alternator pulley diameter

[Dany]

good morning chaps

 

i know this subject has been dicused in the past but despite my researches i can't find the right answer

 

my engine is a 1.9 k series fitted with std crankshaft pulley

alternator is a denso 45 amp

 

i have been adviced in the past to use a 80mm pulley, but my car is used mainly on road and obviously in traffic and i am aware i could have thickover issue because the alternator won't be spinning fast enough and battery could drain.

 

i am in the process of making a custom pulley and i would like to know what pulley size BOBC use

 

cheers

 

[Nigel Riches]

Dany if you don't have a "charge" warning light issue, then you probably don't need to worry about the battery draining whilst idling.

 

Provided the alternator is "on line" it will be fine, and a 45 amp alternator is plenty for a 7.

 

Nigel.

 

 

[SLR No.77]

Dany, when I rebuilt my 1900K I fitted an R500 crank pulley, running with the original SLR (standard?) alternator. No charging issues whatsoever but it did mean sourcing the correct belt size, although not really an issue.

 

Stu.

[LesG]

Hi Dany,

I fitted a Denso alternator off a Kubota machine, and it had a vee-belt pulley on it, which I'm guessing yours has too?

As a Kubota only revs to about 4000 rpm, and my K does (about) twice that 😬, I decided on a pulley approx half the original size. Conveniently, CC online store listed an alloy pulley of near enough diameter (for lhd R500 iirc), and I just milled a little off the end to get the spacing correct. Works a treat!

[Dany]

thanks for your advices guys

 

i have received my new denso alternator today, it comes with a data sheet that says maximum output (59 amp) is reached at 6000 rpm, whereas lower output is 24amp at 1600 rpm

 

i don't know if 6000 rpm is the maximum allowed, but i guess it would be wise to gear the alternator as close as possible to this specs.

 

i can't change the crank pulley because my engine has been balanced so i'll try to find a oversized pulley

 

cheers

[Mankee]

I have found the below simulator fun/useful when messing around with pulley sizes, belt lengths and rotation speeds, especially as I can't be arsed to sit there trying to work it all out for myself:

 

http://www.blocklayer.com/pulley-belt.aspx

[Dany]

Usefull link!

What is the maximum rpm for a small alternator? IICR it is something around 15/16000 rpm, isn ´t it?

[ECR]

I guess that depends on the quality of the bearings/alternator. Brise (which is based on a Denso alternator AFAIK) say 16000 RPM max and advised me the following:

 

. As long as the alternator does not over rev it will be ok. It is sensible to specify pulley sizes such that the alternator is running at 1500 or more rpm or thereabouts for most of the time. This is the revs where the alternator starts to deliver useful power to the battery. If the car was only going to be driven round town at low revs then a smaller pulley on the alternator would keep it in the power producing area. On the other hand a small pulley on a race engine might well over rev the alternator. Within reason the pulley size is not super critical

[Dany]

i'll go for a 75 mm pulley. will give around 27 amp at idle and max alternator RPM will be Under 15 000 rpm

we will see if the alternator last a little longer this way.