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Discussion Starter #1
I have heard that 8" rears are good until about 400 hp and that 9" 31 spline rears are almost undestructible for street applications. How about a 28 spline 9"? How much power do you think it would take before letting loose? I have one from a Granada that had disc brakes and the axles seem a littl larger in diameter than normal. Thanks for the help and peace of mind.
 

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Yeah, but by your own admission, you can break anything.
 

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If the component you're worried about breaking is the axle and you're running
the same 28spline axle in both the 8" and 9" rearend then I would think the
failure point would be identical (assuming the 9" is not a full floater).
I believe the advantage of the 9" as far as axle strength is that there are
31spline and 35spline carriers available, which are not for 8". Stepping up
from 28spline(1.2") to 31spline(1.315") provides 34% increase in torsional
strength, and 28spline to 35spline(1.5") provides 76% increase in strength.
The 31 and 35 spline axles are also available in higher strength alloys,
which also greatly increase strength.
 

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Given the right circumstances, most anything can be broken, but generally in an 8-inch (even with the revised C7OW '67-up case) the 3rd member is the weakest link. The failure generally occurs with either the differential side bearing mains/bolts breaking off: http://img89.imageshack.us/img89/9885/broken8inch3rdmemberop1.jpg

...or, the pinion support tears out like this: http://img503.imageshack.us/img503/6577/broken8inch3rdmemberfl2.jpg


In a 9-inch --particularly with 28-spline axles, the driver's (short side) axle, is the most likely part to break. If the axle breaks, it can also tear the housing up like the '59 Fairlane 9-inch rear end housing shown in the center of this picture (notice tube of the middle housing leans toward the right where the welds of the tube broke, --where it used to be solidly attached to the center portion of the housing) : http://ultrastang.com/Images/Scanned/Rears/25-s.jpg

All the failures shown in the above photos occured with manual transmissions. An automatic is more forgiving on the rear end than a manual, but even an automatic can still break parts, given enough induced shock-load and traction.
 

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Discussion Starter #8
So I can assume I am good for about 400 hp. I guess I should baby it a bit when everything comes together. The tuner thinks they should be able to get about 450 rwhp. I think they are a bit high but I guess I will find out in about 6 weeks. :p
 

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I run about 375 tq to the wheels.

No problem with my 9" but it's built a little with a detroit locker and
-31 spline strange axles, and a
-1350 series yoke.

I'm told it should break somewhere around 1,500 hp and it could be years before I get there.

Next weakest link would be driveshaft, I run a custom aluminum. You're talking about $700 in upgrades, not including the detroit. I'd rather pay up front than pay a second time.
 

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Discussion Starter #10
I have a new trac lock unit with new gears. My problem is I originally didn't plan on over 400 hp but along came the supercharger. I think my t5 will give up before the rear though. I will just have to drive it with a little respect I suppose.
 

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The biggest reason I chose the T56 viper spec was because I was told the T5 would not last.

I am still very curious what it's breaking point would be and if that would have been a wise, money saving mod.

How long has your T5 lasted? And what power was it seeing?
 

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Discussion Starter #12
I haven't driven the car yet. Well that is not entirely true. It had a 6 cyl and 3spd when I bought it and I drove it on the trailer to go to the body shop. I have been building it for 3+ years. The project just keeps getting bigger the longer it takes. I think if the T5 blows I will just go with a G Force T5. I don't have to get different brackets for the JMC clutch and I know I won't have to modify anything else with the exception of a slip yolk.
 
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