Mercruiser 4.3L question

Hello everyone, I am new here so if I ask a stupid question, please forgive me.

So long story short, I had my boat (93 Webbcraft w/Alpha 1) that I purchased last year serviced earlier this year and the mechanic said my 4.3L has a lot of blow-by and will need rebuilt/replaced. So I thought what I would do instead is run it this year and just build my own Vortec 4.3L to update it.

Now I know that I can't just slap a 4.3 together with automotive stuff and drop it in, even if it will never see salt. I guess what I am wondering is specifically, which parts have to be marine? My understanding is the bare blocks would be the same. I know that the carburetor, exhaust manifolds, and gaskets need to be marine specific along with the electric components involved. When it comes to things like camshafts, lifters, valves and valve springs, are those all marine specific or can automotive components be used?

I ask all this because I recently read an article where Hot Rod magazine build a 300hp 4.3L and thought "hey, that would be fun". I know that is right on the threshold of what the Alpha 1 can handle, so I thought I might try to stop before the 300 mark but well above the 205 I currently have.

In my mind it would be more expense efficient to build my own instead of buying a crate motor, but my ego is not so big to believe that I am absolutely right and that is the end of it. That's why I'm asking you all.

I would genuinely appreciate any advice you guys would be willing to share.

Thank you.

As far as the engine only, no additiinal add ons. BLOCK ONLY.

The only differences are the head gaskets (marine) and core plugs (brass).

Everything else is the same.
So builing a higher horse power 4.3 is doable.

You would need a marine specific carb or marine rated efi/tbi and ignition.

kghost said:

As far as the engine only, no additiinal add ons. BLOCK ONLY.

The only differences are the head gaskets (marine) and core plugs (brass).

Everything else is the same.
So builing a higher horse power 4.3 is doable.

You would need a marine specific carb or marine rated efi/tbi and ignition.

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Thank you kghost. That makes me feel a lot better about undertaking a build.

If you do this, you most likely will need to change prop pitch.
Also, A hot rod build may have a rpm redline in excess of 5500rpm.
99% of standard marine builds max out at 5000 +/- rpm.
So cam shaft for high torque at low rpm is a key with marine.
So if you can get one to support 300 hp below 5500 rpm you will have a good set up.

Also there are Heavy duty gear sets available for outdrives which are often used on higher hp and heavy boats...
May add $1000+ to your build but would be worth it in the end!

kghost said:

If you do this, you most likely will need to change prop pitch.
Also, A hot rod build may have a rpm redline in excess of 5500rpm.
99% of standard marine builds max out at 5000 +/- rpm.
So cam shaft for high torque at low rpm is a key with marine.
So if you can get one to support 300 hp below 5500 rpm you will have a good set up.

Also there are Heavy duty gear sets available for outdrives which are often used on higher hp and heavy boats...
May add $1000+ to your build but would be worth it in the end!

Click to expand...

This particular build they did had max torque of 316.6 @ 4,400RPM and max HP of 301.2 @ 5,500 RPM. So it would be right in that ball park. My 4.3 right now with a 19 pitch maxes out at 43mph at 5,200 rpm.

With all this said though, since you mentioned heavy duty gear sets (didn't know those existed) would it just be easier to build a SBC 350 and do the gears? Realisticall speaking, a 300hp 4.3L will be working a lot harder than a 300hp 5.7L, and probably be more expensive to build/rebuild with shorter rebuild intervals because its essentially built to its limit vs a V8 with a few minor changes. My only real thought about doing the 4.3L was that its a direct drop in, fewer cylinders = less fuel used, and I didn't know about the heavy duty drive gears. But now since I have been looking, finding a bare 4.3L block is almost impossible. They have long block assemblies available, that are marine, but from a financial standpoint it makes little sense to me to buy a motor where the lower is already built and then take it apart to the bare block to rebuild with different parts. I mean if I was looking to spend that kind of money I wouldn't own a 93 Webbcraft, The boat nobody outside of Oklahoma has heard of.

Yes, and you can buy a remaufactured 320 hp 383 stroker..
do heavy duty gears for the appropriate ratio and now you have a 65-70+ mph boat.
Of course anything smaller than a 18-19 ft boat wont work.
Smaller boats dont do well at high speeds

kghost said:

Yes, and you can buy a remaufactured 320 hp 383 stroker..
do heavy duty gears for the appropriate ratio and now you have a 65-70+ mph boat.
Of course anything smaller than a 18-19 ft boat wont work.
Smaller boats dont do well at high speeds

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Wonder how long a tank of gas would last...

I think the 305/350 drive is 1.48 and the 4.3 is 1.62 and the 3.0 is 1.84 maybe. I'd have to double check. In such a case, how does one determine the correct prop pitch and diameter? I would assume (probably incorrectly) that the extra torque of a V8 would allow for a 4 blade instead of my current 3 which would help with getting on plane quicker...right?

With the correct ratio, say 1.5:1
A 19 pitch 3 blade is ideal. For a max rpm of spec say 4800

The way to determine is, wide open throttle, half tank of gas, you, outdrive trimmed all the way up for max speed, now your rpm should be to spec.
Say 4800 rpm.
If lower then decrease pitch, if high say 5400 increase pitch.

Once that is determined you can change to a multi blade that would equate the same rpm.

Man thank you so much for your help. I've learned a lot today.

It's surprising how FEW SB Chevy V-8 parts can be used in a 4.3 V-6. (Pistons, heads, rods, cam and other parts are all different.) I was going to build one for my 1986 1/2 Chrysler Conquest.

Jeff

fastjeff said:

It's surprising how FEW SB Chevy V-8 parts can be used in a 4.3 V-6. (Pistons, heads, rods, cam and other parts are all different.) I was going to build one for my 1986 1/2 Chrysler Conquest.

Jeff

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I agree. In reading that hot rod article they talked about how cylinder bore was the same between the 4.3 and the 5.7 and that even the lifters were heavy duty 5.7 units. In talking with my dad though he thinks he may have access to a free 305 block that he said he would be happy to give me so I may be switching to a 305 build.

Not worth the swap.
305 is not that much more than a 4.3 LX which is ~205-210 hp.

305 is 230 at best.
A lot of money for correct parts including ratio change in outdrive for 20-25 hp.

Better off modifying the 4.3