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4.3 DD Prop

Posted: Wed Apr 19, 2017 10:43 pm
by Sarahj
Anyone have a 4.3 DD setup?? Putting a boat together and wondered if anyone has a favorite prop. The engine is a vortec marine engine with an RV cam and its 262hp. Any help would be appreciated. Thanks! :old_glory:

Re: 4.3 DD Prop

Posted: Thu Apr 20, 2017 7:09 am
by happy harold
the problem with your 262 hp engine is that it makes that hp at 5000 rpm's. if you are going direct drive, cut your hp figure in half since you will be turning half or less rpm's. probably something like a 66X32 i would guess. good little cruising engine. won't burn much fuel. make a good frog boat. good luck. harold

Re: 4.3 DD Prop

Posted: Thu Apr 20, 2017 7:43 am
by Scotty1
:rebel: what no volkswagon belt drive :rebel:

Re: 4.3 DD Prop

Posted: Thu Apr 20, 2017 11:28 am
by joshua357
I am building my first airboat on a 12" glass hull and plan on using the same engine. Ill be interested in what the community has to offer for advice on this particular setup.

Re: 4.3 DD Prop

Posted: Fri Apr 21, 2017 7:53 am
by SWAMPHUNTER45
Are you building this engine from scratch or is the plan to install a take out?

Re: 4.3 DD Prop

Posted: Fri Apr 21, 2017 8:02 am
by joshua357
The motor is from an older Maxim that had to be put out to pasture due to hull issues.

Re: 4.3 DD Prop

Posted: Fri Apr 21, 2017 9:36 am
by Deano
So, is that to imply that it was formerly mated to a Mercruiser stern drive?
If so, it would behoove you to at least swap the cam for something more productive at lower rpms.

Swamphunter's question is beyond passively relevant because you will need everything you can get out of it without leaving anything on the table, so to speak. The closer you can get to balls out @ 3200 rpms, the better your opportunity for success will be.

Re: 4.3 DD Prop

Posted: Fri Apr 21, 2017 11:08 am
by joshua357
Deano wrote:So, is that to imply that it was formerly mated to a Mercruiser stern drive?
If so, it would behoove you to at least swap the cam for something more productive at lower rpms.

Swamphunter's question is beyond passively relevant because you will need everything you can get out of it without leaving anything on the table, so to speak. The closer you can get to balls out @ 3200 rpms, the better your opportunity for success will be.

You are correct. The build is in its infancy. Just picked up the hull and a good bit of rigging last week. Appreciate the advice, I'm sure I'll be back for more. My brother in law done gave me the airboat bug a few weeks ago, only took one ride to convince the wife that it is a must have.


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Re: 4.3 DD Prop

Posted: Fri Apr 21, 2017 3:17 pm
by scottyroll
Also, you may want to check and make sure that the engine is not a counter rotation setup.

I've heard that it is sometimes found to be the case at least in dual engine boats where one engine spins clockwise and the other goes counterclockwise (not familiar with the particular one that was mentioned).

Just a thought, it would suck to put the whole thing together just to end up blowing air in the wrong direction :lol:

Re: 4.3 DD Prop

Posted: Fri Apr 21, 2017 3:52 pm
by SWAMPHUNTER45
Was it a raw water cooled engine or did it have a sealed cooling system?

I ask because if it was a raw water cooled engine used in salt or brackish water the chance of rust and scale would be very concerning. An airboat uses a sealed system configuration where corrosion and scale will restrict the exchange of heat. Just something for you to consider.

Re: 4.3 DD Prop

Posted: Fri Apr 21, 2017 4:54 pm
by joshua357
scottyroll wrote:Also, you may want to check and make sure that the engine is not a counter rotation setup.

I've heard that it is sometimes found to be the case at least in dual engine boats where one engine spins clockwise and the other goes counterclockwise (not familiar with the particular one that was mentioned).

Just a thought, it would suck to put the whole thing together just to end up blowing air in the wrong direction :lol:





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Re: 4.3 DD Prop

Posted: Fri Apr 21, 2017 4:56 pm
by joshua357
Excellent point, the boat it came out of was single engine. Spent my younger years skiing behind it. I will check it out to be safe. Would not look real cool going backwards.


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Re: 4.3 DD Prop

Posted: Fri Apr 21, 2017 4:58 pm
by joshua357
SWAMPHUNTER45 wrote:Was it a raw water cooled engine or did it have a sealed cooling system?

I ask because if it was a raw water cooled engine used in salt or brackish water the chance of rust and scale would be very concerning. An airboat uses a sealed system configuration where corrosion and scale will restrict the exchange of heat. Just something for you to consider.

It was raw water cooled used mainly in fresh water. I plan on going through it with a fine tooth comb before installing it. It would be then I would soup it up some more to get the most I can out of it.


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Re: 4.3 DD Prop

Posted: Fri Apr 21, 2017 7:07 pm
by SWAMPHUNTER45
It will not be an issue if you are going to do a quality rebuild.

When the block and heads go to the machine shop and they hot tank it that scale and corrosion should dissolve away.

If I understand the Mercruiser 4.3 platform it is essentially a SBC with 2 cylinders removed. As such my best direct drive advice to you would be to select a small very light weight boat and look to avoid adding weight at every turn in rigging it. You will not have much reserve power so weight is your enemy more than someone who would use a old Cadillac or 1973 Chevy 400 small block. Those big inch and longer stroke auto engines tend to be better platforms to use for direct drive airboat use because they produce big torque in a lower rpm range.

With your 4.3 engine your cam selection is going to be a very critical decision. You will be running in a very narrow power range 1400 to 3100apx. and every part needs to compliment each other for you to have success. I am not very familiar with the valve configuration but discuss the possibility of un-shrouding them so they will flow immediately after they come off their seat.

When you start your build, set up a thread and post some pics so we can follow your progress. I bet there are several knowledgeable DD folks on here that would be more than willing to offer up some advice or if nothing else discuss your thoughts with on the build.

Re: 4.3 DD Prop

Posted: Fri Apr 21, 2017 7:39 pm
by joshua357
SWAMPHUNTER45 wrote:It will not be an issue if you are going to do a quality rebuild.

When the block and heads go to the machine shop and they hot tank it that scale and corrosion should dissolve away.

If I understand the Mercruiser 4.3 platform it is essentially a SBC with 2 cylinders removed. As such my best direct drive advice to you would be to select a small very light weight boat and look to avoid adding weight at every turn in rigging it. You will not have much reserve power so weight is your enemy more than someone who would use a old Cadillac or 1973 Chevy 400 small block. Those big inch and longer stroke auto engines tend to be better platforms to use for direct drive airboat use because they produce big torque in a lower rpm range.

With your 4.3 engine your cam selection is going to be a very critical decision. You will be running in a very narrow power range 1400 to 3100apx. and every part needs to compliment each other for you to have success. I am not very familiar with the valve configuration but discuss the possibility of un-shrouding them so they will flow immediately after they come off their seat.

When you start your build, set up a thread and post some pics so we can follow your progress. I bet there are several knowledgeable DD folks on here that would be more than willing to offer up some advice or if nothing else discuss your thoughts with on the build.

The 4.3 is like a small block just with the 3rd and 6th cylinder removed. I'm no expert but have my brothers advice and help on this one. He has a big block on his 13' and it's pretty bad ass. I'm attempting to do this on the low end of the budget. You know how it works, you gotta talk the wife in a bit at a time. I appreciate the advice and will be sure to start a thread once I'm underway. Thanks y'all.


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Re: 4.3 DD Prop

Posted: Fri Apr 21, 2017 9:17 pm
by joshua357
joshua357 wrote:
SWAMPHUNTER45 wrote:It will not be an issue if you are going to do a quality rebuild.

When the block and heads go to the machine shop and they hot tank it that scale and corrosion should dissolve away.

If I understand the Mercruiser 4.3 platform it is essentially a SBC with 2 cylinders removed. As such my best direct drive advice to you would be to select a small very light weight boat and look to avoid adding weight at every turn in rigging it. You will not have much reserve power so weight is your enemy more than someone who would use a old Cadillac or 1973 Chevy 400 small block. Those big inch and longer stroke auto engines tend to be better platforms to use for direct drive airboat use because they produce big torque in a lower rpm range.

With your 4.3 engine your cam selection is going to be a very critical decision. You will be running in a very narrow power range 1400 to 3100apx. and every part needs to compliment each other for you to have success. I am not very familiar with the valve configuration but discuss the possibility of un-shrouding them so they will flow immediately after they come off their seat.

When you start your build, set up a thread and post some pics so we can follow your progress. I bet there are several knowledgeable DD folks on here that would be more than willing to offer up some advice or if nothing else discuss your thoughts with on the build.

The 4.3 is like a small block just with the 3rd and 6th cylinder removed. I'm no expert but have my brothers advice and help on this one. He has a big block on his 13' and it's pretty bad ass. I'm attempting to do this on the low end of the budget. You know how it works, you gotta talk the wife in a bit at a time. I appreciate the advice and will be sure to start a thread once I'm underway. Thanks y'all.


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This is what I am starting with. It looks a bit rough but my thing is function before fashion. Gonna be a fun build. Image


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Re: 4.3 DD Prop

Posted: Fri Apr 21, 2017 9:29 pm
by SWAMPHUNTER45
There you go! You got a starting point

Good luck with it and make sure that what ever you do with the build that it is safe, that's what matters most.

Re: 4.3 DD Prop

Posted: Fri Apr 21, 2017 9:32 pm
by joshua357
SWAMPHUNTER45 wrote:There you go! You got a starting point

Good luck with it and make sure that what ever you do with the build that it is safe, that's what matters most.

Will do. Slow and steady. I'll be leaning on y'all for advice along the way.


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Re: 4.3 DD Prop

Posted: Sat Apr 22, 2017 7:27 am
by jimbo_jwc
happy harold wrote:the problem with your 262 hp engine is that it makes that hp at 5000 rpm's. if you are going direct drive, cut your hp figure in half since you will be turning half or less rpm's. probably something like a 66X32 i would guess. good little cruising engine. won't burn much fuel. make a good frog boat. good luck. harold


It appears they made 3 power ratings of 4.3 from 1987-1991 . Mercruiser Marine Engine WOT 4400- 4800 rpm and gear reduced by lower unit less than 2-1 . I didn't know they had RV or Truck cams . I'm not sure how they would hold up with end thrust loading . Is his a 1 man boat ?

175hp 2 bbl
190hp 4 bbl
205HP 4 BBL lx

4.3 DD Prop

Posted: Sat Apr 22, 2017 12:53 pm
by nismo2197
Here is my 12' Taylor hull with a dd 4.3. Does pretty good. Will run some dry. It's my first boat so I'm learning.


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Re: 4.3 DD Prop

Posted: Sat Apr 22, 2017 2:38 pm
by joshua357
nismo2197 wrote:Here is my 12' Taylor hull with a dd 4.3. Does pretty good. Will run some dry. It's my first boat so I'm learning.


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Any mods to the engine?


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Re: 4.3 DD Prop

Posted: Sat Apr 22, 2017 3:03 pm
by nismo2197
No its stock. It's a non vortec. I'm going to replace the carb on it to a better one. But I'm enjoying getting out meeting new people and getting some stick time.


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Re: 4.3 DD Prop

Posted: Sat Apr 22, 2017 3:08 pm
by joshua357
nismo2197 wrote:No its stock. It's a non vortec. I'm going to replace the carb on it to a better one. But I'm enjoying getting out meeting new people and getting some stick time.


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Good to know. My engine came out of an older ski boat. Wanting to do the build on the cheap. My brother got me hooked on blow boats and once you have the itch, well you know. Were in central Florida. Maybe one day we will bump into each other.


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Re: 4.3 DD Prop

Posted: Sat Apr 22, 2017 3:11 pm
by joshua357
joshua357 wrote:
nismo2197 wrote:No its stock. It's a non vortec. I'm going to replace the carb on it to a better one. But I'm enjoying getting out meeting new people and getting some stick time.


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Good to know. My engine came out of an older ski boat. Wanting to do the build on the cheap. My brother got me hooked on blow boats and once you have the itch, well you know. Were in central Florida. Maybe one day we will bump into each other.


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What size and type of prop are you running?


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Re: 4.3 DD Prop

Posted: Sat Apr 22, 2017 4:00 pm
by SWAMPHUNTER45
With a direct drive you want a smaller cfm carb, it looks like who ever set up that 4.3 boat knew about direct drive needs because the tall carb adapter will aid in building torque. It also looks like a small carb up top as well which will give good response at lower rpm.

I would leave it alone!