Wondering required parts to swap mercruiser 4.3 v6 to v8

I recently bought a large 27ft boat that has two mercruiser 4.3 v6's, and since I'm crazy I would like to swap them for v8's. I know it's not a simple task, but any help or a parts list would be greatly appreciated. Which v8 doesn't really matter to me, if it's simpler to swap it for an LS or something I'll do that, if a mercruiser swap would be easier, that's fine as well.

 

You'll need to stay in the Mercruiser family of engines for outdrive compatibility. Those look like Alpha drives, so 5.0L or 5.7L would be the engine sizes to consider. Anything bigger will be too much for the Alphas.

Your best bet would be to get a pair of complete 'bobtail' engines with bell housing and coupler. Nothing of significance would be able to 'swap' over to the V8's. Decent used engines in that size range would run about $5,ooo each.

FWIW - I do have a pair of Mercruiser 6.2L 350 HP engines & outdrives that I am selling . . . Check FB Marketplace for details (Massachusetts).

 

If you have enough space forward of the engine, it is very easy. It will attach to the same transom shield. The gear ration of the drive will have to be changed to 1: 1.50. The last generation 5.7 was rated at 315 HP

 

Makes sense, makes sense. Now if I wanted to supercharge it as well, what modifications would I need to make outside of the motor itself? I'm a car guy not a boat guy, so I don't know a lot about impellers and all that

 

How fast do you want to go? A cabin cruiser will start getting unstable at higher speeds because of the superstructure windage. A moderate increase of power would be OK. However, double the HP will give you maybe 30-40% more speed.

 

I want to go too fast

 

The bigger question may be how much money do you have to re-power this boat?

BTW - how is the boat's structure? Any signs of rot in the engine bay?

 

OK. In that case, with a supercharger and a conservative 500HP on each engine, you will need different drives. A Bravo 1 will handle 600 HP. However, you will have to modify the structure of the boat too. It is not likely that it would be able to use that much power when new.

 

Structurally sound, and I have enough as it will take for it, I suppose I will do the procharger kit for the 5.7 mercruisers, but what supporting modifications will I need to make to the rest of the propulsion system excluding the engine?

 

Ahh, yeah that makes sense. Would you recommend reinforcing the engine area, or does the entire hull need metal reinforcements

 

Boats that age usually have at least some rot in the transom and stringers. You should check those first. Also, the lower part of the bulkheads.

 

More importantly . . . There are some SAE standards that apply to boats with inboard/sterndrive engines. SAE J1171 will be the most common one you will see. You should Google the others. Additionally, the US Coast Guard (USCG) has some regulations about fuel systems, and other aspects about boat construction. It is always nice to be familiar with those.

 

Haha well I've never been one to follow laws anyways, it'll be alright, but thanks

 

Another smart guy that didn't follow laws, the other day at the ramp. He got blown off the boat.

 

Oof