how much horsepower can one expect to gain from porting and polishing stock heads on a 98 LS1 engine?

 

well 98 heads are as good as paper weights and not worth the time of porting and polishing. You would be better off buying some ls6 heads and putting them on stock.

but you would probably pick up around 15rwhp.

 

If you have a good port done on them, they can clean up to work pretty well. Better bowls/cathedrals out of it. I would mill them also for more potential. If it is more than 800$ then you would be better off getting a new set of LS6 heads that have 100% cnc.