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Old 26-08-2018, 16:34   #1
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Pulley Ratio for Yanmar 1GM10

Does anyone know the alternator pulley ratio for a Yanmar 1GM10 or similar engine?
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Old 26-08-2018, 17:05   #2
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Re: Pulley Ratio for Yanmar 1GM10

I can check our YSM8 which is the predecessor to yr engine but wont be on boat for maybe a week so sum1 else may chime in 1st.
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Old 26-08-2018, 22:42   #3
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Re: Pulley Ratio for Yanmar 1GM10

The ratio of maximum engine to maximum alternator RPMs should be the same as the ratio of pully diameters. Since the one on the engine is often part of the balancing system you need to change the one on the alternator.
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Old 27-08-2018, 11:07   #4
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Re: Pulley Ratio for Yanmar 1GM10

Thanks for responding, but what I really need is the pulley ratio that Yanmar uses for their alternator.
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Old 27-08-2018, 14:28   #5
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Re: Pulley Ratio for Yanmar 1GM10

Yanmar YSE8 is a similar engine and mine has a 85mm engine pulley with a 65mm pulley on the alternator BUT I don't know if the alternator is original.
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Old 27-08-2018, 14:43   #6
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Re: Pulley Ratio for Yanmar 1GM10

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Originally Posted by Locquatious View Post
Thanks for responding, but what I really need is the pulley ratio that Yanmar uses for their alternator.
Yanmar designs the pulley ratios for the maximum speeds of the engine and the alternator. If the alternator is not as original the ratio needs recalculating for the different alternator.
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Old 27-08-2018, 15:51   #7
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Re: Pulley Ratio for Yanmar 1GM10

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Yanmar YSE8 is a similar engine and mine has a 85mm engine pulley with a 65mm pulley on the alternator.
That ratio seems too low and on another thread someone pulled the design of the 1GM and scaled that the engine pulley was ~148-150mm. If your pulley diameter is right that would give a ratio of ~2.3 which is in the right range.
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Old 27-08-2018, 16:04   #8
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Re: Pulley Ratio for Yanmar 1GM10

Yep, I agree the 85 / 65 ratio does seem too low. I know the engine pulley (85) is original so I am surmising the alternator pulley (65) is not original fit.

Here is picture of a 2GM20, all original and this has the same rpm characteristics as the 1GM10. It may help you in someway.
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Old 27-08-2018, 16:17   #9
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Re: Pulley Ratio for Yanmar 1GM10

Great. That scales closer to 2:1, which seems reasonable.
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Old 27-08-2018, 19:55   #10
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Pulley Ratio for Yanmar 1GM10

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Originally Posted by Locquatious View Post
Great. That scales closer to 2:1, which seems reasonable.


Look up optimum alternator RPM and you may be surprised.
2 to 1 is probably good for a 7000 RPM redline car.
Itís not so much that they canít make power at less RPM, itís the cooling they get from spinning the fan faster that your after.
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Old 27-08-2018, 21:02   #11
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Re: Pulley Ratio for Yanmar 1GM10

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Look up optimum alternator RPM and you may be surprised.
2 to 1 is probably good for a 7000 RPM redline car.
Itís not so much that they canít make power at less RPM, itís the cooling they get from spinning the fan faster that your after.
Actually, it's the centrifugal forces they're trying to avoid in a high redline car. Above 14,000 rpm the alternator parts want to flee. The latin for flee is fuga, hence fugal.

But in a small 9hp diesel for a sailboat, what's wanted is to recharge the starter battery and, when the craft it recharges is taken out for a few nights, can it recharge the battery enough to power the anchor light all night along with some incidentals in the starlit evening.

From what I've seen, alternators are burned up trying to charge batteries with low voltage, batteries that are run down too far, or batteries that are going bad. Alternators can generate more current at low voltage. The heat that you need to cool off in an alternator is I^2*R.

I have a way to obtain more power from a small alternator at idle. Raise your voice if you'd rather recharge your battery with the engine burping at idle instead of with the engine throttled up, yelling down the quiet night, and vibrating the hull.
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Old 23-09-2018, 03:58   #12
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Re: Pulley Ratio for Yanmar 1GM10

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Originally Posted by Locquatious View Post
Does anyone know the alternator pulley ratio for a Yanmar 1GM10 or similar engine?
roughly 150mm/6" for engine and 65mm/2 1/2"ish for altenator
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Old 26-09-2018, 05:48   #13
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Re: Pulley Ratio for Yanmar 1GM10

I am currently replacing a 12V alternator with a 24V alternator and wondered what pulley I should put on the alternator. I looked at graphs of alternator output/rpm and found they were much the same whatever the make of alternator. (Google "alternator output curve"or something like that)

Since I expect my motor RPM at cruising speed will be out 2500 RPM I was able to work out the required alternator pulley diameter to get optimum alternator output.

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