Can penalize google for exchange links with pages of different thematic. For example a website about dogs exchange links with a web about cars, will penalize?

Those links will have virtually no value. Never exchange links with unrelated sites. They never help and can possibly hurt (such as if they are part of a link farm).

I agree google takes 2 things into conscideration when determining the value of your links. First what is the pr of the link and second how is the link related to yours. So in this case a dog website has nothing to do with cars other than drool and rides. It is better to get a link that has a lower pr from a site that is relevant to yours than a link from a page with a higher pr that is irrelevant to your theme.

I agree google takes 2 things into conscideration when determining the value of your links. First what is the pr of the link and second how is the link related to yours. So in this case a dog website has nothing to do with cars other than drool and rides. It is better to get a link that has a lower pr from a site that is relevant to yours than a link from a page with a higher pr that is irrelevant to your theme.

Interesting opinion. Thanks for yours comments