OK, so I found an 8 inch third member with a 3.55 gear and a trac loc that I can get for just under $500. However, it is only a 2 pinion trac loc. Also, I found a nine inch out of a71-73 mach 1. It has a 3.00 gear, and I find out tomorrow if it is a trac loc or not. Now... if it is an open diff, then Im definately not going to get it because getting a posi is the whole point of doing this anyway. I have a 3.50 gear that I can put in it that a buddy will give me if I go with the 9, but I would still need to get all of the brake parts, new trac loc clutches, and possibly bearings, so worst case scenario maybe $800, which is a lot more than I want to spend. So it could end up being good or bad as far as what the 9 will cost me.

I am leaning heavily towards the 8 inch because of several reasons. First of all, I know it is good to go, and Im getting clutches with it, so it'll be good as new after I rebuild the posi. Secondly, I am wanting to go on some of the 45th anniversary mustangs across America run, and would rather do that on a 3.00 gear (current ratio) than a 3.55 (or 3.50 with the 9 if I went that route). So when that time comes, I could simply swap out the 3rd members in my 8 inch in a couple of hours and be good to go on the 1000+ mile trip with the higher gear.

Now, the pros of the 9: Stronger Ring gear and pinion. Slightly stronger 3rd member. Possible 4 pinion posi.
Cons of the 9: Couldn't easily swap out 3rd members for long trips without buying another 3rd member. Would have a lot more parts to buy to make it useable.

So... what do you all think? Like I said, I am leaning heavily towards the 8 inch third member because it could be swapped out in a few hours, should be strong enough [&:], and would be a lot cheaper.

 

what rear end do you have now?

you can skip all that business and worn out clutch packs and used parts that dont fit and just buy a tru trac for 500 and put a set of 3.00 gears on it (guessing you have an 8" now) Its plenty tough and if your under 350hp i dont see an issue.

Why do you want to ditch your current rear end so badly did I miss something?

-Gun

 

+1 on the true trac.

what is the size difference between stock 8" and stock 9" axles? Is this something you've looked into at all?

 

I have an 8 now, yes. I also am making about 400-425 HP. Also, it does see the strip occasionally, and it will be having drag radials on some spare rims now to run at the track only, so it should hook really well. There is also a possibility of a 100 shot of nitrous too. I want a 3.50-3.55 gear which will help out at the track and be better for back road driving. So, if I buy a true trac for $500 and then a gear for $150, and the different bearings that the true trac requires for $100, then I have $750 in a MY current third member. Why do that when I can get a complete 3rd member with a trac loc and a 3.55 already set up for just under $500? It just doesn't make sense to build mine. If I go this route I can just swap out the "trac locked" 3.55 3rd member for the open 3.00 stock one for long trips that I may want to take.

The axle shaft size is exactly the same for both 8 and 9 inch axles (28 spline stock). The strength difference is in the third member itself.

There is just alot of hype about how the 9 is sooo much better than an 8, but I personally think the 8 will be fine. Thhis post is to basically see what you all would do if you were in my shoes and were as tight on money as I am.

 

that sounds reasonable.

The 8" might hold up with slicks i have no idea.

Think the clutch pack for the 8" will hold up? I have heard different stories some say they lasted a while others had them burn out after a bit of normal street driving.

-Gun

 

Well, its not a daily driver for me, and I am wanting to try and put an extra plate and clutch in it to make it tighter, so it should last as far as that goes.

 

I don't think so... not with the combination of parts listed in your sig line.... maybe 325-350.

But anyways, here's my thoughts...

If you don't get the 9" rearend now, you're wasting your time/money. The 8" can withstand a pretty good beating in stock form. It can be beefed up to handle torque similar to a 9". But in the end, it's still going to be an 8" rearend. You've sunk considerable time and money into the rest of the drivetrain, why start "skimping" now?

If you build the 8"... within a year or two, you'll be right back here talking about upgrading to a 9". Save yourself the money in the long run and do it right, do it once.

Yes, the 8" is "sooo much better". Chevy guys use the Ford 9" in their high HP/TQ hot-rods.... they don't swap in the 8".

Dave

 

You dont think so huh??? I think thats pretty funny! Hot rod magazine made 400 Hp on a bone stcok302 with AFR165 heads and a stock cam. As a matter of fact, the only things it had done was the heads, rocker arms (1.7 ratio), and explorerupper/lower intake.I have 50 more Cubic inches and a very nice cam, intake, and carb. Not to mention the compression difference.So yeah, pretty sure that its at the 400 mark or just above .

 

tyler i have faith in 400 to the crank.

dave i have faith at 325-350 at the wheels.

tyler i think you should save or wait for a nine. it weems that youve got quite the performance streak as it is and daves right eventually youll want the nine. if its not your daily driver why dont you wait and just keep looking for a deal.

i think with the nine youll save money down the road. after a while i can see the 8 inch wearing out over repeated strip runs with drag radials. maybe not break, but wear out,

 

Exactly.

I have to call the guy back here in about half an hour to find out if the nine inch is open or trac loc. If it is a trac loc, is $500 a good deal for a complete drum to drum axle that will bolt right in? How about if its an open diff, is $500 still a good price? If it is open I am going to make him a low ball offer, maybe $300, and see what he says.