Thread: Ford 8" and 9" Diffs what horsepower will they handle ?

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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 ?

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" ?

So everybody that runs them must have a different opinion of them.

Guess who and what technics are used to build it. I'm no pro but from what i think i know is a stock nine will hold 600hp and the 8 will 400 hp, But i also know i heard they all break. I also heard the 28 and 31 are fine for any street build but racing is a different game. I like to be corrected because i learn more. I guess to much HP everything breaks .

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.

No mater what I build I always put a 9" under it. Why put in an 8" and later wish I'd gone biger. Besides I think the 9"s are easier to get parts for, and I hate having different types around it just confuses me so I stay with what I know works.

The number 1 thing that kills the rear end is big tires.You could put a 8 inch behind Kenny bernstiens dragster long as you had 4 inch wide tires

Its easier to get gears for the 9s [4.10s &4.57s came stock in older Broncos]
+ they are way more durable than the 8s.

Bob

I have an 8 inch in the Studebaker. The Maverick 8 inch is about as narrow as they come. When I was shopping for limited slip and gears, I found that stuff for the 8 inch were harder to come by and more expensive. The 9 inch is heavier, so if unsprung weight is an issue for you, take that into consideration. If I was starting from scratch I believe I would go with a 9 inch for the reasons the guys stated above. Just like a small block Chevy, there is more stuff available for them at a lower price.

Pat