Can I compensate for wrong I/O drive ratio with a different prop?

[asdasc]

Hi Guys,
2 years back, I replaced the lower unit with one from a 5L instead of the 4.3L I have. The final drive ratio is slightly off. I accidentally got to 5500 RPM the other day before I realized the mistake. I believe red-line on the 4.3HO is 4800 RPM. Can I just get a more aggressive prop to compensate or is there some reason that isn't a good idea?

 

[Philster]

Re: Can I compensate for wrong I/O drive ratio with a different prop?

Need more info, but it might be "no", considering the RPM you need to knock off, but it might be do-able.

What prop is on there now -- make, blade count, material, pitch?

What was/is the drive ratios?

I mean, if you were spinning a 19p 3-blade, I guess you could work it out w/ a 24p 4-blader..... ::shrug::

 

[Maclin]

Re: Can I compensate for wrong I/O drive ratio with a different prop?

The 5.0l drive should have been a lower ratio than the 4.3l drive, (like 1.5 for the 5.0l to 1.65 for the 4.3l) so your rpms should have been lower and you should have had a hard time coming out of the hole and reaching top rpm. Or did I miss something?

 

[Maclin]

Re: Can I compensate for wrong I/O drive ratio with a different prop?

Sorry, I tend to do a play-by-play type of posting...

To answer your initial question, it depends on how close the ratios are and what boat this was done to and the power plant.

In my opinion the 5.0l to 4.3l "swap" can be patched up so to speak with a prop change, though obviously it always best to have the correct ratio.

Going to a lower numerical ratio in a smaller boat is easier to compensate for than in a larger boat, that kind of thing.

I am confused though on the results of you going from a 4.3l drive to a 5.0l drive, RPM's should have gone down and the engine should have struggled more.

 

[UncleWillie]

Re: Can I compensate for wrong I/O drive ratio with a different prop?

The 5.0l drive should have been a lower ratio than the 4.3l drive, (like 1.5 for the 5.0l to 1.65 for the 4.3l) so your rpms should have been lower and you should have had a hard time coming out of the hole and reaching top rpm. Or did I miss something?

I though the Gear ratio is how much it is geared DOWN.

4800/1.5 = 3200rpm at the prop for a 5.0l
4800/1.65 = 2909rpm at the prop for a 4.3l
4800/2.0 = 2400rmp at the prop for a 3.0l

With a 1.65 ratio on a 5l the engine needs to make 3200*1.65= 5115rpm to get that same prop up to 3200rpm.

The 5l is capable of spinning the same prop to a higher rpm and goes over red line.
It needs to be able to spin the prop at the higher rpm without spinning the engine at the higher rpm.
Hence, the LOWER ratio.

 

[Bondo]

Re: Can I compensate for wrong I/O drive ratio with a different prop?

Hi Guys,
2 years back, I replaced the lower unit with one from a 5L instead of the 4.3L I have. The final drive ratio is slightly off. I accidentally got to 5500 RPM the other day before I realized the mistake. I believe red-line on the 4.3HO is 4800 RPM. Can I just get a more aggressive prop to compensate or is there some reason that isn't a good idea?

Ayuh,.... By yer explanation, it sounds like ya got a drive from a 3.0l, Not a 5.0l...

If that's the case,...
No,.... ya just Can't find a prop to buy that's Big enough to make it work...
it'd take something over 30"s of pitch...

 

[asdasc]

Re: Can I compensate for wrong I/O drive ratio with a different prop?

Very interesting. The guy told me 'it's off a 5 liter, but the ratios are pretty close'. I didn't question it enough. It does run fine, I just need to watch the RPM. I have my original prop, but I don't remember what it is - maybe 17x19? Although 14x17 sounds familiar also...

It is a 1990 20' Chriscraft Concept 198

 

[steelespike]

Re: Can I compensate for wrong I/O drive ratio with a different prop?

The ratio may be stamped on the outdrive.If its about 18 or 19 feet I think a 4.3 usually runs about a 23".
If you real have a 17 or 19 pitch a 23 could lower rpm. about 800. from the 19.

 

[444]

Re: Can I compensate for wrong I/O drive ratio with a different prop?

I though the Gear ratio is how much it is geared DOWN.

4800/1.5 = 3200rpm at the prop for a 5.0l
4800/1.65 = 2909rpm at the prop for a 4.3l
4800/2.0 = 2400rmp at the prop for a 3.0l

With a 1.65 ratio on a 5l the engine needs to make 3200*1.65= 5115rpm to get that same prop up to 3200rpm.

The 5l is capable of spinning the same prop to a higher rpm and goes over red line.
It needs to be able to spin the prop at the higher rpm without spinning the engine at the higher rpm.
Hence, the LOWER ratio.

Other way around. A numerically higher ratio requires your engine spin faster at a given mph, all else being equal.

 

[gozierdt]

Re: Can I compensate for wrong I/O drive ratio with a different prop?

Look at it this way. Take the same prop, and it would spin, say 2400 rpm at 25 mph. The engine
rpm would depend on the gear ratio

2400 x 1.5 = 3600 this would be the bigger engine, producing more horsepower at this low rpm
2400 x 1.65 = 3960 This would be the 4.3l, needing more rpm to match the V8
2400 x 2.0 = 4800 this would be the 3.0l, needing even more rpm to match either of the above.

Sounds like you have the last case outdrive, with the middle engine, which is what Bond-O
suggested.

 

[Maclin]

Re: Can I compensate for wrong I/O drive ratio with a different prop?

redundant post deleted

 

[Maclin]

Re: Can I compensate for wrong I/O drive ratio with a different prop?

I was wondering if the "5l" was a typo, and should have been "3l", then the rest of the post makes sense.

The Tach could be off also.