Been a lot of talk in different areas about how much horsepower these diffs will handle. The 8" is a lighter duty than the 9" but works well under a light and medium weight car. True ?


Yes. A lot of street rod guys run 8 inch rear ends.


The 9" seems to have a big difference in various setups like grey cast iron, nodular, open, positraction, locker. Then there are spline sizes like 28, 31, 35 and 40 spline with numerous different carrier bearing sizes. Where does a guy start ? And what kind of car/racing exactly demands "heavy duty" ?


I run a 9 inch in my street rod with a True-Track limited slip. Because of the light weight of the car and the fact I don't drag race it, I used a 28 spline axle. But if I was using it as a street/strip car, I would go with a 31 spline. There would be no need to go with a 35 spline unless it was an all-out strip car with a massive amounts of torque. There are a lot more gear ratio and third member choices with a 9 inch.