House System Alternator Pulley

[Gerry H]

OK: Bought new a military surplus 24V 110A Alternator for my house batteries. Now I need to figure out what size pulley I need for it to drive it off my 8V92 where my AUX AC compressor was mounted. If I should go somewhere for help, that would be the way I'd prefer to go. I don't need the formula or the math. Suggestions? Thanks Gerry H

[luvrbus]

If you are driving off the cam pulley all you need is a 3 inch dia pulley on the alternator for most.if you want more output at idle or low rpm go to a 2-1/2 inch on the alternator 

good luck

[Gerry H]

Thank You Clifford. Now to find one! Gerry H

[bevans6]

All the pulleys on a two stroke run at crank speed, including the cam pulleys, so alternator speed is an issue.  You need to try to find the smallest pulley for the alternator you can, to get it to spin as fast as possible.  And use the biggest pulley on the engine that you have access to.

If you have an rpm-output chart for a particular alternator you can use the pulley diameters to find out the speed the alternator will be running.  If you have a 7" engine pulley and a 3" alternator pulley the alternator is running at 2.3 times crank speed.  Most larger alternators start to have some output by 1,000 rpm shaft speed, have decent output (maybe 60% of rated) by 2,000 rpm and are close to maximum output by 4,000 rpm.

Brian

[luvrbus]

His 8v92 has a 6-in cam pulley the blower drive pulley will be 5- in the same dia as a cam pulley on a 8v71 if I remember the 110 amp military alternator is a 6000 rpm alternator

[bevans6]

If it has or he can get a blower drive pulley wouldn't that be around twice crank speed?  That would work out well.  My MCI has a rear pulley but it's drive by the cam.  I didn't know that you could get a blower drive pulley.

Brian

[Fredward]

I drive a standard, internally regulated 12 volt alternator off the 871 cam pulley and it seems to work fine. I've never checked RPM at the alternator. It has a 2-1/2" pulley on it I suppose.

[Gerry H]

Thanks to all for your input: Here's more details. Measuring the outside dimensions of my cam pulley, I get 6-1/4" OD. Don't know how the outside diameter equates to true size. My alt specs say turn on speed 1400 rpm and stable 26V output. The graph shows 110A at 5000 rpm with not much more output at 6000 rpm. I was figuring 5K as the top end, so if the top engine rpm was 2500 rpm and I need the alt to go 5k, then I need a 2 to 1 ratio. So 3" seems about right except at 600 rpm idle, alt would be at 1200 rpm and not on. It would be on at high idle 1000 rpm with alt at 2000 rpm. A 2-1/2" pulley would be a 2.5 ratio and would spin at 6250 rpm when engine at 2500 rpm. ? Maybe too fast? Not planning on that high rpm , but SH. At low idle (600 rpm), the alt would be spinning at 1500 rpm and on. I like. With engine at 2100 rpm, it would spin at 5250 rpm or max output speed. That is if my logic is sound. What do you think? I think Clifford's recommendation seems valid. So where do I find a 2-1/2" x 7/8" Pulley, preferably a quality one, not a cheapy?  Thanks Gerry H

[bevans6]

http://www.surpluscenter.com/Power-Transmission/Pulleys/Finished-Bore-Pulleys/2-75-O-D-7-8-BORE-2-GROOVE-PULLEY-1-2BK27-D.axd

Working diameter is measured down where the belt runs, this pulley is 2.5" with a 5/8" belt, which is what you would probably run.  2500 rpm on your 8V92 you maybe don't have a little overspeed of an alternator as your biggest worry...

Brian

[Gerry H]

I knew somebody would take that wrong (2500 rpm) Gerry H

[bevans6]

My 8V71T is set at 2450 no-load, 2300 under load...  Sometimes they gotta spin...  I figured you left off the  

Seriously, I think the pulley I put a link to would do you fine.  Might need to be trimmed a bit.

Brian  

[TomC]

My 1980 KW 8V-92TA had the alternator running off the blower drive which is about 2:1. Hence both pulleys were the same size.  The Leece-Neville alternator was only replaced one in 800,000mi. Good Luck, TomC