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Aussie XAXB, help!!
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Posted by: siXXXd7coupe

ok from reading some of your posts and responses to other members posts...you know your stuff...i want to know if i made a mistake in the choice of the rear i bought for my 67. a while back i found a 67 fastback 289 4 speed car sittin in a junkyard while insearch of a rear. i quickly bought it and took it home. what i though was a 9 inch rear turned out to be an 8 inch rear. the ONLY difference i found in my researches was that it was one inch smaller and only had one dimple instead of 2. i also saw that this rear was used in hotrods for ground clearance reasons for lowering..tho i dont know if thats true or not. i recently spend alittle over a grand on an auburn posi center section, precision gear 3:55's, a precision gear complete overhaul kit, end bearings, and seals. i take everything i have to a local speed shop where i became concerned with my decision, which i thought were good at the time. i was told that this rear basically sucked. i was told it would NOT handle over 400 HP even built up like mine will be. i was told that if i try to powerbreak/dump the clutch/burnout that it will not hold up. i estimate that my car will be making around 500hp sprayed 350-370 all motor from a worked 289 backed by a T-5 trans with a dual friction centerforce clutch setup. is this all true? will it not work with my cars power/torque output? the part i dony get it...its not a stock rear any more. the internals are racing parts made to handle power. a housing is a housing...its whats inside that counts..isnt it? please help me out with this. thanks in advance.
Nick

PS i know that there are alot of guys/gals on here that know their stangs..ANY help is greatly appreciated...please help me in getting one step closer to bringing my classic 67 stang back to life.

Posted by: Aussie XAXB

I just ran across your post. Been busy the past few days too.

It is commonly accepted that if you are going to build an engine with more than 300 HP that you should go to a 9" rear. This is what most people will tell you and is probably what they had heard as well, though few people probably actually have experience with an 8" rear going out on them.

There may be a difference in the diameter of the axles between an 8 and 9 incher, off the top of my head I don't know. There are also different shafts available in each, 28 and 31 spline. From a machinists point of view, though the 31 spline is considered stronger there is a question of surface contact being less compared to a 28 splined shaft which could have deeper splines than the 31, that is unless the 31 splines are cut into a larger diameter shaft. That's another issue though.

Here is a link to everything you ever wanted to know about 8 and 9 inch rears. Get something to drink and then click on it and be ready to read.

http://www.kevinstang.com/Ninecase.htm

At the bottom of that page is even more links to more info.

Back to your concern. I have a friend that drag raced a Mustang with an 8 inch rear. It was a stick and he launched at 7,000 RPM when he dumped the clutch. His engine was a 351 Cleveland built to the teeth. The rear axle lasted 4 seasons before he broke it. I think that is some serious duty.

The Auburn posi unit is a very quality piece. It is what I put in my wife's axle. (thought I was going to say rear, eh?) With all new top quality parts used in the rebuild it should be able to hold up to quite a bit. It would handle the most power behind an auto tranny but I see you have a manual. I wouldn't recommend 7,000 RPM clutch drops but the rear should be able to hold up otherwise. Your biggest concern with the 8 incher is twisting the axles while using slicks, if you get that kind of traction. If you really want to be able to sleep well at night get a set of aftermarket axles made from a stronger material.

If your engine is going to make 500 HP that will be a peak rating at higher RPM. If you launch at the RPM that you make peak power, with slicks, dumping the clutch, and get serious traction, yes, it will be rough on factory axles in your 8 incher.

What are the components that you used to build your 289?

Steve

Posted by: siXXXd7coupe

my 289 is bored .30 over with trw forged pistons with a ford motorsports cam. i do not know any other specs on the engine. i know its worked pretty good....id say a hair more the mild. the person who built it builds up 289's and races with them. im sure i can get the exact specs on the cam and all but it might take a while. thanks for the help im going to read everything u sent me tomorrow..itsd 2:33 am here lol...i better get some sleep. thanks again..Nick

Posted by: Aussie XAXB

You're welcome.

The 289 is a pretty good little powerhouse but it needs to rev in order to make up what it lacks in displacement. If you are putting out close to 500 HP (with nitrous) you would be well advised to find out what kind of rods and pistons, and crank, are in there. For a 289 to be making 350 HP is quite an achievement if the engine was built for the street. If it is a race engine it is easier to get 350 HP out of it but is doesn't fare too well on the street. This is why I was asking for engine spec's. If it is running well on the street, in city traffic, and with a smooth idle that can idle all day I wouldn't think it to be in the 350 HP range. If it has a rough idle and idle's a little fast i would think it to be over 300 HP. Even the 289 Hi-po put out 271 HP from the factory, but that was rated "gross" at the flywheel with no accessories, compared to how engines are rate now, "net", at the tranny output shaft with all accessories running.

Steve