Prop pitch and number of blades 4.3LX Gen II TEST

4.3mudder

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1994 Crownline 182. 4.3LX Gen II with 1.81 gear ratio I think. Serial number of drive is 0F339808 if anyone knows for sure the gear ratio. Did some test with various props and for some reason these numbers are either high and low of what I come across with engines and drives with similar as mine. These were all taken on the same day. Calm water. Hardly anyone out. So ride was smooth one end to the other. Props [Alpha 4 14 X 20 Max 5300 RPM 38 MPH @ 4800RPM 2 adults Good hole shot] [High five 13 1/4 x 19P Max 5400RPM 36 MPH @ 4800 RPM 2 adults Awesome hole shot] [High five 13 1/4 X 23P Max 5200 RPM 44 MPH @ 4800 RPM 2 adults Lose of hole shot] I'm looking into getting a 3 blade and looks to me that either a 25 or 27 falls into the range of it. But it doesn't seem right. I figured boats like mine and that power train setup is only good for 21 pitch max. Maybe someone can chime in or has the same thing of what I'm looking for.
 

steelespike

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I'm pretty sure your dealing with a tach that is not accurate, reading high. All the props show the same incorrect slip of about 24% typical is usually about 13%
with a setup like yours. If you were actually at 5200 with a 23" prop speed would be about 54 mph.Is your speed by gps? The only accurate speed indication.
It is normal for the 23 to produce the best speed and poor hole shot. 23" is about the highest you would see for a typical 4.3 setup.
There should be a selector switch on the back of the tach; note the position and operate it a few times to clean the contacts.
It also may be set for a 4 cylinder raising the rpm.
 

4.3mudder

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I know it has the selector switch from 4-6-8. But it's been at 6. When I get home from work I will verify.
 

4.3mudder

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Is there a table or calculator that has the different rpms for 4,6, and 8 cylinder engines that would correspond with one another? Say if the tach is reading 4000 for a 4 cylinder. What would it read for a 6 cylinder at 4000 RPM. I assume 2000 for an 8 cylinder if it's on the 4 cylinder switch
 

steelespike

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If its been on 6 then try cleaning the contacts as described. Can't help with a formula.
I can tell you that a 20" prop at 4800 rpm and 13% slip produces about 43 mph.
and a 23" at 4800 13% makes about 50. Your actual rpm with the 20" your selected 4800 setting is actually about 4174,
your 23 at your selected 4800 is actually about 4200. I think its safe to say the tach is off about 600.
This assumes a 13% slip for all the calculations.
Tach error is not necessarily lineal.
 

4.3mudder

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I know that at idle it sits at 700. But I guess it's kind of like larger or smaller tires on a vehicle with the speedo sensor off. It will read all kinds of different speeds. I borrowed and photo laser tachometer to try. Will put a piece of aluminum tape on the harmonic balancer and see what comes up.
 

4.3mudder

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As far as mph. For the hell of it I tried with my phone against the dash gauge. It was pretty close.
 

4.3mudder

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If it sits at idle at 600 or 700, Then it must be a percentage that it's off rather a certain number. Because 600 extra at idle would be 1300. And the needle goes straight to 0 when engine is off and key is on.
 

4.3mudder

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Well, did some test and come to find out tach is off. Not by a certain number but a percentage. As the rpm increases so does difference between the actual rpm by a photo and dash tach. Here are my results Will have to buy an aftermarket tach and put on the dash. Hope this helps someone for future reference. I do thank you steelespike. Thanks!
 

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steelespike

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Glad to help,thanks for the chart.
Did you operate the selector switch a few times to clean the contacts?
 

4.3mudder

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I did go back and forth with the Selector switch and did not help. So. I ordered a new set of faria gauges since mine are 20 years old and figured a little updating can be done. And on the other hand. Did more research and found out that on the back of the tach and I guess all of the faria Tachs there is a potentiometer hex adjustment screw in the back behind the paper tag. So. I borrowed the RPM gauge again and ran it. And tuned in that set screw so that it's near perfect. Now. How long it will stay like that I don't know. Just funny it got out of calibration because it hadn't been messed with.
 

steelespike

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The marine environment is not kind to electronics I wonder how long your adjustment will hold.
Perhaps it needed to be operated a little to clean up any corrosion/oxidation present before recalibrating.
 

4.3mudder

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Well. Here soon I will put the new gauges in to be on the safe side. Due to this tach issue it has made me spent unneeded money on props that are out of range! So I figured of asking questions to verify.
 

steelespike

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I assume you will be getting us a lightly loaded wot rpm and gps speed test with your new instruments.
Lots of folks sell their extra props on Craigs list or ebay.
 

4.3mudder

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For sure. Probably just be me and one other person. I got plenty of props to choose from. I hope I have some in the range. So far what I have to test is mercury laser 21, 23, and 25 pitch. Vengeance 21 pitch as well. I'm not sure how that 25 will preform though. Boat says 2700 unloaded. Then 550 for 2 people. On top that. We shall see.
 

4.3mudder

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And right now is the time to buy since boating season is almost over with and no one is looking for props!
 
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