MPI vs Carb

212rowboat

So... I know some of you fella's have used both on the water.  Care to share appreciable differences?  I'm most interested in economy... theoretically the MPI is superior, and certainly so on the street... but.... how much improvement in economy does one enjoy on the water with MPI if any? 

thanks

btw.. I blew my engine into shrapnel.  oops. 

212rowboat

Will a new catalyst engine plug and play using standard 10 pin and linkage? 

69fastback

I had a crownline 240LS with a 5.7 carb engine, and now my 232 is MPI. I've never done any real scientific research into either boats real fuel economy, but just by a rough estimation on how much gas it uses in the water, the difference "seems" negligible. BUT, the extra 50 hp is very noticeable. So it seems similar fuel consumption, but very different performance.   

212rowboat

Roger... sweet. 

Another question... is the fwc full system worth It? I'm in the salt. 

Black_Diamond

Dang. Blew up your new engine...get too happy with it?  

rasbury

My boat is fwc and I'm in fresh....but it was a salt water boat when I bought it, lift kept with 100 hours. It was on my list of must have-it only made sense to me to have a block that never saw water if that was an option. Perhaps Al can chime in on the value of it- perhaps I put more stock in it than I should have. Obviously you still have raw water for your heads so it's not like the motor does not see some raw water.....will be curious to check back and see what others say.

212rowboat

yeah Ras

I wouldn't mess with a half kit- but a full kit is another tale- it pulls raw to the water:water exchange, and then through the risers to be injected into the exhaust stream... so, basically a person would still be replacing risers every 3~5 years, or maybe every 8~10 years if they went with something like the ceramic coated ones, and they'd be replacing the heat exchange every 3~5..... but the engine or manifolds would never see anything but fresh/coolant water.  

BD-

I don't know whether i should take ownership of the issue or not- but... i windowed the block under the starboard head with what has to be a rod- the crankcase is full of debris, too.  the block was a 305 bored to 314, which is as far as you can 'safely' take a 305- and the rotating assembly was suspect to me in the first place... i kept going all the same but i had a nagging feeling our union would be a short one- and it was.  

I'll pull the engine and drop a new one in, this time doing it right with an engine nowhere near the edge like that last one was.  I wager that engine was over 300hp at the crank, and it was enough to push that hull 61mph w/ a 19p pleasure five prop.  though carb'd, it was a full roller valve train and i could easily hit 5500RPM's.... i went above that a few times, actually.  

the exhaust manifolds looked great from the outside and from the port sides, but separating the riser you could see substantial corrosion- and if I were to guess (which I am guessing) it was water intrusion into cylinder via exhaust manifold, and enough that cylinder pressure caused something to give- that something was a connecting rod. 

i've got a lead on a short block a guy started to build and then stopped- he's always building something... I'll transfer all the parts to it from the blown engine that should be transferred (which is to say everything but the exhaust/risers), and attempt to recoup some losses- if I could get $1500 for that engine, and another $1500 for the running 4.3LX, I'd be just under halfway to a new mercruiser 5.7MPI, which is what I'm going to drop I'm pretty sure... catalyst engine or not; full fwc or not- those are teh questions now.  the boat itself is solid as a rock and perfect for what i use it for- i just can't see getting rid of a 'paid for' platform because of shenanigans with engines which is mostly self induced... 

rasbury

Do they actually do a "full" closed cooling? I have a heat exchanger that cools from raw water and yes goes through the risers....I thought that was as complete a FWC system you could have?

SeaHare

Mine are carbed 5.7's.. While i have never owned an MPI boat, i sure do appreciate how easy it is to work on a carbed engine....just a simple dinosaur

212rowboat

From what I get, the half system cools the exhaust manifold with raw, where the full kit is just the risers. 

J3ff

that sounds right 212, I believe that's what mercruiser shipped me..

212rowboat

i went and checked, and that's exactly the difference- half kit doesn't cool exhaust anything except with raw water, while the full kit cools manifolds in the closed circuit and the risers with raw.... risers are pretty cheap- may be worth it.  

Seahare:  it took me a while to accept MPI on terra, but man o man there is no comparison once they worked the bugs out of the system... an engine can run almost perfect stoichiometric ratios in any weather, temperature, altitude or condition of fuel or air supply... it's astounding to watch how fast sensor data is processed and how often the lambda crosses zero.  that said.......... in this application and lacking o2 sensors (which means the engine is running purely off table and little environmental information- UNLESS it's a catalyst engine that DOES have o2 sensors) I question just how much difference it makes.

I won't argue with experience, though- which is why i asked.  

212rowboat

here's the deal- 

I can land a brand spanking new Mercruiser (reconditioned block) 5.7 350mag 4v + full FWC for right at the same price as a 300hp 5.7MPI, also a mercruiser and also reconditioned... 

or... the same MPI engine turn key with a full FWC system for the same price as a 5.7 300hp catalyst engine.  

see how easy it is to reason away a few thousand dollars?  

put it out there like this: 

$7400 4v 275hp;
$8200 5.7MPI complete ~or~ $8200 above engine + full FWC system
$9100 5.7MPI catalyst engine ~or~ engine above + full FWC system

can y'all see how a man parts with $2k and a smile?  it's too easy. 

@Alswagg, 

Can a catalyst engine be had with a FULL FWC system?  those manifolds integrated with the risers seems to indicate only a half system... is this true? 

rasbury

So how do I tell what I have then? I thought it was manifold and risers, did not know there was something in-between?

212rowboat

in between the riser and manifold there will be a stainless steel blocker plate that looks a lot like a gasket but doesn't have the blanks allowing water to leave the manifold and run into the riser. 

how you'll know is if there is a hose into both the riser AND the manifold.  if it's just through the manifold, it's a half system and raw water is running through the manifold into the riser.  if there are two- one running into the manifold is on the closed circuit, the other running to the riser is on the raw circuit. 

randy56

Having  many carb boats, when shopping for this one, a fuel ejected MPI was a must on my shopping list. Rebuilt to many carbs, in the past, tired of it, Not caring what so ever about economy. However roomer has it Mpi is more efficient.

skennelly

What @randy56 said ^

212rowboat

i've had my hands on both.  i prefer MPI, and even TBI over a carb... but. 

MPI, when it isn't reconciled trims fuel no better than a properly tuned carb.... 

MPI NOT monitored uses hard tables scripted on the computer, which monitors MAP or MAF, temperature, intake temperature... a Carb, despite it's humble status, does the same.  it relies on vacuum which is also a type of MAP and MAF- it relies on temperature evidenced by the choke operated by coiled element... a properly tuned carb is just as good as MPI.... but- that carb is hellaciously complicated and can't adjust for wild ambient temperature swings or altitude, and it has a hard time metering on a cold engine.  a carb as less problems firing on a warm engine... a reconciled MPI has no problem in either case. 

this is why my ears perked up @ a $8200 5.7/300hp catalysts engine.. it IS monitored- by o2 sensors sniffing lambda.  that isn't a small thing; it's a pretty important advantage... there is zero chance a carb engine can compete against a MPI reconciled engine.    

^all that is crap against experience...  experience on/in boats is what I lack, and why I asked... and it looks like it's akin to a oil conversation...