Rear End: Ford 9" OR change gears on my posi?

[dougfather]

Ok, I was doing some investigation this weekend on my Grand Prix.

I have:

7.5" rear with 2.3x?? gears and a POSI unit. I want to keep the posi, but need to change gears to either a 3.50 or 3.73.

Do I need gears specific for a POSI unit? If so, what is all involved and how much $$$ will I have in this rear? It needs sand blasted and probably rebuilt.

The other option is a Ford 9" from QuickPerformance. A buddy has a 3.50 POSI center unit for a Ford 9" that I could get. http://www.quickperformance.com/Products/housings.htm#Axles Anyone with experience from these guys?

 

[82355supreme]

First off, the only thing you have to make sure of is the series of gears you need. If you look in a Summit or Jegs you'll see Series 1 or Series 2. You have sub 3.08 factory gears, so you'll need Series 2. Series 1 is for factory gears of 3.23 & UP.

As far as price goes, if your engine isn't putting out over 400/400 combo, I'd just go a set of Motive Gear, or shop eBay for gears. You should be able to get the fears & an install kit for $300. I personally have Auburn guts in mine, but it wasn't neccesary.

Lastly, a Ford 9" is a strong unit yes, but what most people don't realize is that a 9" takes another 30 horsepower on average to turn vs your stocker. So, unless you have such a powerful engine that you'll never miss that 30HP, you'll have a stronger rearend, but the car will be a little slower.

 

[KrisW]

You know, for not too much money, you can get a used set of 4th gen f-body gears with a zexel torsen posi (the meat grinder style) for just a few hundred bucks, with many selling for about 100 bucks.

eBay and various parts for sale classified sections have them, even local junkyards. You will need 28 spline axles. I usually buy axles aftermarket from Jeg's but on a severe budget the advice about the series 1 or 2 gears is the best bet.

So far I have broken 3 left side axle shafts in g-body cars with 7.5 axles, none with 8.5 or larger...

 

[Nightmare388]

Ok, I was doing some investigation this weekend on my Grand Prix.

I have:

7.5" rear with 2.3x?? gears and a POSI unit. I want to keep the posi, but need to change gears to either a 3.50 or 3.73.

Do I need gears specific for a POSI unit? If so, what is all involved and how much $$$ will I have in this rear? It needs sand blasted and probably rebuilt.

The other option is a Ford 9" from QuickPerformance. A buddy has a 3.50 POSI center unit for a Ford 9" that I could get. http://www.quickperformance.com/Products/housings.htm#Axles Anyone with experience from these guys?

For the 7.5 you just need to look for gears for the right carrier. Should be able to get off ebay cheap. I got a new rear with axles and gears from Quick Performance of Ebay. The gears and rear with axles are excellent quality and the rear and gears were shipped very fast. I highly recommend them. Here is a couple of pics. I painted it flat black with a skull on housing for the Nightmare theme. I haven't got new pics of it installed yet.

 

[85 Cutlass Brougham]

There are several ways to go about this, but I will tell you how to do it with your original concept of using a 2 series posi carrier with a 3 series gear. You basically have 2 choices. You can either buy the gears new but order them with the thicker ring gear that compensates for the 2 series case, or use a 3 series gear with a ring gear spacer. The thick gear is the better choice of the two as it cuts down on complications and possible failure points. The spacer is a budget choice if you have a open diff axle with a good gear and happen to have a 2 series posi carrier lying around. In that case, you MAY be able to get away with just swapping the case as long as you keep the same case spacers, etc and the wear pattern is not disturbed. I plan to try it myself as I have a $50 3.23 axle and a posi from another $50 axle that had a 2.41. If it breaks, I hope it doesn't hurt anything else, but then again, I am willing to take some educated risks and suffer the consequences of my mistakes if there are any. You may not be. My car also runs street tires only and is not very powerful-only around 350hp/400-425 ft lbs of torque. If you are running anything with a lot of torque, skip doing anything to the 7.5 and just build the 9 inch or get an 8.5. All you are doing with a powerful engine and a 7.5 in a car that hooks is wasting time and money because it's gonna break.

 

[dougfather]

I assume this is what I need?

http://cgi.ebay.com/ebaymotors/GM-Chevy ... enameZWD1V

 

[82355supreme]

That's exactly what you need there, plus an install kit. Don't forget that!! :wink: