RPM?

[lahiggs]

Does anyone know what the tach should read on a 19' Citation w/ a 1987 4.3L Merc at WOT? I am reading 5500 with a 19 pitch prop. I would like to go to a 21 pitch to gain a few more mph's. I am thinking 5500 is a little on the high side. would i be correct? thanks

 

[lenny2113]

Re: RPM?

wvboater, <br />I don't know exactly the WOT on 4.3L Merc, but I am sure 5500 is high. I think the correct is around 4500-4800. I say you can easily switch to 21 pitch prop. If I recall correctly, you will reduce RPM by 400 for evry 1 increment of pitch. <br />Contrary, 1 decrement of pitch will increase RPM by 400.

 

[tommays]

Re: RPM?

you need to check the tack first with a shop tack many boat tacks are off<br /><br />if its reading high and you go to a bigger prop it will be very hard on the motor<br /><br />tommays

 

[lahiggs]

Re: RPM?

I think the tach is pretty close. My cruise speed is around 30 and thats what I am doing at about 3600 rpm, which I am pretty sure thats about right for cruising.

 

[summit1]

Re: RPM?

WOT for my 4.3 is 4400-4800 rpm (per manual)

 

[rodbolt]

Re: RPM?

check the tach

 

[CTD]

Re: RPM?

My Chris Craft 18 ft 7 inches long with a 4.3 and a 14.5 X 21 prop. It turns 4700 rpm max at 48.6 mph. I cruise at 3000 rpm at about 33mph. Thats with a full tank and 400lb people. My book says for a 4.3 with a 2bbl max rpm is 4600 and with a 4bbl it is 4800. Operating range 4200-4600 and 4400-4800. If you carry a lot of weight or tow skiers at times you may want to use your 19 for that and just watch your rpms.

 

[lahiggs]

Re: RPM?

I think I may try a 21 then. Thanks. I have to buy a new prop for next season no matter what, I ruined my current 19.

 

[achris]

Re: RPM?

What's the gear ratio in the box, 1.84 or 1.65:1? and what speed are you getting at WOT (5500rpm).<br /><br /><br />So far I know :<br /><br />RPM at WOT : 5500<br />Prop pitch :19"<br /><br />Let me know and I'll run the figures in my program (written specifically for this application) and come back with some more info for you.<br /><br />Chris.............

 

[lahiggs]

Re: RPM?

achris:<br /><br />i dont know what the gear ratio is. How would I find that out? Speed at WOT is around 45mph, give or take a few due to fuel and passenger differences.

 

[Don S]

Re: RPM?

Assuming the gears have not been changed, the gear ratio is stamped on the drive (see picture)<br />If the gears have been changed to something else, you would have to dissassemble the drive and count the gear teeth to be accurate, or turn the input shaft and see how many turns it takes to turn the prop shaft 1 complete revolution. It may be 1.32 turns, or 1.5 turns or 1.65 turns or 1.84 or 1.98. And that may be real hard to do accuratly.<br />But before going to all that trouble, check the tach with a shop tach. Thinking your tach is accurate is a real waste of time and money if you start changing gear ratio's and props to find out your tach was off.<br /><br />

 

[Boatist]

Re: RPM?

Wvboater<br />I also think it a good idea to check your tach. Do you have a dwell meter or something that has a tach built in you can compare it too?<br />For some reason on this board many think it's ok to turn 9000 rmps but if your 100 rpms under the max that will kill your engine. <br /><br />I think your WOT RPMS should be in the recommended rpms of 4400 to 4800. If your tach is correct you should go to a 23 pitch prop. This should put your WOT Rpms at about 4700 rpms which would be perfect in my book. I think this will gain you about 5 or 6 MPH and give you better fuel mileage as well. Your hole shot will be a little slower but still acceptable. I think will also lower your 30 mph cruise rpms to about 3000 and that is a good thing. If you go with the 21 pitch you will still turn 5100 or 300 rpms over redline.<br /><br />When you buy your new prop give the dealer your spec and ask about the 23 pitch. Also ask if you can exchange the new prop for a different pitch if it does not turn out right.<br /> <br />My boats is a 1980 3.0L 181 cu in 4 cylinder recommended rpm range 4200 to 4600 rpms. I have two props a 17 pitch that turn 4600 rpms at 35 mph and a 19 pitch that turns 4200 rpms at 38 mph. I use the 19 mostly in the ocean where we often curise 25 miles offshore and do not do a lot of starting and stopping until we get to the fishing banks. I made this trip many times with both props and at the end of the day will have traveled about 70 miles mesasured by GPS, Loran C and Depth Finder with paddle wheel. With the 19 it will take 18 gallons average to fill up. With the 17 pitch it will take 21 gallons average to fill up. In the ocean I never run over 25 mph and if the swells are over 5 feet we will not go that far out. <br />I use the 17 pitch in Lakes and Rivers where do tend to do a lot of stoping and starting. Seem there is always a Marina, Group of fishing boats, Water skier down. Also inland seldom we cruise over 5 or 6 miles. Having two props ensures I pull the prop and grease the shaft a couple times a year.