Brianr390

So I have a 65 mustang coupe with its original 8 inch rear end. I have been building a 347 stroker motor for the car and have been wondering if i will need to change the rear end. My goal with the motor is to have around 450/450 horsepower/torque at the crank and its going to be backed up by a c4 transmission that is built to handle that power with either a 2800 or 3200 stall converter. Now i have been wondering about the durability of my rear end. Will the stock 8 inch be strong enough to handle the power of the motor. If the rear end isn't strong enough do i need to get a bigger one like an 8.8 or a 9 inch or can i just swap some parts and make it stronger. Thanks for any input.

MonsterBilly

It depends on how you drive it and what tires you are running. the 8 inch rear ends are strong. Lots of people swap to a heavier rear end without the need to. However, if you are going to run slicks and launch really hard, it will most likely be the week link. Will it break.....who knows. The chances of it breaking are greater if you run slicks. As long as your running street tires, even sticky street tires you should be fine.

Brianr390

This is going to be a 100% street car with normal street tires. Sorry I forgot to add that. I will only really want to launch hard maybe rarely at a red light but other than that probably no. I have read so many people switching to 9 inch rears that i thought i would too because of the torque. Can you explain to me why it would be bad to run slicks? I won't but I think it would be good to know.

noroof66

450/450 is an awful lot of power.... If you are only going to launch once in awhile and this is a 100% street car, I don't see a problem with slicks.

Stangman70

Don't mean to hijack this thread, but I had thoughts of swapping to a 9" as well, I don't plan on doing any crazy HP just a reliable DD. I currently have the trusty 8" with 3.00 gears and will be swapping to a t5 shortly. I do have a 9" already out of a 67 with 3.55s but for those of you who have t5's and kept the 8" what gears are you running and should I bother doing the swap if its easier keeping the gears that I have now or should I go ahead and do it?

MonsterBilly

without spending too much time on it, Stangman-if you are switching to the T5 you will definitely want the 3.55 gears or lower. I am certainly not saying you need the 9". but if you have one with those gears, you will definitely want to use it. I have a T5 with 3.73 gears and it is a nice set up. I did have the stock 3.0 gears at first and 5th gear was way too long legged. The gear ratio reduction was a must do.

Brian, the reason the slicks will lead to problems is the added traction will transfer more torque through the rear end. This will stress it much more than if you just have street tires. See when you launch hard the street tires will spin easily. this relieves the torque to some extent on the differential. slick will hook up and force more of that energy through the rear end to move the car. This action will create much more resistance on the differential gears.

One last thought. For everyone out there using T5 transmissions out of a factory car, the 8" rear end has more than enough strength for you. Even the strongest factory T5 transmissions which came from the 1990-92 cobra mustangs were only rated to 310 ftlbs. of torque. The 8" rear end should handle 400ftlbs. Therefore the weak link in this situation would be the T5.
Now if you have a T5 and a factory 8" rear end as stated above, It would be a good idea to switch to a better gear ratio like 3.55 or 3.73's. when doing so you may be better off finding a 9" that will fit your car and going that route other that switching out the ring and pinion in the rear end you already have. (must do cost comparision)
I opted to do go a whole other direction and install a 8.8 out of a ford explorer. I did it because of price. For the Approx. $300 i spent on parts, I ended up with 3.73 gears, trac-loc differential and disc brakes. It was little cash but a good amount of work. back then, I had time.

Stangman70

Monster thanks for the reply, again sorry if I jacked your thread. When you said gear reduction, did you mean just going to the lower rear end gears, or you went to a z-spec trans, if yours was not one already. I ask because the budget will only allow one part to be rebuilt, the rear end or the transmission, but not both. The t5 i have is a 86-87 trans from a low mile car.

palerider

I have an 8" in my 70 with c4. Builder (fordstrokers) rated it at about 400. In any case I got a new center section with trac-loc from a person on ebay - very reputable and good to work with. Its rated to 550. I also just bought one for my 66 8" have not installed yet.

http://www.ebay.com/itm/FORD-REAR-EN...ec5f49&vxp=mtr

MonsterBilly

See Brian you are in about the same situation as palerider. You should be okay. Not to knock it or anything but 450hp/450t is a lot to get out of a 347. It is doable but is stretching it and it will cost you $. I honestly think you will actually be more like 380/380. even so, that is a ****load of power for a 65-66 lightweight car. I can surely tell you that unless you at least mini tub it, you will spend more time spinning than anything else.

I have a very well built 289 and I am pushing about 325 hp and i think my car rips. when i hammer it off the line it doesnt really stop spinning until 3rd gear. If i want to take off fast i have to take off normally then once moving floor the pedal. I am running 245x45x17 rear nitto's and they have a hard time hooking up. They are pretty soft tires too.

Stangman, yes I was referring to dropping down to a 3.55 or 3.73 gear from the stock 3.0. depending on the final gear of your T5. which is most likely .68 or close to it even a lower gear than the 3.73 is okay I dont know what the 5th gear ratio is in the z spec.

120mm

I just learned more about the transmission aft during this thread than in my 38 years of owning and driving Mustangs. Thank you so very much.