2bbl to 4bbl conversion on 1990 4.3L

[QBhoy]

I’m not sure the merc 4.3 4 barrel has ever been 205hp. The VP 431, AQ205 and similar is 205. I think generally the merc 4.3 carb is either 175 or 190.
The vortec head makes a difference for sure.
Unless you get the parts cheap as chips, I’m not certain it would be worth it. Might be as well selling your 4.3 and getting a 5.0 or 5.7 maybe ?

 

[achris]

I’m not sure the merc 4.3 4 barrel has ever been 205hp. ....

Until the advent of rating engines as 'propshaft horsepower', all 4.3 4bbl were 205hp.... Then it was 175 propshaft hp. Once they went to vortec head, TBI and MPI it eventually got to 220hp...

All these were 205 crankshaft hp.
MCM 205MR 1984 to 1985
MCM 205Alpha 1 1986 to 1988
MCM 4.3LX Alpha 1 1989 to 1991 (180 propshaft hp)
MCM 4.3LX Alpha 1 Gen II 1992 to 1996

After that, vortec heads added about 15 to 25 hp across the range
With Vortec heads the 2bbl (called '4.3LX Gen +') was outputting 190 propshaft hp... and the 4bbl (4.3LXH Gen +)and TBI was 205 propshaft hp.
The MPI, with vortec heads is making 220propshaft hp...

Chris.........

 

[QBhoy]

Until the advent of rating engines as 'propshaft horsepower', all 4.3 4bbl were 205hp.... Then it was 175 propshaft hp. Once they went to vortec head, TBI and MPI it eventually got to 220hp...

All these were 205 crankshaft hp.
MCM 205MR 1984 to 1985
MCM 205Alpha 1 1986 to 1988
MCM 4.3LX Alpha 1 1989 to 1991 (180 propshaft hp)
MCM 4.3LX Alpha 1 Gen II 1992 to 1996

After that, vortec heads added about 15 to 25 hp across the range
With Vortec heads the 2bbl (called '4.3LX Gen +') was outputting 190 propshaft hp... and the 4bbl (4.3LXH Gen +)and TBI was 205 propshaft hp.
The MPI, with vortec heads is making 220propshaft hp...

Chris.........

Cheers Chris. It must be different over here for some reason. I’ve had a few variants on the GM 4.3. The only ones we see (I’m sure) are;
mercruiser 4.3 2 barrel at 175hp
mercruiser 4.3 4 barrel at 190hp (lx and similar)
mercruiser efi at 210hp
mercruiser mpi at 220hp

VP 431/aq205/4.3Gl at 205hp
VP 4.3 GXI at 225/230hp (depending what you read)

thats all all I know of at least.

 

[Bmaverick]

Something to ponder, a car runs most of its life at 1/4 throttle or less. A boat engine can run wide open at full throttle all day long at 4800 rpm. There are a lot of differences between a car engine and a boat engine. Some of which are heavy duty crank and cam bearings with wider clearances, that's why they run 30w oil. Brass core plugs. Special marine cams that give low end and keep the valve openings from overlapping and keep the water in the marine exhaust from getting sucked back into the engine with the water in it. Heavy duty double valve springs. Marine designed carbs and fuel pumps, to keep fuel from leaking into the bilge area and being a source of explosion. Marine starters, distributors, and alternators to prevent sparks from igniting any fuel vapors that might be in the bilge area.
Mercruiser Vortec engines run considerably stronger than the non vortec ones. One year they made a Gen + 4.3L, that had a 2bbl Mercarb on it. It would walk the dog on any older style 4bbl.

 

[Chris51280]

from the mercruiser manual #7. Now like Scott said, Prop HP or Engine HP. Interesting fact is that the 4.3 GM engines in the car are rated between 140 and 165HP. I think the difference is in the cam setup.

 

[achris]

Cheers Chris. It must be different over here for some reason. I’ve had a few variants on the GM 4.3. The only ones we see (I’m sure) are;
mercruiser 4.3 2 barrel at 175hp
mercruiser 4.3 4 barrel at 190hp (lx and similar)
mercruiser efi at 210hp
mercruiser mpi at 220hp

VP 431/aq205/4.3Gl at 205hp
VP 4.3 GXI at 225/230hp (depending what you read)

thats all all I know of at least.

Over here???

We get the same engines from Mercruiser as you do...

All those powers you listed are propshaft HP, for the vortec engines. Pre vortec, the 4bbl was 205 crankshaft HP....

Volvo are rating their engines at Crankshaft.... Which means the power is the same as the Merc engines.

Chris.......

 

[Scott Danforth]

Something to ponder, a car runs most of its life at 1/4 throttle or less. A boat engine can run wide open at full throttle all day long at 4800 rpm. There are a lot of differences between a car engine and a boat engine. Some of which are heavy duty crank and cam bearings with wider clearances, that's why they run 30w oil. Brass core plugs. Special marine cams that give low end and keep the valve openings from overlapping and keep the water in the marine exhaust from getting sucked back into the engine with the water in it. Heavy duty double valve springs.

you really should look into the truth on the block and heads. other than cam, core plugs and head gaskets the long blocks in boats are the exact same long blocks use in trucks. no heavy duty anything. same block, heads, cast iron crank, bearings, pistons, rods rings and oil pump..... all pulled from the same parts bin in the Tonawanda engine plant. for the 4.3 the cam came from the truck parts bin as well.Only recently did Mercruiser started making their own motors again. Volvo still gets all their stuff from GM

the fuel system and ignition system are SAE J1171 compliant, and therefore are proprietary to Mercruiser or Volvo

some light reading on the 4.3 https://forums.iboats.com/forum/eng...history-of-the-4-3-liter-with-casting-numbers

 

[Rick Stephens]

you really should look into the truth on the block and heads. other than cam, core plugs and head gaskets the long blocks in boats are the exact same long blocks use in trucks. no heavy duty anything. same block, heads, cast iron crank, bearings, pistons, rods rings and oil pump..... all pulled from the same parts bin in the Tonawanda engine plant. for the 4.3 the cam came from the truck parts bin as well.Only recently did Mercruiser started making their own motors again. Volvo still gets all their stuff from GM

the fuel system and ignition system are SAE J1171 compliant, and therefore are proprietary to Mercruiser or Volvo

some light reading on the 4.3 https://forums.iboats.com/forum/eng...history-of-the-4-3-liter-with-casting-numbers

Ditto to that. Marine GM motor is no heavier duty than a factory pickup motor. Same GM line pumping them out.

 

[Walt T]

For me, the entire point of modifying a boat motor is to be able to turn a steeper pitch prop. My 88 Hydroswift Cuddy went from a stock 350 turning B5 aluminum props to a 383 stroker turning steel F6s up here at altitude in Colorado. 1:95 290 Volvo drive, it works well. Gained about 6 mph top end, pulls skiers well. At Powell, lower elevation it's awesome. if you're gonna drop a manifold and 4bbl carb on it well I hope you got the parts cheap so you're only out a bunch of time. That said, It's your boat and I know I'd like to hear the results if you do it.

 

[achris]

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?.

I think my questions are...

What are you hoping to gain from any changes?
How do you normally use the boat?

Chris....

 

[Walt T]

Just because the manufacturer says it doesn't mean your particular motor is gonna do it. achris asks a very good question.

 

[Chris51280]

Boating with the kids. Tubing skiing and cruising. Its a 1990 century 20ft. I thought i could use the intake manifold from the 4bbl monte Carlo. But if that is not the case, then I may pass on the conversion.

 

[achris]

Is the Monte Carlo intake cast iron? If it is, then you can use it. If it's aluminium, it needs brass water passage liners...

With cruising, the 4bbl will make no difference...
Tubing and skiing, it may actually be a disadvantage... Here's how it works...
When we select a propeller for a boat is based on the power at WOT... If you increase the engine maximum power you would probably go up a pitch with the prop... That will have a negative effect on the 'hole shot'. Something you don't want when skiing...

Chris....

 

[dlogvine]

the manifold doesnt care if there is a balance shaft or no balance shaft in the motor.

Actually the intake manifold for the engine without a balanced shaft has a little shield riveted inside. It should be removed for the balanced shaft version, since this cover wont fit.

 

[Chris51280]

Since it doesn't have the balance shaft I should be good. Where are the liners? I would be surprised if everything on tbis engine was made out of cast iron or steel that the intake was made out of aluminum

 

[Chris51280]

I think I know what chris is saying. The water passage has brass casted in. Is this correct

 

[Scott Danforth]

correct. marine intakes are dual alloy castings with bronze (not brass) inserts cast in.

this was developed by Buddy Barr castings about 50 years ago for the marine market (OEM caster for Mercruiser, Volvo, PCM and Edelbrock)