Max RPM's at WOT?

S

smithrock

Petty Officer 1st Class
Joined
Jun 11, 2007
Messages
399
'85 Mercruiser 4.3L- I was wondering what the max RPM's should be at WOT?
 
QC

QC

Supreme Mariner
Joined
Mar 22, 2005
Messages
22,783
Re: Max RPM's at WOT?

I'm finding 4400 - 4800.
 
S

smithrock

Petty Officer 1st Class
Joined
Jun 11, 2007
Messages
399
Re: Max RPM's at WOT?

OK, thanks. I've had the RPM's as high as 5400 at times. I had a feeling that was too high:eek:
 
QC

QC

Supreme Mariner
Joined
Mar 22, 2005
Messages
22,783
Re: Max RPM's at WOT?

Do yourself a favor and double check your tach. None of this is good if the tach is reading wrong. Most marine tachs can be used for OBs, 4, 6 and 8 cyl engines, and each uses a different setting. There is usually a switch in the back of the tach for this. 5400 seems like screaming. What prop pitch allowed you to do that?
 
S

smithrock

Petty Officer 1st Class
Joined
Jun 11, 2007
Messages
399
Re: Max RPM's at WOT?

Well I just had the motor rebuilt a few weeks back (had a cracked block) My prop is a 14 1/2 19P. I'm experiencing blowout at times as well? Prop does have a couple dings in it
 
QC

QC

Supreme Mariner
Joined
Mar 22, 2005
Messages
22,783
Re: Max RPM's at WOT?

How fast in MPH? GPS maybe? I am thinking your tach may be right. 1.84 ratio is what I find. What boat? Length, weight etc.
 
S

smithrock

Petty Officer 1st Class
Joined
Jun 11, 2007
Messages
399
Re: Max RPM's at WOT?

Approx 44-45 mph (non gps) '85 Chaparral 198 XL approx 20' not sure of the weight. It's an open bow, seats 9
 
QC

QC

Supreme Mariner
Joined
Mar 22, 2005
Messages
22,783
Re: Max RPM's at WOT?

Your tach may be right. If so, and your load was typical, and hole shot was good, then she should easily pull a 21 inch pitch. Confirm tach. Go from there.
 
S

smithrock

Petty Officer 1st Class
Joined
Jun 11, 2007
Messages
399
Re: Max RPM's at WOT?

OK, I'll give that a try. It does take a little longer to plane than I would like. Would a 21 fix that? Thank you!:)
 
QC

QC

Supreme Mariner
Joined
Mar 22, 2005
Messages
22,783
Re: Max RPM's at WOT?

No! Wrong direction for hole shot. Now I am back to checking that tach, and also verify speed with GPS. I would NOT go up in pitch if she struggles to plane. Clean up the 19 you have, retest with accurate tach and GPS speed (phone app is fine). We have to start with an accurate baseline. Good luck!

Do you know how to use your trim to assist planing and then top speed?
 
S

smithrock

Petty Officer 1st Class
Joined
Jun 11, 2007
Messages
399
Re: Max RPM's at WOT?

QC said:
No! Wrong direction for hole shot. Now I am back to checking that tach, and also verify speed with GPS. I would NOT go up in pitch if she struggles to plane. Clean up the 19 you have, retest with accurate tach and GPS speed (phone app is fine). We have to start with an accurate baseline. Good luck!

Do you know how to use your trim to assist planing and then top speed?
Click to expand...

Will do with the GPS. And yes, I know how to use the trim. My trim gauge does not work, maybe I had it trimmed too far up and thats why the RPM's went up? But I can tell by the wake if I have it too far up, and that wasn't the case at that time:confused:

I'll check with the GPS and go from there- won't be able to get on the water till next week some time
 
Don S

Don S

Honorary Moderator Emeritus
Joined
Aug 31, 2004
Messages
62,321
Re: Max RPM's at WOT?

When you trim too high, the rpm goes up and the speed goes down. You want highest speed and highest rpm at the same time.
 
S

smithrock

Petty Officer 1st Class
Joined
Jun 11, 2007
Messages
399
Re: Max RPM's at WOT?

Don S said:
When you trim too high, the rpm goes up and the speed goes down. You want highest speed and highest rpm at the same time.
Click to expand...


Yes, I'm going to check it again to make sure before I do anything. The boat is old and has the original gauges, which could be faulty? But I could tell by the engine that my RPM's were too high.
 
You must log in or register to reply here.
Top