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post #11 of 17 Old 10-09-2011
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I'm running a 3/8" belt with a 100A alternator.
We're doing so as well with a 105 amp alternator. When we first start up the motor, I watch for the oil pressure light to go out and as soon as it does, I turn off the ignition to deactivate the alternator charging circuit. Only after the motor has warmed up a bit will I turn the ignition key back on to activate the alternator and then rev the motor up. I found that I get less belt squeal and better charging at a higher RPM.

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post #12 of 17 Old 10-09-2011
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That's one way to do it, but if you have an hour meter it's not running.

If you have an external regulator, try the Small Engine Mode idea.

But the real gist of my message to you is that if you have a 105 A alternator now, putting a 125 on it will not necessarily increase the load on the pulley and belt, since the amount of charge is based on the house bank requirements and acceptance.

We've fund that even with our nominal 400 ah house bank at 50%, the 100A alternator is only able to get up to 60A charging due to bank acceptance, which Maine Sail has noted is a certain percentage of the bank ah capacity - I forget what it is specifically, sorry.

So you'll essentially have the same belt load and output, given the same size house bank, which is why it's a good idea to have a larger size alternator, so it works "less hard."

Belt squeal could be from a loose belt, misaligned one, or just an old one. I put a new belt on recently and it made a world of difference.

Good luck.

Stu Jackson, C34, 1986, M25 engine, Rocna 10 (22#)

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post #13 of 17 Old 10-09-2011
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Originally Posted by hellosailor View Post
Trash, I'd vote against twin belts. With twin belts there's a continuous problem because one belt always carries more load than the other, they get unbalanced and you wind up going through lots of belts. This is one reason the auto industry went to serpentine belts to replace the dual-belt applications for police and taxi use.

Odds are that you will need to go to a machine shop and have two pulleys fabricated. Depending on your local market, what metal they have on hand or you want to use, that pulley could cost you a couple of hundred bucks but it is a simple lathe turning job that any machine shop can do.

There ARE many different v-belt profiles, "a half inch" is meaningless. If you are buying an alternator with a specific pulley already on it, find out what the belt profile is and make sure the new flywheel is made with the same profile.

While you're at it, check the pulley diameters to make sure you'll be turning the new alternator at the optimum speed for your use, and within the speed range it is rated for. Won't cost ay more, will ensure better charging system.
Absolutely correct as there is no way that two belts will be 'riding' in their grooves equally even with 'automatic' tensioners.
A 1/2" belt will be at its near total load capacity at about 90-100 amps. For 120 amps your really should be running larger "B" sized belts ... and the pulleys will have to modified/changed for a "B" series belt (or metric equivalent).
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post #14 of 17 Old 10-09-2011
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If you have an external regulator, try the Small Engine Mode idea.
Stu, I've done that as well and it does keep the amperage down for the first couple of minutes when it's in the "charge" cycle. When it switches over to "Acceptance" we get a full 100 amps coming out of the alternator which taxes our single belt system. With a new belt we do okay. As the belt starts to wear, and I start to lose travel on the adjustment due to the small size of the engine compartment, I have to be a little tricky to get the most out of an old belt, just because it's easier than replacing the belt more often.

Ray
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1983 Fraser 41
La Conner, WA


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post #15 of 17 Old 10-10-2011
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I had some of the same problems. First I deglazed the pullys,then installed a heavier adjusting bracked. My alternator is a 100 amp Ford style. The regulator is set to ramp up slowly. No noise or dusting from the belt and charges the two 4ds fine.
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post #16 of 17 Old 10-11-2011 Thread Starter
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Again, Thanks everyone for your input. Mike Fosil, I will check with Electromaax. Bellatrix, converting to a serpentine system might be what I have to do. HelloSailor, good advice on having them turned out on a lathe. I'm trying to do the most simple that will be the most bullet-proof installation. I feel like I want a system that I don't have to worry about and having a spare belt is just an insurance policy that will never be utilized.
I believe my first hurdle is to get the engine identified.
Again, thanks everyone.
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post #17 of 17 Old 10-15-2011
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You can solve a lot of the uneven stretch in double belts by ensuring you buy identical belts (same manufactur) and change them in pairs.
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