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Old 09-02-2011, 04:29 PM   #1
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Smaller alternator pulley

Don't know if anyone else does this but I have been able to coax a lot more juice out of my alts by putting on smaller pulleys. I cruise at 1400rpm instead of the max 2500 so putting on a smaller pulley got the reps up more to the rated numbers. I had great success with it on our last boat and have ordered new pulleys for the new boat. With the Lehmans I use a 2 1/2"OD with a 17MM bore 1/2"belt for the original equipment Motorola alts.
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Old 09-03-2011, 03:54 AM   #2
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RE: Smaller alternator pulley

I traded out the motorola for a 110 amp large frama leese neville I found on ebay for $45 it produces full output at very low rpm worked out very well
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Old 09-03-2011, 07:43 AM   #3
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RE: Smaller alternator pulley

Keep in mind that if your tach runs off the alternator and you change the pulley size, your tach is no longer correct and will at the least need to be re-calibrated. Chuck
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Old 09-03-2011, 08:52 AM   #4
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RE: Smaller alternator pulley

There has to be a reason the pulley is designed the way it is. Nothing is "free". If it worked better with a smaller pulley, it would have come with a smaller pulley. You might be overheating the windings, diodes, etc. The higher capacity aftermarket alternators we see don't just have smaller pulleys.

Now if the manufacturer of the alternator says it's OK, fine, do it. Otherwise, prepare for a problem somewhere down the road.
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Old 09-03-2011, 10:08 AM   #5
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RE: Smaller alternator pulley

Quote:
motion30 wrote:
I traded out the motorola for a 110 amp large frama leese neville I found on ebay for $45 it produces full output at very low rpm worked out very well
*wow you scored on that Alt... those are great units and are bullet proof.. the key to a pulley swap is to make sure that the unit will handle the rpm if you need to use full rpm in a emergency.

HOLLYWOOD
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Old 09-04-2011, 04:09 AM   #6
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RE: Smaller alternator pulley

The hassle with a smaller pulley is the question of belt wrap.

IF the drive belt is close to 160deg or so a smaller pulley will be fine.

If a bunch of goodies are driven by the same belt and the wrap is only 90 deg or so, higher belt tension will be needed.

The cure is frequently dual belts, on a large frame alt.
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Old 09-04-2011, 02:58 PM   #7
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RE: Smaller alternator pulley

when I ordered a new 100 amp alternator the engineer insisted that I go with a smaller pully. He based this on my max governed engine rpm.
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Old 09-05-2011, 08:37 AM   #8
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RE: Smaller alternator pulley

Quote:
rwidman wrote:
There has to be a reason the pulley is designed the way it is. Nothing is "free". If it worked better with a smaller pulley, it would have come with a smaller pulley. You might be overheating the windings, diodes, etc. The higher capacity aftermarket alternators we see don't just have smaller pulleys.

Now if the manufacturer of the alternator says it's OK, fine, do it. Otherwise, prepare for a problem somewhere down the road.
*The alt pulley is sized for max RPM which could be as high as 3600 in a turbo. I run at 1400 that is why I can spin the alt faster
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Old 09-05-2011, 11:15 PM   #9
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RE: Smaller alternator pulley

Quote "If it worked better with a smaller pulley, it would have come with a smaller pulley"

You have to take into consideration that the alternator manufacturers are mainly selling to gasoline power vehicles with very small numbers used in marine applications. Even in marine applications it is not unusual to see vessels operating with gasoline engine speeds in excess of 3000 RPM's but since most of us have diesel engines that are being max'd out at 2500 RPM and usually run at say 1500 RPM there is a large window of available RPM's to be increased with the alternator.

I too am running a large frame Leese Neville alternator that was sold into the emergency vehicle market originally where the engine was spinning upwards of 3500 RPM regularly during an emergency run. When I installed the alternator in my boat and ran the engine at 1400 - 1600 RPM I could not achieve the rated output no matter what load I put on it. Like Daddyo I installed a smaller pulley and got a much greater output but even then it was not up to specs.
I did some careful calculations and found that in order to get the alternator spinning at its rated output RPM I had to further decrease the size of the pulley and then I got the rated output. I have dual pulleys and the alt. is driven by its own set of belts so I get lots of "pulley grip".

Since I only rarely run the engine at anything over 1800 RPM and then only to its governed 2200 RPM for short bursts, the alternator is still not being over run and has been on the boat in this setup for over 10 years and two sets of batteries so I personally think it is still a very reliable alternator despite my changes.

John "MV Penta"
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Old 09-06-2011, 03:37 AM   #10
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RE: Smaller alternator pulley

Most alts installed in boats are simple car units.

Even cheap car engines can be spun 5500 to 7500 rpm these days , so over speeding an alt is seldom a problem.

The only folks that might are those with alts that are flywheel driven , as on the old Volvos with a 16 inch flywheel.
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Old 09-12-2011, 06:57 AM   #11
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Smaller alternator pulley

I understand the part about normally cruising at a lower RPM so you're not damaging the alternator, but what about when you do operate at a higher RPM, say to outrun a storm or even WOT for testing?

I usually cruise at 2,000 RPM, but I'll take it to 3,900 RPM (the max) for a couple minutes to test the engine and the hull and I will usually run at 3,000 RPM for a few minutes to bring the engine up to normal operating temperature.

How would one decide what size pulley would be best, other than the one that comes on the alternator or boat?

And how do you determine the output of the alternator?

*BTW: The alternator on my boat was replaced by the PO.* I don't know why, but I figured this out when I realized that it was the only "non-metric" bolt on the entire boat!* It's also not painted Volvo green.



-- Edited by rwidman on Monday 12th of September 2011 07:00:55 AM


-- Edited by rwidman on Monday 12th of September 2011 07:10:44 AM
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Old 09-12-2011, 08:08 AM   #12
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RE: Smaller alternator pulley

Like FF said these things are designed to handle vehicle RPM's which can be over twice what you run at WOT. I did this on my last boat and by sizing down from a 3 1/2" to a 2 1/2" never had a problem. Any smaller then 2 1/2" you start having a belt wrap problem with wear I do believe. To test output you need an amp meter or pull it and take it to an alternator shop.
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Old 09-12-2011, 08:46 AM   #13
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RE: Smaller alternator pulley

Most small frame alternators can run at up to around 8000 rpm, larger units run at somewhat less around 6000 rpm. You should size the pulley so that you don't exceed that figure at max engine rpm but still produce a few amps at idle.

Contact the alternator manufacturer and ask them what the max rpm for your specific model is. They are the only source for a definitive answer. They will tell you how to calculate pulley sizes to obtain the best performance and longest life.
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Old 09-12-2011, 03:45 PM   #14
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RE: Smaller alternator pulley

Quote:
RickB wrote:
Most small frame alternators can run at up to around 8000 rpm, larger units run at somewhat less around 6000 rpm. You should size the pulley so that you don't exceed that figure at max engine rpm but still produce a few amps at idle.

Contact the alternator manufacturer and ask them what the max rpm for your specific model is. They are the only source for a definitive answer. They will tell you how to calculate pulley sizes to obtain the best performance and longest life.

*

If I only knew.* It's been replaced.* I suppose there is a name plate on it somewhere, probably where I can't see it.
*
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