New alternator has a smaller pulley

J

Jiminwatford

Member
I'm replacing my original 45amp alternator with a 65 amp one

The new pulley is around 2cm smaller

Should I swap pulleys or get a new fanbelt?

Thankingyou
 
Not necessarily an issue. What rpm is the new one rated for? Spinning faster will give you better output while idling obviously. I got a pieve of paper with mine showing the details.

I assume the question means you are now beyond the adjustment.
 
Yes i'm at the limit of adjustment

I can't see what rpm the new one is rated for. What it does say is if the old alternator has a different pulley to swap them over

The fan belt 130mm but i see there is a 1125mm belt fitted to some models so i have ordered one of those

I thought to get the alternator fitted (with the longer belt) i would swap pulleys for now. Unfortunately i couldn't remove the new pulley. Although a local mechanic has a windy gun which may get it off

For the time being i have put the original alternator back on

So i have the choice of new alternator with small pulley and new belt or new alternator with large pulley and original belt

I'm tempted by the higher output of the smaller pulley so unless i get advised against it i will probably fit the new belt when it arrives and use the new alternator with the smaller pulley
 
What are the actual sizes of the old vs new pulley? And the second question is, are you in the habit of winding the engine up to the redline when you drive the car? The first question is so we can figure out how much faster the small pulley will turn. And if the answer to the second question is no, then it’s unlikely there is any danger of you damaging the alternator by overspeeding it.

Yours
Vern
 
According to a Lucas document I have the old 11AC alternator could be run up to 12500 rpm & I seem to remember & I might well be wrong that the Lucas 18ACR or similar can be run up to about 17000 rpm.
 
The best way I have found to size a fanbelt is as follows.

Mount the alternator with the adjustment a third up from fully slack. take some 1" low tack tape ( masking tape ) and run a strip round the OUTSIDE of all the pulleys on top of the vee.
Where the tape overlaps at the join mark with a pen on both parts of the tape. Remove the tape and lay out straight on a bench, measure the length mark to mark, this is the belt length.
By setting a third up from fully slack you will have room to back off the alt to fit the belt, and enough adjustment left to allow taking up any stretch.
 
Vern Klukas said:
What are the actual sizes of the old vs new pulley? And the second question is, are you in the habit of winding the engine up to the redline when you drive the car? The first question is so we can figure out how much faster the small pulley will turn. And if the answer to the second question is no, then it’s unlikely there is any danger of you damaging the alternator by overspeeding it.

Yours
Vern
Click to expand...

The old pulley is 8cm in diameter and the new one 6cm

I do like to take it to the max :)

A couple of pics for side by side comparison and info

IMG_20231124_112630_629.jpgIMG_20231124_112649_178.jpgIMG_20231124_112428_254.jpg
 
colnerov said:
...Or just use a tape rule round the pulleys.
Click to expand...
Been there done that.......... A steel tape won't work as it buckles and kinks and slips off, a fabric dressmakers tape slips off the tops of the vee, low tack tape stays where you put it.
 
cobraboy said:
Been there done that.......... A steel tape won't work as it buckles and kinks and slips off, a fabric dressmakers tape slips off the tops of the vee, low tack tape stays where you put it.
Click to expand...

OK, you do whatever works for you, can't argue with that.:thumb:
 
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