4.3 Merc to 5.7 Merc swap

[stevekw_3]

I have a 91 Rinker with a 4.3 Merc. I am considering swapping to a 5.7 after this season. Ihave the Thunderbolt IV ignition setup. I am wondering if this system will work on the 5.7, with different distributor of course, so that I can use my existing wiring harness. Also, may switch to thru hull exhaust as well. Are there any modifications that need to be done or any real cons for doing this? Thanks for any input

 

[achris]

You would need to change the ignition module over to a V8 one. The one on the 4.3 is only for a V6... Through hull exhaust... There has been plenty written about the pros and cons, mostly cons... Claims of more power available (and that would be the only reason to go 'though hull') are wildly exaggerated. If you get 2 or 3 hp more I would be surprised. And that will not relate to any improvement in speed or acceleration. Most people who put them in are after the 'noise' factor. And most of them report that the noise is great, for about 5 minutes, then it just causes a headache, literally...

 

[stevekw_3]

Ha ha, yea I admit it was a "noise thing" the main reason for wanting that. I'm sure my wife would rather it not be that loud. So do you know if the v8 module has the same plug as my v6 module? I just would love to use my wiring harness because it has the one big plug and would make the switch easy along those lines. I have considered just upgrading to a newer Vortec 4.3 and maybe 4 barrel carb, would it be safe to say I'd be in the 225 horse range that way?

 

[achris]

Yes the plug is the same, but you can't buy the older TB IV modules any longer. You have to update to the TB V module, and that includes a plug in harness with the new module. As for the noise thing, most report it gets old very quickly, and it's not cheap to do, and you've gotta knock a couple of thunking great holes (4" diameter) through the transom... Not easy to cover up when you change back to the quiet option (because the local LEO booked you and you can't use the boat again until you go back to through prop)........