Should I upgrade to a four barrel on a 4.3L?

[gixxerjim]

My 1992 Century Mustang II runs great. The engine was rebuilt in 2007, with flat top pistons, roller lifters but stayed with a 2barrel. The outdrive was replaced last year. I would like a little more top speed as I am only getting 40mph at 4600rpm with a MW 17P 3blade aluminum prop with a moderate to heavy (normal) load.

Whattya think would a 4barrel wake it up or wait and try the 19P prop first? My theory is it may be running out of breath at 4600. Justa theory!

 

[90stingray]

Re: Should I upgrade to a four barrel on a 4.3L?

40 sounds about right loaded down... but some boats it is what it is. My old stingray with 4.3 ran 40 mph with a 17p prop loaded or unloaded. My glastron likes a 23p and runs at 50 w/ 4.3. I have messed with some props and i like my 21p 4 blade. Anywho.... Sometimes when you are lower in the rpm range and prop up to a higher pitched prop... you will lose mph. Some say the 4bbl will just suck more gas w/o gaining anything. Some have good luck when doing edelbrock alum intake, vortec heads, and the 4bbl. I wouldn't spend the cash on a 4bbl... but find a prop that gets you to the 4800... and you might find that you will pick up some mph.

 

[achris]

Re: Should I upgrade to a four barrel on a 4.3L?

4600rpm is exactly right for that engine. The only thing putting a 19" prop on will do is drop the WOT engine speed to around 4200... Outside the recommended range, and that can lead to detonated pistons and all kinds of un-nice things.

A 4bbl carb will up the power by about 20hp (with the right manifold). That will give you about 3-4mph quicker on the top end, which may necessitate a change to a 19" prop, then you will lose low-end 'hole-shot'... It's all a big trade off.

The question then becomes, is all the expense and effort worth 3-4mph? (I know it will only give you that much because I changed from a 180hp 4.3LX to a 220hp 4.3MPI and only gained 5 mph. But speed was not the reason for the change).

Chris.....

 

[NHGuy]

Re: Should I upgrade to a four barrel on a 4.3L?

40 MPH seems too slow. I assume your motor is a V6. If it's a 5.0 or a 5.7 that's way too slow for a 19 foot, 2300 lb boat.
Flat top pistons & roller lifters, hmmm. It's my understanding that you can rev roller motors up higher.
So yes, maybe you would benefit with a marine 4 barrel. Use the 17" pitch prop to start with. You will need an intake manifold and a flame arrestor to go with the carburetor.
Then let us know what you get for WOT rpms and speed. I'd spin the motor past 5000 with roller stuff.
What ratio is in that replacement drive?

 

[achris]

Re: Should I upgrade to a four barrel on a 4.3L?

... I'd spin the motor past 5000 with roller stuff.

That's all well and good... The roller stuff will spin much higher BUT the camshaft is designed and ground to make maximum power at 4600, hence the recommended rev range of 4400 to 4800... Yes you can spin it higher, but you'll be producing LESS power, not more.

My calculator says all the numbers are right for that engine with a 1.84:1 drive (ratio Merc were supplying with those engine back then).

Chris.......

 

[gixxerjim]

Re: Should I upgrade to a four barrel on a 4.3L?

Thanks guys I feel better about what she's doing. I will try the 19 when ice goes out and go from there. It is a 1.84 ratio.

 

[Howard Sterndrive]

Re: Should I upgrade to a four barrel on a 4.3L?

I would switch to a stainless prop before worrying about the carb. Similar gain without tearing into a motor.

 

[gixxerjim]

Re: Should I upgrade to a four barrel on a 4.3L?

I will Look into that. I am leery of SS for two reasons: potential damage to out drive and higher cost. Need to see if there are proven hubs that will protect the drive with submerged contact...

 

[achris]

Re: Should I upgrade to a four barrel on a 4.3L?

I will Look into that. I am leery of SS for two reasons: potential damage to out drive and higher cost. Need to see if there are proven hubs that will protect the drive with submerged contact...

That is the biggest load of BS in the marine industry!!! I don't know who started it, but I would love to get my hands around his throat. It's up there with calling Merc stern drives built before 1985, 'pre-Alpha'.....

A SS prop will NOT cause damage to your drive. I have seen plenty of 'rock strikes' and the extent of internal damage bears absolutely no correlation to the material the propeller is made of. I've seen gears cracked and drive-shafts sheared with ali props that show only the slightest indication of impact, and I've seen SS props folded up like a piece of paper without any damage inside. And I've seen the opposite. So get this notion of SS props damaging drives well and truly out of your head....

Chris...........
(Rant over )

 

[Hoosmatroos]

Re: Should I upgrade to a four barrel on a 4.3L?

Chris, just my point. You can't get it out of people's heads. You hear this story all over the world. The guy who started this knows how to get a rumor out

Cost and performance are the two major differences between aluminum and SS props.

 

[achris]

Re: Should I upgrade to a four barrel on a 4.3L?

One more thing. Changing to a 19" prop (ali or SS) will only achieve one thing. You'll be over-propped. That will mean slower acceleration, higher load on the engine (through the entire rev range) and LESS speed at the top end. You will also run a very real risk of engine damage through detonation.... The current 17" prop is EXACTLY the right prop for that engine/boat combination. Change to 17" SS would be a good option. You might get an additional 1 or 2 mph, that's all.

The correct prop for my boat is a 19". It runs 43.1 knots at 4600rpm. If I change to either a 21" or a 17" prop (one size up or one size down) I lose about 4 knots at the top end. I know this because I tried out all sorts of different props when I re-powered....

Chris...........