HP limit for the 8" Rear End

Blue67fastback · Mar 7, 2008

Hi Guys,

Anyone had any info on how much horse power my 67 Mustang fastback's stock 8" rear end can handle? I'm not planning on slicks or 1/4 mile. The car has a 5.0 currently at 375HP with the Redneck C4 auto and Richmond 3.50 gears with a Auburn posi unit. So far so good, the Cooper "Cobra" tires are kind of hard, so there is plenty of slip or smoke. I have a Edelbrock nitro kit I want to add for another 100HP shot. Am I about to say goodbye to this unit under a hard accel? Thanks, Bill

SadbutTrue · Mar 7, 2008

Blue67fastback said:
Hi Guys,

Anyone had any info on how much horse power my 67 Mustang fastback's stock 8" rear end can handle? I'm not planning on slicks or 1/4 mile. The car has a 5.0 currently at 375HP with the Redneck C4 auto and Richmond 3.50 gears with a Auburn posi unit. So far so good, the Cooper "Cobra" tires are kind of hard, so there is plenty of slip or smoke. I have a Edelbrock nitro kit I want to add for another 100HP shot. Am I about to say goodbye to this unit under a hard accel? Thanks, Bill

100 shot is a bad idea on the 8". If you aren't using slicks/traction aids or powershifting (which you won't be with the auto) the 8" will probably be okay. The nitrous changes that, though.

Vinyl66 · Mar 7, 2008

Nitrous and slicks do not get along with 8" rears. Saying that, I have MANY runs in the 11's on my 8" rear. A best of 11.47 this past weekend. But I am running drag radials, 3.55 gears, and an automatic. So it will not be nearly as harsh as a similarly built stick car on slicks.

Edbert · Mar 7, 2008

HP doesn't break drive train parts as readily as traction does. General rule of thumb though is the 8" is good out to 300rwhp if it is built with quality components. A stock 8" would be somewhat less than that.

Blue67fastback · Mar 7, 2008

Many thanks for the input. I may have to upgrade to a 9" to do this right I guess. Those rear ends are getting hard to find these days. Bill

jspagna1 · Mar 7, 2008

What I have heard is an 8" will take up to a 400 HP motor if set up right.
The plus is you have an Auto and not a stick.
You do not have slicks,so your ok there.
If your not going to pound the car every weekend.
So I would wait till it blows up on ya, then upgrade at that point. You will be surprised at how much they will take. Mine is going on 12 years and it's still alive.

Blue67fastback · Mar 7, 2008

Yeah, I don't plan to do roll backs! I can turn on the HP when I'm moving at least 15-20 mph. I think that would relieve a large part of the heavy shock that the rear end would normally handle. Besides, if I blow it, it was going to sit in the back yard anyway if I was lucky to find a 9" unit.

D.Hearne · Mar 7, 2008

As others have eluded to, it's not the horsepower you worry about, it's the sudden application of torque, combined with traction that'll do it in. I'd not worry about it. You could though look for a 9" to either narrow in the future or keep your eyes peeled for a bolt in 9 for a good price, and later have it ready to go.

rbohm · Mar 7, 2008

without the nitrous your 8" will do fine. if you use the nitrous as intended, IE coming on at about 2500-3000 rpm, and NOT off the line, the 8" will be stressed and wont last as long. now if you bring the nitrous on in stages, 50hp for the first stage, followed closely by the rest coming in about 100 rpm later, the 8" will handle that for much longer.

Blue67fastback · Mar 7, 2008

That sounds even better. I would be less likely to forget or show off and then blow the rear end. I'm not looking to spend what is needed right now for a 9" unit. But I already have the nitrous kit. So in it will go and then go easy.

ultrastang · Mar 8, 2008

If you're going to the trouble to hop up then engine (at least in my mind) it only makes sense to beef up the rest of the driveline so you don't have to worry about a catastrophic rear end failure. Once bitten by the horsepower bug, it just tends to grow.

Increased engine horse power is a great temptation to show out whenever and wherever possible. It only takes one time to do the wrong thing and then suddenly the guy driving the cool car doesn't look so cool with his car's rear end exploded in front of a group of on-lookers.

Running on the idea "I'll replace it if blows up" might not be a wise idea. A lot of 8-inch rear end failures look like this:
http://img369.imageshack.us/img369/1348/broken8inch3rdmemberfl7.jpg

In this type of failure, you're most likely going to have more than just rear end damage, since the drive shaft is going to be wielding around under the car tearing up the exhaust system and beating the $#!& out of the floor pans in the process.

But, this is your car and your call in what you chose to change or not change. It's only money, parts and sheet metal.

htwheelz67 · Mar 8, 2008

I've been running my 525hp 408 stroker on my 170.000 mile 8" until I install my 9" so far so good but all it does is spin the TIRE thru all gears, if there was traction I'd probably break it already.

The 8" can be built to withstand about 400-450 hp max with aftermarket gears and axles. Beyond that its a crapshoot.

ultrastang · Mar 8, 2008

Another problem with 8-inch breakage doesn't just come from the tires hooking up. Breakage will also often occur when the rear tires goes into wheel-hop under hard acceleration. It's the shock forces from the tire gabbing, then losing traction, then grabbing again (loaded, unloaded, loaded, unloaded) in rapid succsession that causes things to snap.

Blue67fastback · Mar 9, 2008

Thanks again for all this info. I did install a drive shaft loop just in case. But my guess is the 8" will fail sooner than later with the added horsepower. I looking online to see where a 9" unit can be found. I have 3 years of work into this car and spent $50,000+ already. So I guess what's another $1000. bucks at this point!

HP limit for the 8" Rear End

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