I was wonder if it is possible to swap motors in a 2008 Tahoe Q7i boat. I currently have a Mercruiser 4.3L and I would like to replace it with a Mercruiser 5.0L. Will the Tahoe boat be able to handle this size motor?
I did a similar swap back in 2015-16 on a 2004 sea ray 200 sport (20-21 ft) that turned out very well.
I bought it with a known cracked 4.3/190 hp carbed engine. The boat was also offered with 4.3 MPI 220 hp, 5.0 carb /220 hp, 5.0 mpi 260 hp options. I went back in with a 5.0 figuring if it needed a 5.7 it would have been offered with one... if I did it again I would just do the 5.7 as it costs the same or actually less. They stopped making 5.0s longer ago so they can be harder and more expensive to come by.
the key thing to look at is space in front of the 4.3. The 4.3 is a 5.7 missing two middle cylinders, so is identical block front and back but the V8s are 4-4.5" longer. So you need 4-5 " in front of the engine clear.
Sea ray built the hull and engine compartments to handle the V6 and V8, there was space in front, and the engine mount stringers on the sides of the engine were about a foot long so the 4.3 mounts sat on the rear of these, and the V8 mounts sit towards the front. If you don't have this you will have to glass in or fab metal to make it work. if you have the space in front and mounts it is basically plug and play.
I made my own bobtail by getting Michigan Motorz partial engine (silver package). Its a new GM base engine with tinware plus a distributor/ignition, intake/carb, and fuel pump /hoses. I bought new exhaust. You actually can use your risers /elbows on the V8 manifolds, just will need the downturn elbow after the riser elbow off a V8 as the V6 is different. The distributor is also the same if you swap cap and rotor and get a V8 module.
So I spent a bunch of time combing through parts diagrams of the 4.3 and 5.0s. I used all the accessories off the 4.3 - PS pump/alternator, wiring harness, flywheel cover / coupler , starter etc. Only snag is if you have a 090M (metric) casting 4.3 you will need to swap out the fasteners for the flywheel cover, starter, and side motor mounts for SAE. These 10 or so bolts are metric, everything else is SAE. Also needed a few cooling hoses (thermostat to one manifold and hose up from drive to thermo) but a lot of them fit. Also i had MM take off the fuel pump and used the factory merc one-tweaked the fuel line length to fit, and had to use the throttle cable bracket off the 357alpha crate engine as it fit my edlebrock 1409.
Bottom line I saved $1500-2 k vs buying a complete bobtail and this was at 2015 prices- whole engine with exhaust and a few other spare parts was $5k delivered. Savings today would be more as long as you are not paying a mechanic to spend a couple hours moving stuff over. If you are paying someone to do this just get a bobtail as it will be the same cost total.
Other thing to negotiate is the drive ratio. My boat came with a 1.81 ratio drive with the 190 hp 4.3, allother engine options use the 1.62. I kept the 1.81 drive as the boat only had 37 hrs. I had to go up 2" in pitch to get the WOT rpm correct. It seems like the drive ratios work with the 2" prop pitch steps to keep most applications within the typical/basic 17"-21" prop range.
Again if you do it just go with the 5.7 as the alpha drives are factory offered with the 5.7 Mag/300 hp. While my set up works great, when I have 8 adults from time to time a few more horses wouldn't hurt
