2bbl to 4bbl conversion on 1990 4.3L

[Chris51280]

I looked at the mercruiser manual and it says I can use the Rochester MCM185 or MCM205 to get 205 HP out of my current 175HP model, Is this correct? Are these two the same 4bbl carburetors? The manufacturer number is 3300-8886 or 3304-9354
What is the difference?

Is there anything else that I need to consider for hooking up the 4bbl instead the 2bbl carb? Throttle linkage, choke? I know how the 2bbl works with the electric choke and the throttle linkage. Just can't make sense of the 4bbl looking at pictures.

I know I need a different manifold for the 4bbl. I can get that one pretty cheap from a 1987 Monte Carlo.

 

[alldodge]

The 4 bbl uses a different bracket (805359T) to hold the throttle cable, other then that its just intake, carb and fuel line

 

[Chris51280]

thanks

 

[Scott Danforth]

if you want more power, get a set of vortec heads (prior to buying an intake)

 

[Chris51280]

with current older heads and 4bbl carb I get 205 HP. How much more is it with the vortec heads and 4bbl carb setup? I believe the vortec heads have the 8 bolt manifold. I'm also not sure what manifold would work since I have a non balanced motor (pre-1992). GM liked to write vortec on the engine even though they were not in the early days. So the later years have a different bolt pattern and different intake size.

I'm not sure if this is still worth it and a lot of changeover parts required.

I need to rebuild my current 4.3 block since I found a welded area. (bought the boat in March this year and was covered by the seat boxes). I found an engine from a 1987 monte carlo that will be the donor. 130k miles on it.

 

[alldodge]

Others may know more, but according to manuals (7 and 18) from 1983 thru 1997 the HP list are the same,175 2bbl and 205 4bbl. Manual 25 list HP as 190 and 205 (2bbl and 4bbl), and also a EFI at 210.

My thought is the 2bbl version makes a difference with vortec heads, but no so much with a 4bbl

 

[Chris51280]

Its all a little confusing since the HP ratings are different throughout the years

 

[alldodge]

Another thing that I'm unsure on is the cam used in the Monte Carlo, the marine motor uses a truck cam for low end torque, but a car motor usually does not

 

[achris]

Difficult to compare horsepower as some are crankshaft and some are propshaft.

Why not replace the non balance shaft engine with a later model, complete with vortec heads?

Chris....

 

[Chris51280]

I asked in a previous post about using a balance shaft motor build after 1992 and swap over parts from my current 1990 motor. Came to the conclusion after Scott told me:

"your short block assembly is the Block, crank, rods, pistons, cam, balancer, timing assembly, flywheel.
that all must remain together. you cant mix and match parts.
and because the timing cover changed and the balancer changed from GM, Mercruiser had to change the accessory drive when they went to a balance shaft motor.
only the heads work on either block until 1995
either stick with all components from a non balance shaft motor which means get a different shortblock or rebuild the one in the boat now
or switch everything over to the mercruiser parts for a balance shaft motor which means getting all the stuff to bolt on the balance shaft short block to make it a mercruiser - accessory drive pulleys, brackets, belts, manual transmission flywheel for a balance shaft 4.3, cam would be close enough to use."

So isn't easier to use the 1987 block with everything?
Just what I can think off like dipstick, front cover, intake manifold, flywheel, damper, starter, distributor etc will all work. I do not know what else was changed for the balance shaft motors. Not sure if the Monte Carlo is a manual or auto setup.

get a rebuild kit for about 200-300, not sure about pistons or just rings is enough.

Maybe I understood Scott wrong

 

[Chris51280]

It wouldn't make much difference in HP I believe. I have the fluid mounts and vibrations aren't really that noticeable. and when I go with the 4bbl manifold and the 4bbl carb, I have 205HP from the current 175HP. I don't think the balance shaft motor is that much better in power. If at all, the balance shaft draws HP according to some hotroders. They tend to disconnect the balance shaft

 

[Walt T]

Adding a 4bbl to your already anemic V6 will do nothing as far as HP increase. Simple math determines what the CFM your motor can actually process at a given rpm.

 

[Chris51280]

are you saying mercruiser lied with the 205hp rating vs 175hp. 2bb vs 4bbl

 

[Scott Danforth]

with current older heads and 4bbl carb I get 205 HP. How much more is it with the vortec heads and 4bbl carb setup? I believe the vortec heads have the 8 bolt manifold. I'm also not sure what manifold would work since I have a non balanced motor (pre-1992). .....

. I found an engine from a 1987 monte carlo that will be the donor. 130k miles on it.

the manifold doesnt care if there is a balance shaft or no balance shaft in the motor. neither do the heads
the heads are good for 20hp

the monte carlo motor will need to be stripped down and the cam replaced. too much over-lap on the cam for a marine use.

as far as ratings go, the jump from 175 conservative to the optimistic 205 may be a stretch for just a 4-barrel. however since you have to pull your donor motor apart anyway to install marine head gasket, you really need to look at the cylinder bores. 130k on the motor may have been a rough 130k

 

[Scott Danforth]

Difficult to compare horsepower as some are crankshaft and some are propshaft.

Why not replace the non balance shaft engine with a later model, complete with vortec heads?

Chris....

Chris, the OP didnt want to invest in all the bolt-on bits required when you jump motor generations. I suggested the same thing in his other post.

 

[Rick Stephens]

I figure I’m somewhere around 230 hp with my ‘90 block, Vortec heads, Performa intake and 1409 4 barrel. It wastes around 15 hp to turn a balance shaft. And I am running a pickup cam.

 

[Walt T]

The HP increase is at the upper RPM range or WOT where the extra cfm is actually being used. For the vast amount of operations such as skiing, cruising, you will never actually use the secondaries. I personally would continue to enjoy the boat until I can buy one that is faster or will do what I want. Adding a 4bbl will not get you extra grunt out of the hole or give you midrange power boost. As some of the guys have suggested, making major modifications to the engine so it can take advantage of the extra cfm may be a way to go. However you will most likely be disappointed. It still will be a V6. I have a 725 cfm Holley on my 383 stroker and it needs 471 cfm at 5000m rpm, therefore my carburetor is about right. Adding an 850 double pumper would do nothing except over fuel it and I would get no extra power unless I ran it over 7000 rpm which aint gonna happen. Thats why I say adding a 4bbl wont help. It's gonna take a lot of HP at 5000 rpm and a prop to match to get 5
mph out of your top end.

 

[thumpar]

I agree with some of the others. Just doing the carb swap will probably not do much for you other than a couple more MPH and being able to use a little more fuel if you want to. If you want to do something worthwhile wait until you are at a point you want to update the motor and go full vortec with the 4 bbl OR maybe by then you will be ready to move to a different boat and can choose the motor you really want then. I really think you will be disappointed in just the carb/intake swap after spending that money.

 

[Chris51280]

Since i will be getting an intake manifold with the monte Carlo, i only need the carb abd I believe i can get one for about 100. A rebuild kit another 25. Im not sure if that is then a good deal or not. Not pkanning on spending 500-1000

 

[Scott Danforth]

I wouldnt run an auto manifold on a boat unless you plan on adding a heat exchanger. there is a reason the marine manifolds have bronze inserts for the water passages.

no way you can get a marine carb for $100 unless your extremely lucky