Just how strong/weak is a 8 inch rear? Im wanting to put together a mild 347 with a 5 speed in a 67 Coupe for the street . Just wondering if it will last or if they are a lost cause.

 

Depends how strong the 3rd member is, and the axles.

Are you talking about stock?

 

A good 4-pinion diff (or a TrueTrac) in an 8" would probably be fine for that motor as long as you're not using slicks.

I'd be more worried about getting a 5sp that will hold up. Even the T5z is only rated to 330ft-lbs, which is easy to exceed with a 347.

 

Get one of ModernDrivelines T5's, they have one rated to 475hp.

 

Also look at a G-Force T5, they are improved as well. That 8in should be just fine with a 347 stroker. The 8in doesn't get the respect it deserves, they can handle 400hp just fine and have seen them in 500hp cars but that is a ticking time bomb.

 

The reading I've seen shows the 8" stands up better against an auto, rather than a manual because of the sudden shock factor of a clutch. Auto's tend to pre-load, so parts don't suddenly break. I think the 400-450hp rating is fine for an 8", but that's probably near the limit. Torque is another story.

 

I actually won't recommend G-Force kits anymore because, to my knowedge, neither will Modern Driveline. They ran into an issue with them where the output shafts (iirc) on the G-Force units were all a hair too large, causing the 5th gear syncro to wear out extremely prematurely. They may have fixed this, but I haven't heard anything about it. Last I heard, Bruce decided to stop offering the G-Force kits.

For this reason, I decided on a TKO600 for my conversion

 

TKO 600, good plan! I am using a TKO600 behind my Boss 347 engine. An 8 inch will hold up unless you are drag racing regularly. Just be prepared for that one time when you decide to get on it at the stop light and you leave a trail of rear end parts behind. A 9 inch is better insurance.

 

My friend has a 347 and a detroit true trac in hers and runs 12.20's and has over 400 passes on it with slicks...it'll hold fine as will the t-5...I have a 347 and a 100 shot in my 85 and I run slicks and it's a stock 300 ft lbs t-5...the t-5 will hold fine as long as you don't do 6k launches, I have easily a 100+ passes on my t-5.....anyway my 66 has a t-5 too and a 4 pinion 8 inch rear and runs low 13's....wheel hop will gernade a 2 pinion as I did that a month ago so as starfury states get a good 4 pinion, I got mine from Randy's ring and pinion and is a HD YUKON 4 PINION

 

Wow I was completely unaware of that. Now I may begin to rethink my t5 conversion. What do you have to do different compared to a t5? Modify the trans tunnel or anything extra?