difference is negligible. I will try to find one. boattest.com has done a few

 

[QBhoy]

http://www.boat-fuel-economy.com/outboard-fuel-consumption-liters

Look at the difference between the 5.0 mpi 260hp and the equivelant 5.7 carb 260hp.
I'll screen shot my figures next time out too. She was running at 2150rpm and 23.5mph at 18 litres per hour earlier.
With a 100l tank...that would give a comfortable range well over 100 miles with reserve to spare. Probably unrivalled by an equivelant carb model.

 

[bruceb58]

LOL...2 different displacement engines? That is not a Mercruiser site. Show me where they have the identical displacement engine in the exact same boat comparing MPI vs carb

Have a look at the Merc figures and the economy calculators for these engines.

Waiting..........

 

[QBhoy]

The 5.0 (even the tks 5.0) is there too.
Is the 5.0mpi not considered the equivelant to the older 5.7 carb ?
It's getting late over here and time for bed soon, but I'm sure the Merc website has always boasted about the fuel efficiency improvements the mpi brings.
The website link posted is tried and tested and completely neutral in its findings.
If anything, it shows a more fuel hungry figure to the reality.

 

[bruceb58]

There are many tests that have been done comparing small experimental aircraft(one of my interests) engines using carbs and fuel injection. The difference in economy is almost not measurable.