3.7 to 4.3 swap?

[nola mike]

Ok, 2am and I've been at work for 14 hours so far...so idle thoughts. Have finished a significant portion of my floor replacement/gel coat/interior reconfiguration after deciding that I'm going to keep this boat for a while. The grand plan was to freshen up the spare 470 that is sitting in my driveway and replace the (probably) tired model currently in my bilge. Never really considered a 4.3l swap because I knew I'd have to fab some motor mounts. However, after doing all this glassing, that now seems much less intimidating. But I don't know if it's worth it. I'd have to find a junk hull most likely. So is it worth it? I'd be taking a 200# weight hit, so would need significant HP gain to make the swap worth doing.
1) What's the max power from a carb'd 4.3? Post '96 (?) with vortec heads and a 4bbl carb tops out at 190hp, or is it more? 190hp probably isn't worth the time/effort.
2) I'd love a FI model, which looks like 220 hp, but those might be tough to find at a reasonable price. When did they come out?
3) What are the dimensions of the 4.3 with everything attached?
4) My engine is 30 years old, but still has 150+ compression across the board. Should I even be thinking about this now?

I know everyone here except stonyloam will recommend the 4.3 as the "better" engine. I'm comfortable with the 3.7 though, and have all the parts I need for it. Might need a new riser at some point, but those are still available. Heat exchanger would be the other failure point that I don't have a replacement for, but I'm sure I could figure something out if the need arises (though it might be pricey).

 

[Scott Danforth]

the fact that you still have good compression, I would worry about it when the 3.7 craps out.

regarding the power output of a 4.3 vs a 3.7, the 4.3 will have more torque at lower RPM's. its the fat torque curve that matters.... much so more than a max hp value.

stock carbed motor, I believe there is a 205hp version. tweeked and you can hit 220 with a carb.

Me, I would skip the V6 and jump into a V8

 

[stonyloam]

Yeah Mike, you probably know and have fixed everything that commonly goes wrong with a 470, 150 on all 4, you have me pegged, stick with it

 

[Rick Stephens]

While 250 HP is eminently doable with a 4.3, I would agree that the swap up to a V8 would be more satisfying for the labor involved. In the end, the timing is probably more a factor of finding the right donor boat setup. Leave the 470 in place and keep your eyes open for the right donor. Have it all built and ready to swap. Sell the 470 and parts, probably come close to pay for itself. Going with a 4.3L is exceedingly expensive without a donor boat source for the parts as well. Dropping a 305 or a 350 in would be a blast, and long term easier to maintain with more horsies than the 470. Sounds like fun.

 

[nola mike]

the fact that you still have good compression, I would worry about it when the 3.7 craps out.

Yeah, just seems like it's losing a few ponies over the years. And I don't want a catastrophic crap out mid season.

regarding the power output of a 4.3 vs a 3.7, the 4.3 will have more torque at lower RPM's. its the fat torque curve that matters.... much so more than a max hp value.

stock carbed motor, I believe there is a 205hp version. tweeked and you can hit 220 with a carb.

Me, I would skip the V6 and jump into a V8

You sure about that? Never seen the torque curves, but I'd be willing to bet the curves of both are pretty flat and peak early. We're not talking s2000s or rx-8s here. Hadn't even considered a v8... Worried about the length of that, not to mention the weight. My hull is 1800# with the motor, that would probably change the handling considerably. Although if I'mgoing that route, why am I messing with a 305
Like I said, idle thoughts. I only need the power to pull a slalom skiier (repropping every time out is a pita). The 3.7 is still quicker than my buddy's new 21' 4winns with the 260hp efi 305...
As far as easier to maintain, that's just not true. Takes me 30sec to winterize my boat, and there's only 1 of everything (or 4 at most) . Most of the crap that goes wrong (fuel system, old wiring, outdrive, ignition ) is common to all the engines.

 

[JASinIL2006]

Go with the V8. How many times have you ever heard anyone wish they had less power on their boat? For an extra 200 lbs. or so over the 4.3, you get a nice jump in power. Especially if you skip over the 5.0l and install a 5.7/350 Mag. My boat was repowered by the previous owner from a 4.3 to a 350 Mag, and it's pretty nice, especially if you're pulling skiers or a tube.

 

[Rick Stephens]

I like, no love, my 4.3 over the 3.0L. Darned near doubled my HP. And definitely changed my weight and balance on the boat. It sits a bit lower and is a little more sensitive to where people sit.

Advantage, in addition to the HP, the 4.3 is a lot smoother and quieter than the 4 cylinder. But trim is noticeably more sensitive now. In the end, I think the entire swap, except for the new Edelbrock carburetor, was completely covered by selling off the 3.0L, outdrive, and trailer out from under the donor boat.

 

[nola mike]

The v8 would be ludicrous speed. I'd like to know the dimensions, but I think that that would just be overkill, and the balance of the boat would be off...but I'm intrigued. I think I'd run out of room forward.

 

[Scott Danforth]

Your 4 banger is about 1 inch LONGER than a 5.7 liter

 

[nola mike]

Hmmmm

 

[nola mike]

Width? What's the distance between stringers needed?

 

[JASinIL2006]

No modification was done to stringer/motor mount width when my boat was switched to from a 4.3 to a 350 Mag.

 

[1ruralmailman]

was going to say a 350 chevy and a 4.3 are the same motor,one just has two cylinders lopped off.strigers wouldnt need to be moved for width,length may be a different matter

 

[Scott06]

The 4.3 is just a 5.7 missing two cylinders just 4" longer so most v6 to V 8 swaps done require glass work.

Dwg of 5.7 engine with mounting dimensions

https://mercurymarine-gsdesign1.net...4v_installation_drawing_865108r85-and-r88.pdf


4.3 engine dimensions
https://mercurymarine-gsdesign1.net...l_alpha_4v_installation_drawing-865108r89.pdf


If the cowling is wide enough may be an easy swap. Would look for a donor boat or complete used engine. Your boat is so light with a stock 5.0 or 5.7 it would fly. I just did a 4.3 to 5.0 swap really like the torque from the V8 when pulling skiers with several people in the boat. Turned out real nice.

 

[Scott06]

That was most v6 to v8 swaps do not require glass work... Friggen auto correct...

 

[nola mike]

I'm coming from a 4 cylinder.
Looks like I have room in there. Width in the lower part of the bilge is 32". Wondering about the height. In the rear I only have 27". There's a lip there that I could get a bit more if need be. I couldn't tell how much height is needed for the riser/elbow back there.
God help me, I'm actually considering this. Someone talk me out of it. Sitting here in 40 mph winds watching crashing waves is giving me too much time to think about it.

 

[Rick Stephens]

Faster quieter smoother beastly power plant isn't for you. You'd make it go too fast. Better stick with your I-4.

 

[nola mike]

Ha, now you sound like my wife. Why did you go with the 4.3?

 

[Rick Stephens]

Ha, now you sound like my wife. Why did you go with the 4.3?

Now you got me laughin - my wife HATED when I opened up the throttle with the 3.0L, even for 15 seconds or so. She would say stuff like "you just had to do that". For her, a pleasure cruise is 15 knots in perfect weather.

​ WOT speed with the 3.0L was about 32 MPH. That 3.0 was 25 years old and needed a refresh. No biggie, but I wouldn't take it into the backside of nowhere. Just didn't trust it that much.

Yesterday, wife and I took the boat to the far end of Dworshak Reservoir to check out the kokanee salmon spawn. We did the run at 'cruise'- I defined that as 35 MPH, about 3500 RPM, for this trip. While at 32 MPH the 3.0L sounded like it was on a racetrack screamin away and putting out everything it could - at 35 MPH the 4.3L sounds like nothing much at all. You couldn't talk over the 3.0L at anything close to high end. Only time the 4.3L is loud is when you open the secondaries, which I almost never do other than as a wild hair moment. Like a different boat. Have to slow down when jumping over another boat's wake with a little lightweight 17 footer, but other than that, the fact is that 40 MPH seems slower than 25 did with the old 3.0L screaming.

Why did I do the swap..... mostly accident I guess. I found a donor boat with the floor torn out. I bought it with the intention of buying, on the same trip, an aluminum boat that had hit something and ripped the outdrive off of it. Idea was to build the two into one, sell it all off and make a few dollars. Turned out, after I bought the donor boat, and only then went and looked at the torn up outdrive boat, it was totally thrashed. I reached in and pulled out handfulls of the transom. Nothing worse than old carpeting left out in the weather, and that boat was a Spectrum, mostly held together with carpeting.

The donor boat just sat there for a year with a new rebuilt 4.3 in it, calling to me..... Besides, it was FUN doing the remodel.

Rick

 

[stonyloam]

Say it aint so Mike LOL! One thing to remember is that a 4.3 will use the same outdrive as the 470, but you would need a gear ratio change for the V8. If you gotta do it sigh.... save your money and go with a fuel injected 4.3. Good luck.