Repower options for 1990 4.3

I have a small 1990 monterey ssc2700 with twin 4.3 liter (non balanced) alpha one I/O's and I need to repower the boat.
I was wondering if it would be better to replace with the original 4.3 type engine or swap over to a 5.0 engine?
I have heard that the are the same bolt pattern and operating range as the 4.3.

The other thing I was looking for information on is, I thought when you have twins one engine is counter rotating, but both of these engines are standard rotation. Is this normal?

Looking forward to seeing what the group thinks.

Re: Repower options for 1990 4.3

Not sure what you mean by non balanced? The counter rotation is in the drive itself not the engine. All or most of I/O engines are left hand rotation, when it gets to the drive then it spins the other way to counter the torque of the other propeller. Are you replacing the drives as well or just the engines? If you can, I would upgrade to the 5.0 (4.3 seems like it would be under powered for that boat) or even the 5.7 if you are replacing the entire power package. Mercruiser has a remanufactured set of engines called the plus series. here is some info on them and a video on them.

new re power:

MerCruiser New Repower Products | Mercury Marine

Plus Series repower:

Remanufactured MerCruiser Products | Mercury Marine

Plus Series video:

Mercury Remanufacturing - MerCruiser - YouTube

The plus series is basically Mercury taking a used block, making sure that it meets factory specs and then filling it with mostly new innards. All the stuff on the outside of the block is brand new. (fuel injection system, all pumps, pulleys, starter, alternator, all wiring, sensors and ECM etc. The best part is they come without those new catalytic converters. Some of the plus series motors are actually completely brand new inside and out. They just were on display at a trade show so can't be sold as new or were sent to trade schools to be disassembled and reassembled. Either way the motors are brought up to factory spec by Mercruiser and then "hot tested" (actually run with water and gas) to make sure they pass all factory specs. I just bought 2 complete 300 mpi 5.7L power packages to re-power my boat. Watch the video, impressive stuff.:happy:

Re: Repower options for 1990 4.3

Some twin engine boats do have a counter rotating engine. If you upgrade to a V-8 you will need to replace the outdrive too, to get the proper gear ratio.

Tazrig: Later 4.3s have a balance shaft under the intake, driven off the cam. Makes for a smoother engine as V-6s usually have a slight lope.

Re: Repower options for 1990 4.3

cr2k said:

Later 4.3s have a balance shaft under the intake, driven off the cam. Makes for a smoother engine as V-6s usually have a slight lope.

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cr2k, Just so I'm clear. Does the balance shaft on the 4.3 act like the harmonic balancer on the v8's?

Re: Repower options for 1990 4.3

In a way yes. It offsets the pistons mass.

Re: Repower options for 1990 4.3

cr2k said:

In a way yes. It offsets the pistons mass.

Click to expand...

OK makes sense, thanks.