502 Merc Build, MPI or Carb, what Cam?

TLDR: Replacing a Gen IV 454 Carb with Gen VI 502, how do I aspirate if I have choices? Boat is 1991 Bayliner Command Bridge, single engine used casually on intracoastal and Chesapeake tributaries...

Copied post to start newer thread.... I am a few months down the road and I need advice. Since the last update, the Volvo 8.2Gsi fuel injection was set aside as there were problems with the fuel pumps and I want a Mercruiser type set-up. We broke everything down and sent everything to the shop. Now, my Gen VI Engine is now a 502 bored .030 over with new pistons/rings, new rod/main/cam bearings all assembled into a short block ready for the bolt on goodies. Stock Roller Cam was checked out a-ok and re-installed.

This is where I need guidance.

I have a 1996 Mercruiser 502 Magnum EFI system with promising, yet not verified, history. Even though I have a MPI set-up in hand, much of the advice I have been given by reputable builders is to go with an Edelbrock carb set up since wiring to the boat would be simple and the Edelbrock system is reliable. Others have said the MPI would pay for itself in reliability and fuel savings, but since this boat will likely be run moderately and not looking for breakneck speed, I'm not so certain.

Since I am starting with a short block, I am for drawn to the relative simplicity of a carb set-up. The stock Mercruiser 7.4 from 1991 was carb and ran reliably for nearly 30 years, so I am expecting similar longevity.

Assuming the cost from now to install would be similar for MPI and Carb, what route would the group take; Used Mercruiser 502 Magnum MPI with unknown history, or for completely new Edelbrock carb'd set-up?
There would be wiring and installing an electric fuel pump going from Gen 4 to Gen 6, what else am i missing for MPI?
Do the carbureted and MPI 502 variants of the Gen VI use the same roller cam?
New Manifolds, Risers, and Cooling system are on the way. Hoping this can all come together in the next month or so.

What else am I missing?

For right now, without looking into the cam specs. My preference would be the Carb.

Pros for Carb use vs MPI,

1. Known reliability.
2. easy to repair/replace
3. Carb and any Parts available thru most Auto stores not just marine
4. Lack of extensive wire harness, sensors, possible fail points and the infamous fuel pump and MANY associated issues with Mercs COOL FUEL never mind the difficult nature to service it.
5. Marine Fuel pump for carb, also accessible at/thru Auto stores, low pressure and simple in design and can be located almost anywhere, NO cool fuel design/set up required.
6. Keep all current wiring
7. Thunderbolt ignition, Bullet proof, Been in use since the 80's Just replace all internal ignition parts including cap as preventative every few years. Coil can be purchased at any Auto store (internal resister) (you could easily and for short money have all spare parts in boat. (Ign pickup, rotor assembly, cap and coil)


Cons,

Other than cleaning/replacing fuel filters and possible higher fuel consumption and lower HP (meaning not as high HP as MPI mostly at higher rpm), Not much else that I can think of.

MPI Pros

1. Consistent/easier/quicker starting.
2. Quicker throttle response
3. improved fuel economy
4. slightly higher power
5. can be more precisely tuned for your application where as carb is limited.

MPI Cons,

1. Extensive wire harness.
2. Multiple sensors to fail
3. multiple connections to cause signal loss for many reasons especially in salt water
4. Ignition is dedicated to MPI and the use of Thunderbolt no longer needed
5. COOL FUEL difficult to service and a long history of issues.
6. May require some different gauges to support certain outputs from ECM
7. ECM, if it goes your screwed. $$$$ to replace.
8. in order to trouble shoot you have to have a Break out/analyzer of OEM or aftermarket
9. LIMP MODE..........how many times have you heard someone heading to shore in LIMP MODE
10, 8 injectors vs One Carb. trouble shooting and repair can be much more difficult and costly (could have a spare carb on board and replacement could be done with hand tools in an hour if carb failed which would be unlikely) Injectors, well they are electro-mechanical.


That is all I can think of and I am sure there are others who know a lot more that I do about MPI and can add to the pros and cons

Further kghost... Actually, I don't think that MPI is more reliable. There are too many parts in it that can fail... and while solid state electronics are theoretically more reliable than mechanical stuff they replace, unless its built to just about MILSPEC for use in a marine environment, its not in practice. Consumer systems manufacturers don't take the care in sourcing parts or in manufacturing, nor in design margins for a "consumer grade" MPI system to be more reliable than a carb. Do carbs plug up (largely thanks to ethanol), yes... Some years ago when I re-engined my boat the yard that did the re-engine wanted me to install an MPI engine. I insisted on carb. I bought my first board ( I/O powered) in 1964 and have always been able to get home ( assuming fuel in the tank!!!) with a few simple tools ( and spare ignition parts).

EFI haters gonna hate. Carburetors are seen as more reliable due to the fact that they are 100 percent mechanical, and issues with them are easily diagnosed and resolved.
I will flip that and say that EFI is more reliable because it is almost zero percent mechanical and issues tend to be easily diagnosed and resolved. You just have to educate yourself a little and understand how and why EFI systems work and why they have taken over as the fuel delivery system on virtually every engine being produced.

What gives these a bad rep is that most driveway mechanics don't know the basics and throw in the towel before even taking a few simple test procedures.
99.999 percent of the time fault lies with something that's easily repaired like a fuel pump, a solenoid, a fuse or a sensor. Sensors rarely go bad, and often you get plenty of warning before they go.
The EFI system you are referring to is fuel-only. Ignition and timing are totally separate however the fuel system needs a signal from the ignition to know when to open the injectors. That's the only interaction between the two systems; a pulse wire.
So by exchanging a carb for injectors you are only giving up a mechanical box full of passageways, vents, springs, diaphragms, linkage, valves, blah blah blah. The ignition remains basically the same. As for sensors and wiring, pah! every connector is waterproofed.
Yes, there are downsides to every choice but for me the EFI decision would be the right one.

One final thought. These are more or less industrial power plants, not automotive engines. They're meant to be run under full load constantly. They don't go up hill or coast down hill. They run (presumably) at sea level, not high altitude. Doesn't need to adapt to your driving style or run in sub-freezing conditions. The difference between computer controlled fuel delivery and carburetor fuel delivery under these conditions is almost nil. Your desire is for reliability. Yep Mercruiser still plops carbs on its engines even today. That's cz they work. They also use EFI. That's cz it works. It's really a toss up.

I have seen numerous post regarding connector related issues causing false/bad signals back to ECM.............Very difficult to troubleshoot and a thorough understanding of the wiring schematic and signal tracing/measuring is mandatory...........If he had a NEW complete wire harness and sensor array then maybe ok, but used and most likely been sitting for a while??????? Can you say OXIDATION/CORROSION?

I would go with the carb.....

If you take the time, you can dial in its calibration and get best efficiency as they rarely come 'dialed in'....

there are also fewer parts so troubleshooting is easier - on the EFI systems, if you don't have a scan tool, you will really never know what's going on.

ECM gets:
Timing pulse from ignition amplifier module inside dizzy
Throttle position (variable voltage from dashpot on butterfly)
Air charge (intake) temp (again variable voltage from heated wire)
Mass air flow (another heated wire variable voltage)
Water (coolant) temperature ( another variable voltage)
Barometric pressure ( what another variable voltage sender?!)
Fuel Temp (yup another)
So, not that many things really and those connectors are all the waterproof type like a 2 pin Bosch or a sealed multi pin.

Meh there's gazillions of boats running around with twenty-odd year old wiring harnesses that are perfectly fine. A little dielectric grease on there and you're pretty good for another twenty. Grounds are the most critical. Those tend to be exposed and often just bolted. They can corrode, come loose, etc.
Jack's statement regarding harness corrosion is pretty much true of ANY harness that's used. You deffo wanna be sure it's all good. Even a carb has a wire on it.
Either way that you decide, there will be things to keep an eye on. Both have the potential to leave you dead in the water.