does anyone have any idea of what the standard ford 9" axles can hold for horsepower? reason i ask is cause im having a local shop build a 9" for my 91 but i want to have an idea of what i need b4 i call...im lookin to have it built for around 800hp, thats not what mine will make but ive got my whole drivetrain set up like that because id like to eventually see it around there someday. what all will i have to have done to this rear end to hold up to that?

 

I wouldn't even consider using factory axles at the HP level.

Axles are rated for torque, not HP. Moser has an axle calculator on their web site. Each axle should be able to handle all the torque produced so with 2 axles, you have 200% safety range.

Engine torque x first gear ratio x diff ratio x .9 = total torque

Lets say you produce 800 foot pounds of torque from the engine through a TH400 with 2.48 first gear and have 4.56 gears in the diff.

800 x 2.48 x 4.56 x .9 = 8142 pounds of torque.

31 spline axles can handle 7000 pounds
33 spline axles can handle 8200 pounds
35 spline axles can handle 9600 pounds

Based on the above example, you would only need 33 spline axles. My car only uses 31 spline because that's all I need.

 

cool thanks alot can you tell this is my first build lol i think im gonna go with 3.73 gears because its a street strip car right now but is there anything else i need to worry about for that kind of hp/tq

 

Upgrade the entire driveline. Upgrade the entire suspension. Install SFC and probably a full cage.

If you want 800HP, you should upgrade the entire car to handle that kind of power before even thinking about the engine. I doubt you'll get that kind of power in a street car without a BBC or some sort of huge power adder.

You should consider more realistic goals like 500HP.

My engine puts about 750HP to the wheels and is in now way streetable.

 

oh no like i said its no where near that right now but my trans will hold up to about 800 with blue clutches and shift kit the motor is done and will probably push somewhere around 500 n/a all forged internals, im just about to order the driveshaft right now from spohn, http://www.spohn.net/?action=product&pid=1108. the sfc are in the suspension is brand new just got done with the restore im on some of the last steps now. in no way shape or form are my goals set there right now but im not looking to halfass this thing just to get a couple runs and blow the rear end or the trans ya know? i dont want ne of those troubles quite yet. everything is brand new its like a new car now. i built the sub frame connectors myself they are something like the umi set but completely bolt on and i built it with some beefier steel and tied them into the trans crossmember