Technical How much power will an 8 inch ford take?

Building a model a coupe for a friend, the car has a really nicely installed four bar 8 inch, very nice work indeed. He wants to put a lincolin Versailles 9 inch with ladder bars, and I think that's a mistake, the car is set up with a 409, three twos, more or less a period style car. My guess is regardless of the horses the engine will make, in a 2800 pound car the tires will boil hard long before any damage to the rear end, but I also don't know how strong an 8 inch is.
The other reason I want to stay with the 8 inch is unsprung weight, the 9 inch is damned heavy, in a light car unsprung weight can be a problem. Also, the 9 inch is a disc brake unit, not exactly fitting the look of the car, it suits drum brakes.
Any thoughts?

 

An 8 inch Ford rear axle was designed for use behind 6 cylinder and low power 8 cylinder engines. 175 hp to 275 hp max, with tor questions to match.

The 409 engine your friend is installing will probably make 350 to 400 hp, and a lot of torque. The 8 inch may be built up to take the abuse, with new gears, center section, and hp axles, but at a cost. A nine inch was designed to be used in hp and heavy duty applications, such as Big p***enger cars, and trucks with engines on a par with the 409. It would definitely hold up better than the 8 invh, with no mods, and with hp parts inside, it will be stupid proof.

Dave

 

an important thing to look at is the gear ratio- sites like project33.com have calculators to help figure out what will work with the transmission being used (?) - - a trac-lock (Ford style of positraction) would help too

 

I think you are correct with your thinking. A light weight car should break the tires loose before you break the rear end. I have a 300h.p. 350 crate, S10 T5 and an eight inch in my 32 sedan. If anything goes in my situation I would think it would be the transmission. I've broke the tires loose several times but I alway have the car rolling a bit and then stab it. I guess I must be getting soft the older I get because doing a burn out isnt at the top of my list.

 

I have a 29 Model A Tudor running a warm 289 V8 & C4 ****** (without shift kit) and a 8" diff, ratio is 3.7:1.
It has served me well, but I have not been too harsh on it.
I can say it did crack between the main housing and the cover plate in the center, but who knows that could have just been a bad weld.
My guess it could handle maximum of 250HP and no outrageous punishment.
You are correct, the weight is way less than the usual choice of diff and the still run reasonable sized rear brake drums
I believe my diff came out of one the last flathead powered bodies, but that is just a guess.
Sorry that I don't have any real specific pic's of the diff, but it does run a parallel 4 link suspension setup.
Just don't treat it like in my last photo and you should be OK.

 

My post here is going to be W-A-Y off topic for THIS site, but it gives some credence to the Ford 8 inch rear end in how the OP wants to use one. Some 20 years ago, Hot Rod Magazine did an edition called, Fast and Cheap. They built a real thrown together Chevette powered by a Cadillac 500 CID/TH400 transmission. For the rear end they mounted it with cut in half leaf springs functioning as trailing arms, coil over shocks, and of all things, a Ford EIGHT inch rear end. Car was on slicks, and at Los Angeles County Raceway it dipped easily into the 11's. I guess it held up to the trashing they subjected the car too. Car was called, the Bad Seed. I am Butch/56sedandelivery.

fiftyv8, that does't look like an 8" to me; there's no separate pinion support that I can see. Looks more like an 8-3/4 MOPAR to me, or possibly a 56 and earlier (back to 37?) Olds/Pont.. The car up on the guard rail obviously has a Corvette IRS or Jag setup, and I know nothing about those. Cool photo though!

 

Unless you plan on a set of sticky slicks and 8000RPM launches, a 8" will last a long time.

 

I'll agree with Butch; if the car pictured is the one you're talking about, that's not a Ford rear, looks more like a Mopar.

And the Ford 8" doesn't like too much torque. Ford didn't use it behind anything bigger than the 302 (except for some late '70 heavily detuned 351s) as heavy torque loads would kill the pinion bearings sooner or later. And the Versailles rear, while a strong unit, has those very problematic disc brakes. Hard to work on, and difficult-to-find and expensive parts...

 

I can tell you that the rear is a Ford Diff and from what I know it came out of a 1952 Ford Customline Ranch body hence the lower ratio.
I have a 2nd diff here identical that I removed from a similar car.
The center installs the same as 9" Ford diff.
It was powered by a Ford 239cui flathead engine and I believe was the last year before they had a change of brake shoe/drums sizes.

 

Anyone know the weight difference between 8 and 9 inch?

 

Might be a shoebox rear end. I have seen a lot of them for sale, billed out to be 8" rear ends.

 

^^^^^Yes, I will agree it looks to be the "pre 9 inch" Ford rear end used 56 and earlier (?I'm NOT a Ford guy, so it could be earlier than 56). I think hearing 8 inch, the later 8 inch is what the OP was referring to that looks like a "smaller" 9 inch, is what we all probably thought of. Not the early shoebox style rear end. 8", 9", 8.8" is what's mostly available Ford wise now-a-days. I have seen several later 8" rear ends in early Mustangs, push the entire pinion support right out the front; usually stick shift, high RPM launches, but I've also seen 9" rears do the same thing. Now, how in the h*ll did you manage to get up on a double wall high armor-all guard rail? I am Butch/56sedandelivery.

 

The Versailles rear end is thinner than other 9" in the tubes. While it has disc brakes, that is not an ***et. The parts for them are out-of-production, and are becoming unobtainable. You can retrofit drums, but they won't be "straight up". The housing ends are clocked a bit.

Brakes aside, I would worry about the housing before anything else.

 

Seems like I read on here that the 9" weighs 35 pounds more than the 8"?
Anybody confirm that?

 

65 Comet Caliente convertible with a 289 4 speed and street tires . I broke 6 chunks in about 6 months . Went to a 9 inch and never looked back. This was in 68 and 69 when you could buy a chunk for about 25 bucks at a junk yard.

 

If the rear end is in good shape, the width is ok and the installation is nice I would just run it as is. If he's going traditional style with tall narrow tires he'll probably end up with a 6"x8" contact patch which will spin quite easily with a 409 up front. And I agree, that Versailles rear end isn't the best choice, these old hot rods don't need rear discs.
Unless he's just dying to spend more money, just get it on the road.

 

I have read on this site the difference is about 29 lbs. I think it depends on which 8" or 9" you are weighing. They both vary in width and the 9" also varies in housing thickness.

Ray

 

Well, 30-35 lbs. isn't too bad. About what I gained over the holidays.

 

Maybe what I have is not called an 8 inch diff then, but definitely a Ford item and probably internals from Borg Warner.
Great little diff all the same and way lighter than a Ford 9 inch.

 

8" uses the same axles as a 9" small bearing 28 spline. That leaves the center section as the weak spot.
SPark

 

If that axle is a pre-57 ford p***enger car axle, I've had extremely BAD luck with them. IMHO, they're not suited for any engine over 85 HP. I've broken axle shafts in them with stock merc flatheads. I've broken spider shafts in them too.
Now, I've used an 8 inch ford rear [not a 56 and earlier rear axle] in just one of my hot rods...a 49 ford coupe with a 260-275 HP 350 chev and an automatic trans. The rear axle was out of a V/8 maverick and had 3.00 gears. I was tough on it many times but only with street radial tires...never saw a drag strip. It never gave me grief.

 

Years ago I swapped a 351W into my early Fairlane and asked at the "Rear Ends Only" shop in Vancouver B.C. if the stock 8" diffy would be okay. They told me it was good up to 400 hp.
An acquaintance of mine had a 1969 Mustang with a 351W and 8", so they did come with bigger engines.

 

Here's a Hotrod Magazine article on the 9" and why its so great.

http://www.hotrod.com/articles/9-inch-ford-rearend-strongest-rearend-due-hypoid-distance/

 

And here's one on the 8" and why it's so great

http://www.hotrod.com/articles/0505cct-ford-8-inch-rearend-engine-tech/

 

Maybe we can break this thing down in to lower denominators.
What diffs fall roughly into the era we are talking about?
I know of an 8 inch and a 8 3/4 inch and a 9 inch diff.
Could it be that what we are discussing is actually an 8 3/4 inch diff as used in the early Mustangs!
Some where here I hope we can find some common ground and see if we are all on the same page with our concept of what is an 8 inch diff...

 

The article below says it's 45 pounds.

According to the article promoting 9 inchers, the power to weight ratio of the 9 inch is hard to beat, once you get past a certain hp point.

I'd like to see a price and weight comparison between an 8" and 9" capable of 400hp and 550pf (for an OT Cadillac 500, in a T bucket, at the dragstrip, application).

 

Just a heinous post regarding shameless self promotion ...... BUT, I do have a set of 9 inch 1970/71 Torino, Dutchman forged 31 spline axles, with the Lincoln Versailles disc brake knuckles still pressed on the ends. Nothing else. Axles, with knuckles. Check your lengths, but if anyone needs 'em ..... well ..... I'll be here.

 

I've had good luck with the 8" rears from Mustangs,Granadas & Mavericks. HRP

 

I am not arguing for or against any diff discussed, but do recognise that generally a 9 inch would be preferred for numerous reasons.
The diff I run is definitely not a Mustang style which I call an 8 3/4 inch diff.
The diff I run just happened to be there and serve my needs and I call that an 8 inch.
As hotrodprimer has expressed the Mustang style diffs are pretty darn good and not expensive and pound for pound if the right width for your project, would be difficult to ignore and I figure would easily handle 300+ HP.
I think with a Mustang style diff, possibly the choice of gears would be more limiting and the range of gears found in these diffs would be mostly only around either side of the 3.0:1 ratio region.

 

It will take all you can give it.
Then it wont.
If its in and looking good, get it runnin, you can always re-engineer stuff later.