Upgraded mercruiser 4.3 prop

Hi. I recently upgraded my 1996 mercruiser vortec 4.3. My current prop is a 13 3/4 x 21 vengeance. The upgrades added 100 hp over the stock 190 hp and I'm figuring a 24" pitch is what I will need. The alpha outdrive has a 1.81:1 gear ratio and the boat is only used with a light load. The boat is a Donzi runabout and weighs 3000 lbs plus one or two people and fuel. The engine is still in breakin period with only 8 hours usage on the 20 hour breakin period. For the first 10 hours, I am keeping the rpm between 1500 and 3600. For the next 10 hours of usage I will go above 3600 rpm for short periods.
I'm looking for a performance prop for top speed, because the boat is always lightly loaded. Props I am considering is a laser, tempest or fury. Does anyone have a fury on a sterndrive? Like above, I figure 24" pitch with a diameter not too small or too big. I can't get any rpm/speed numbers yet, because I'm not operating the engine at WOT. With the current prop I figured I will have 5,500 rpm at WOT. I feel I don't want to operate the engine 700 rpm over recommended WOT of 4800. I'm thinking of purchasing a cheap prop with a 24 or 25 pitch and see what numbers I have after the 20 hour breakin period. Any prop suggestions if my initial numbers are correct?

might be better off with a 1.48 ratio drive off a V8

Thanks for your answer. I have thought about a ratio change, but I am trying to avoid additional cost, unless there is more than a slight improvement in performance.

Ayuh,....... If yer at 300 hp, ya need the 1.5:1,......

Why not lower my WOT rpm by approximately 500 rpm by using a prop with 3" more pitch?

To me it makes more sense to adjust my WOT to 4800 by changing my prop pitch and not doing this by a gear ratio change. go-fast.com says, "It my experience, a boat will run fastest with a higher numerical drive ratio assuming you can find a prop with enough pitch to limit the engine to the desired RPM at full throttle. A slower turning prop with more pitch is more efficient than a faster turning prop with less pitch, even though the calculation for theoretical speed is the same for both." My stock mercruiser vortec 4.3 had a WOT of 4900 rpm with a vengeance 13 3/4 x 21 and 190 hp. My mercruiser vortec (now 4.4) has 290 hp and I must lower my WOT rpm by 500-600. I can easily do that with a prop with 3-4 more inches of pitch.

And it's much cheaper.

And it's much cheaper.

Jeff

PS: Sorry for the double answer.

funny funny

you have a 100 extra hp at the same RPM as before the mod? what's the HP difference at your rated WOT RPM?? Do you have the before and after torque curves?

I'd suggest looking at your intented cruise RPM to see what the torque difference is there. I'd bet the difference wouldn't warrant the gear change....

You can allways just keep the engine speed at max spark advance and keep and eye on the plugs. One concern would be the engine running too lean. Fuel is the only thing that cools the valves and pistons. You risk burning up valves and pistons if your under propped. If the plugs come out powdery white you must change the overall gear reduction.

The stock number before upgrades according to mercruiser is 190 hp. With the stock 13.75 x 21 prop I had 4900 rpm at WOT. Now after upgrades I will have 290 hp and probably about 500 rpm more with the same size prop

You wont know unil sea trials 100 ponies is alot to just speculate the outcome. Is it a planing hull or a displacement hull is the big factor. But there are other variables to consider also. Give it a shot then you can make the necessary adjustments to extend engine life. Is the engine running close to 100% and what is WOT rpm?

It's still in 20 hour break in period and I am not giving it more than 3600 rpm. Now I am in the last 10 hours of break in period and will go above 3600 rpm for short periods. A lot of people may question an extra 100 hp from a mercruiser 4.3, but I'm sure this number is correct from engine remanufacturers, speed shop and my research. Compared to before, it is unbelievable how fast the boat goes to plane with just hitting the throttle lightly. I'm sure I will have around around 5500 rpm at WOT with the current 21 inch pitch prop. I will buy a cheap 24 inch prop for testing and after testing get a good performance prop.

The original rpm was about 100 over spec if you get it around 4800 With a stainless you may need to add a spoiler to hold it on the water

I hear you about running too lean and risk of detonation. I use only non-ethanol premium gas. Edelbrock recommended that I do not use the calibration kit for the V6 and leave it setup for the 5.7. They also said I should tune the edelbrock 1409 marine carb on the rich side. I already checked all plugs after 10 hours of break in period and they look good. It is recommended to go one heat range colder on the plugs when adding 0.70 compression and the extra hp, which I did. The pistons are KB bored 0.30 over and the valves or Manely severe duty.

Spelling correction, the valves are Manley severe duty valves. The spark plugs are one heat range colder and are Brisk multi spark.