Strength of 9 inch?

[Stangboy67]

I have been searching but have not run across an answer to this yet. Based on info I found here, I'm about to acquire a 9 inch out of a Lincoln to drop into (under rather) my '67 stang. My question is how much power (assuming decent traction) can the 28 spline axles handle vs upgrading to 31 spline axles? I'm not concerned in the least with my current combo, however future plans include a decent 347 stroker, which is my reason in going to a 9 inch in the first place. Thoughts??

 

[Bullitt]

i broke a 9" with 325 WHP/317 TQ.

5spd car, on launch, with 3.73s and Nitto DRs.

car held up for roughly 120 drag strip passes up to that point though (about 90 of those on DRs).

 

[302 coupe]

the lincoln axles are some of the beefier ones, they don't taper down like most others (mine don't at least). I'd say that with an auto trans and real slicks it would safely handle 400-450 hp. With a manual trans and real slicks, mayeb 375-425 hp, shock loads will kill axles/diffs. If you're running faux slicks (drag radials), you can beat up an axle pretty quick. The short and stiff sidewalls, coupled with decent traction, would kill 'em pretty quick, like Bullits experience above.

Note that hardened 28 spline axles are available and will work with the stock 28 spline diff. To go with 31 splines you'll have to run a 31 spline diff. You can figure new axles to cost around $300 a set, depending on who/where you buy them.

 

[brianj5600]

Let's not forget first gear ratio and rear gear ratio when it comes to axle and pumpkin strength. Moser has a calculator for axle torque. A 5 speed with a 4.11 rear will be alot harder on axles than my c4 and 3.08 gears. 400 ft/lbs of torque put through the above combos will transfer 5343 lbs for the first combo (400x3.25x4.11) and 3030 lbs for the second (400x2.46x3.08). 5 speeds in general have much lower first gears than autos and will be harder on parts even w/o shock loads. Math is your friend.

 

[302 coupe]

good point, hadn't thought of that.

 

[gzminiz]

hum, i really thought a 9" with the larger main bearing (non tapered axle) would handle more power. I am putting together a motor that should be around 450-500 hp (at the crank i believe) and figured the 9" would handle that with a manual tranny. I really don't want to break the rear end.

Bullitt - did you have any indications as to why it failed besides too much power (325 hp is far from what i am expecting at the the wheels)? Was it rev'd to high on launch? I am concerned that after a few launches i will blow out the rear end. Getting used to the car will surely include to high rpm launches.

 

[steel1212]

If your worried about it get some hardened 31 splined axels and center and you should be good to go. In my opinion if your building a HP/TQ monster it should be a no brainer. I'm probably going to get some hardened 28 splined axels for mine and have the whole rear rebuilt and it won't be anywhere near 400hp.

 

[gzminiz]

that was the plan but just worried after bullitt said he blew one with a motor running 325hp.

 

[10secgoal]

Stock is stock. You will be fine with good 28 spline. They are good for more HP than anything most of us can afford to build.

 

[brianj5600]

Bullit, what broke?
Stangboy, If you are seriously going to build a stout 347, I don't think 28s will last. If you buy anything other than a complete rear end, spend money once and get good stuff, 31 or bigger axles, aftermarket chunk, big bearing pinion support, and quality diff. Sometimes a failure causes total distruction and you have to start all over. Oh, and use a rev limiter. It is not good when a motor free revs when something breaks.

 

[blown65]

Well, Ive been on my twisted 28 spline axle for about 4 yrs now. LOL

Ive put plenty of power thru those poor axles, still going ok. (Splines barely twisted)

Plan on upgrading soon but not till im done with the other few things I have planned.


I should add, are you planning an automatic or stick. Auto is a tad more friendly than a stick when your dumping the clutch. Ive done numerous full boost brake launches with my car (500lbft tq at 2500 rpm)

 

[10secgoal]

Bullit, what broke ?
Stangboy, If you are seriously going to build a stout 347, I don't think 28s will last. If you buy anything other than a complete rear end, spend money once and get good stuff, 31 or bigger axles

I say that because John Calvert is running 9's with his small bearing rear in factory stock.
Bullit broke his stock axles in a versilles rear end.

 

[brianj5600]

I say that because John Calvert is running 9's with his small bearing rear in factory stock.

Are you saying it is a production center? And 28 spline axles? An "N" case won't be any cheaper than an aftermarket case. If the case fails it will atleast take the gears out. If it pushes the pinion out the trans is in danger and the gears are trash. IMO, standard cases are a crap shoot. Some live forever, other don't.

 

[10secgoal]

Stock center....dunno. He did say he was running 28 splines. Imagine he can't stray too far from factory stock. Even at the time I talked to him, no one made a bearing that would take a 31 spline to fit a small case, he said. I have only heard of one person breaking a stock center. But he was also running 5.30's in the 1/8 with 31 splines.

 

[Bullitt]

well, 2 things broke in what order i don't know. it was on launch (5000 RPM) on drag radials.

the splined section of the left axle broke off. however when this happened the bearing cap on the 3rd member broke... and then the diff internals destroyed themselves. the only parts to survive unscathed were the gears.

this was a Currie built 3rd member with 3.73 gears with ~20k miles on it.

i'd known the axles were slightly bent, but never enough to make me worry about it that much. i guess that is probably what screwed me though.

my advice is to thoroughly get your junkyard parts checked out. it'll save you money in the long run... my $900 diff broke, i had to buy $400 Moser axles and i cheaped out and got a $450 used diff.

expensive lesson! be careful of boneyard parts!

EDIT: it's 325 to the wheels... it's a roughly 390-400 HP motor at the crank. with 380ish torque.

 

[Stangboy67]

Wow! Thanks for all the responses. The rear assembly I look at yesterday was far too wide and had to pass it up. I will keep looking for the right housing. From "listening" to your inputs, maybe 31 spline should be the way to go. I am(and will) be running a stick with the car. Even though it will be mainly a street car, it will be making some trips down the track, and I can envision the use of slicks, just to see what it will do (just one more tenth, oh maybe one more .)

Also, a side question. Anyone have experience with Versiallias (sp) disk breaks and whether or not they will fit inside 14" wheels. Thats what are currently on the car. This I plan to change down the road as well as you can't get decent tires in 14"?

Again thanks for all your help.

 

[Bullitt]

i think you'll have to run 15s with versailles brakes.

but i haven't measured or anything like that.

 

[Stangboy67]

After my experience Friday, I was kinda assuming that. When I end up with a new set of wheels, I'll probably get 16". Big enuf for good rubber, but not too large.

Thanks

 

[my66coupe]

Im running a 56 Farilane 9" with a stock cast center, 28 spliners and a full spool. Ive done about 20 passes on full slicks and nitrous (100 shot) off the line. Still holding up fine. Ive been told that the full spool adds alot more strength to the entire system...

Mike

 

[Stangboy67]

Ive been told that the full spool adds alot more strength to the entire system

I wonder if that is due to the spool eliminating and slack in the system? Interesting.