Repowered UP-gctid404745

[Guest]

My 1992 Bayliner Ciera Sunbridge came stock with a I/O 305 5.0L v8, I believe. There is a new engine in the boat.....installed by the previous owner. The engine is an Edelbrock 350 5.7L Thunderbolt, naturally aspirated.

When a boat is re-powered upward as this one is, I am assuming that the stern drive must also be upgraded? The original stern drive lower unit is 1.84R (sn: 0D730874) which is still on the boat, and the boat has sluggish response in the water.

Questions: I am assuming that I will need to change the stern drive lower unit to 1.50R, which is what a 350 would require? If so, can I use the old stern drive upper unit or do I need to replace it as well? What about the prop?

Thanks for your help.

[Guest]

Edelbrock is just a manifold and carb maker. Thunderbolt is just the ignition system. Neither tells much about the engine itself.

As long as it's a normal marine build, it won't be too powerful for your drive, which I believe is an Alpha One. Can you verify that?

What exactly do you mean by sluggish response in the water? Slow to accelerate and get on plane?

What is the pitch of the prop you have? What is the RPM of the engine at full throttle?

[Guest]

I am assuming that I will need to change the stern drive lower unit to 1.50R, which is what a 350 would require?

Also, that is an incorrect assumption.

[88fourwinns]

While some engine/drive combos specified a different drive ratio going from the 5.0 to the 5.7, as long as the engine is able to reach its specified max rpm and it performs well I would not change the drive. IF anything, the smaller engine will be geared to reach peak rpm quicker, so with a bigger engine it should perform even better. Just make sure the engine does not over-rev at wide open throttle.....

if your OE drive ratio was a 1.84:1 that means that the engine is turning 1.84 turns for each turn of the prop

if the one you are considering is a 1.50:1, the engine turns 1.5 turns for every turn of the prop

the ratio you have is better for performance than the one you are considering, in that it will let the engine run faster for the same water speed. The reason that they use different drive ratios with different engines is that the larger displacement engines have more low end torque and can pull that load at lower rpms.

If the boat feels sluggish then there are other problems, wrong prop, fuel delivery issues or engine not reaching full ignition advance.

All other things being equal, the 350 or 5.7 is going to be running at a higher rpm than originally spec'd with the drive you have, so you may need to go up in prop pitch if the engine over-revs at wide open throttle.

[Guest]

SwampNut wrote:
Edelbrock is just a manifold and carb maker. Thunderbolt is just the ignition system. Neither tells much about the engine itself.

As long as it's a normal marine build, it won't be too powerful for your drive, which I believe is an Alpha One. Can you verify that?

THE SERIAL NUMBER ON THE ENGINE/DRIVE INDICATES ALPHA ONE GEN TWO - AFTER 1991.

What exactly do you mean by sluggish response in the water? Slow to accelerate and get on plane?

YES, BUT THIS COULD BE DUE TO THE FACT THAT IT HAS NOT HAD A TUNE UP LATELY. IDLE SEEMS LOW. NO BIG DEAL, I THINK.

What is the pitch of the prop you have?

I NEED TO CHECK THIS OUT.

What is the RPM of the engine at full throttle?

4400 RPM AT MAX SPEED IS WHAT THE MERCRUISER MANUAL RECOMMENDS. CRUISE SPEED AT 2200 RPM. AT 2500 RPM WE WERE DOING ONLY ABOUT 10MPH. THE BOAT IS RATED AT 25MPH CRUISE AND 40MPH MAX.

[Guest]

88fourwinns wrote:
while some engine/drive combos specified a different drive ratio going from the 5.0 to the 5.7, as long as the engine is able to reach its specified max rpm and it performs well i would not change the drive. If anything, the smaller engine will be geared to reach peak rpm quicker, so with a bigger engine it should perform even better. Just make sure the engine does not over-rev at wide open throttle.....

If your oe drive ratio was a 1.84:1 that means that the engine is turning 1.84 turns for each turn of the prop

if the one you are considering is a 1.50:1, the engine turns 1.5 turns for every turn of the prop

the ratio you have is better for performance than the one you are considering, in that it will let the engine run faster for the same water speed. The reason that they use different drive ratios with different engines is that the larger displacement engines have more low end torque and can pull that load at lower rpms.

If the boat feels sluggish then there are other problems, wrong prop, fuel delivery issues or engine not reaching full ignition advance.

All other things being equal, the 350 or 5.7 is going to be running at a higher rpm than originally spec'd with the drive you have, so you may need to go up in prop pitch if the engine over-revs at wide open throttle.

okay. Thanks, i will plan to leave the ratio as is and check the prop, ignition timing, fuel system.

[yachtman]

Your outdrive is not the problem. You have other problems that is causing this . Check your prop that its not too big.

You said its idling low. That couls be because your timing is retarded. That would cause it to be sluggish. I would check that first. Be sure to check also for total advance timing at higher rpm and that the timing curve is correct.

is it moored? Is the bottom clean? If not that could be part of the problem also

[Guest]

yachtman wrote:
your outdrive is not the problem. You have other problems that is causing this . Check your prop that its not too big.

You said its idling low. That couls be because your timing is retarded. That would cause it to be sluggish. I would check that first. Be sure to check also for total advance timing at higher rpm and that the timing curve is correct.

Is it moored? Is the bottom clean? If not that could be part of the problem also

from this and other replies i think that we can eliminate the outdrive - except for prop. Bottom is clean and freshly painted, on a trailer. Timing issues will need to be corrected i am sure. Will purchase a tune up kit. Thanks for your input.

[2850Bounty]

There is NO substitute for a correct ratio for the engine HP and Torque.

We can't prop ourselves into a correct WOT RPM..... it just can't be done correctly.

You can try props until the cows come home, but if the final drive ratio is incorrect, you won't be able to get it dialed in.

Check with Mercruiser to see what the recommendation would be.

The ratio of 1.84:1 sounds too low for this new engine.

.

[Guest]

2850Bounty wrote:
There is NO substitute for a correct ratio for the engine HP and Torque.

We can't prop ourselves into a correct WOT RPM..... it just can't be done correctly.

You can try props until the cows come home, but if the final drive ratio is incorrect, you won't be able to get it dialed in.

Check with Mercruiser to see what the recommendation would be.

The ratio of 1.84:1 sounds too low for this new engine.

.

Too low or too high? Here is a chart provided by Merc Aftermarkets, an OEM:

350, 5.7L, V8 1.50R (Formerly 1.50)

305, 5.0L V8 1.62R (Formerly 1.65)

4.3L V6 1.81R (Formerly 1.84)

3.7L and 470 1.81R (Formerly 1.84)

Inline 6 1.62R (Formerly 1.65)

120, 140, 3.0L 4cyl 1.94R (Formerly 1.98)

The prop selector app on https://www.mercurymarine.com/propel...rop-selector/#

indicates that I need a 21.0 inch pitch prop for my intended application with the current power plant.

[2850Bounty]

92CieraSB wrote:

• 1 wrote:
• Too low or too high?
• indicates that I need a 21.0 inch pitch prop for my intended application with the current power plant.

• 1 wrote:
• In terms of reduction, it would be too low.
  
  In terms of numbers, it would be numerically too high.
• A 21.0 inch pitch prop is pretty tough to pull off on anything much larger than a ski boat.

[Guest]

2850Bounty wrote:

• 1 wrote:
• In terms of reduction, it would be too low.
  
  In terms of numbers, it would be numerically too high.
  
  Copy that.
• A 21.0 inch pitch prop is pretty tough to pull off on anything much larger than a ski boat.

Okay. Will check the current prop numbers this weekend.

[Guest]

92CieraSB wrote:
Okay. Will check the current prop numbers this weekend.

This appears to be the stock prop - 19" pitch, 3 blade standard rotation. It is in excellent condition with no signs of cavitation. I think that I should stay with this prop to begin with when I upgrade the lower unit, and go from there. Need to take care of the other issues first though.