RPM Mystery

karista · November 23, 2010, 05:22:47 PM

I have always been cruising at 2500 RPM at about 6 knots, now I just replaced my original 51 AMP Prestolite Alternator with a same 51 AMP Prestolite replacement. Now my cruising speed of 6 knots is at 2200 RPM and I can't even get to 2600 RPM at full throttle, yet the idle rpm is still the same at 750 RPM!
Any ideas what could be the problem?

Ron Hill · November 23, 2010, 06:04:32 PM

Bernd : Not surprised at all.
The new alternator has a slightly differant pulley and the belt is seated in it differently - than the old pulley.

Your engine rpm is generated from your alternator RPM. Then data like the number of poles in the alternator and the diameter of the alternator pulley are factored in to the dipswitch settings on the back of your tach instrument - to give you a "engine" rpm reading !!!

scotty · November 23, 2010, 07:34:09 PM

Ron, why does it affect the RPM in that manner?

Stu Jackson · November 23, 2010, 09:51:39 PM

'Cuz the tach works electrically off the alternator not mechanically off the engine.

Bernd, you have two choices: adjust the tach from the back or do the math in your head for max rpms. I think we have a Teleflex manual on the wiki.

Stu Jackson · November 24, 2010, 07:56:57 AM

Yes, Teleflex Manuals, 2 parts, under Misc Manuals: http://www.c34.org/wiki/index.php?title=Manuals

Ron Hill · November 24, 2010, 02:36:13 PM

Bernd : The simplest thing to do is to purchase or borrow a laser or mechanical tach. Then and ONLY then will you know the actual/true engine RPM.

Make your self a small chart at some selected points showing actual rpm and what the tach gage read. I'd suggest 1000, 1200 1500 1800 2000 2200 2400 as points. Then when the tach gage reads ie 1500rpm, from your laser tach reading you'll know what your engine is actually turning.

BTW, you'll find a small difference as the new belt wears and rides lower in the tach pulley.
A few thoughts

karista · November 25, 2010, 06:35:14 AM

Quote from: Ron Hill on November 23, 2010, 06:04:32 PM
Bernd : Not surprised at all.
The new alternator has a slightly differant pulley and the belt is seated in it differently - than the old pulley.

Your engine rpm is generated from your alternator RPM. Then data like the number of poles in the alternator and the diameter of the alternator pulley are factored in to the dipswitch settings on the back of your tach instrument - to give you a "engine" rpm reading !!!

Ron!
I understand that the RPM's would be different with a different pulley, however I ordered a direct replacement alternator so I wouldn't have a RPM issue. Also if the pulley was different would it then not also change the RPM at idle? My RPM is still the same at idle (750 RPM)!

Ron Hill · November 25, 2010, 01:04:28 PM

Bernd : I've try ed to give you some ideas of why your tach reads differently with the new alternator.
1. Are you sure that your new alternator has the same number of poles?
2. You are certain that the pulley is identically the same as your OLD 20 years ago OEM alternator?

If someone asked me I'd say that 750 rpm (tach reading) at idle is in error as a diesel that slow should shake your teeth out. Maybe you have an extraordinarily smooth idling engine?

I believe there are 14 dip switches you can play with. I still believe your best bet is to make the comparason chart of actual rpm to the tach reading rpm.

A few thoughts and as Stu says, your boat your choice !!

lazybone · November 26, 2010, 07:01:34 AM

Ron,
What is the proper idle rpm for the 25xp?

Ken Juul · November 26, 2010, 07:48:29 AM

Idle depends on many things...condition of motor mounts, hump hose, etc. I generally don't idle mine below 1000, that provides the smoothest engine with smooth shifting on my boat.

Ron Hill · November 26, 2010, 11:11:37 AM

Ciao : A good idle (actual engine rpm) is between 900 and 1000rpm. I preffer the higher of the two like Ken, as the engine is smoother and it's easier for the engine to spin the alternator (so you don't have a lug down).
Again you need to use a laser or mechanical tach to fine the actual engine rpm. Then note what your tach engine instrument reads.

lazybone · November 26, 2010, 12:52:02 PM

Thanks for confirming my suspicion. Our engine laser tached at about 700 and shook and sounded like crap. Raising the idle to about 1000 smoothed things right out but I was worried that I might be shock loading the trans? Sounds like I'll be ok, so thanks again.

Stu Jackson · November 26, 2010, 04:45:48 PM

Aldo, IIRC the transmission is good up to about 1500 rpm. We keep idle at 1100 or else, like Ron says, it shakes my few remaining teeth out.

karista · November 29, 2010, 05:06:08 PM

Quote from: Ron Hill on November 25, 2010, 01:04:28 PM
Bernd : I've try ed to give you some ideas of why your tach reads differently with the new alternator.
1. Are you sure that your new alternator has the same number of poles?
2. You are certain that the pulley is identically the same as your OLD 20 years ago OEM alternator?

If someone asked me I'd say that 750 rpm (tach reading) at idle is in error as a diesel that slow should shake your teeth out. Maybe you have an extraordinarily smooth idling engine?

I believe there are 14 dip switches you can play with. I still believe your best bet is to make the comparason chart of actual rpm to the tach reading rpm.

A few thoughts and as Stu says, your boat your choice !!