Why no 8.5" rear conversions for us?

Someone needs to fabricate an "Xtreme Street Duty" 8.5" replacement rear for our cars. Should be aimed at average to mid level (11's) consumers - economically priced ($1500-$1700 entry) as a competitor to, well, nothing.

The 12bolt/9" option is very nice for guys looking to run 8's,9's,10's but what about the average joe that may or may not race - or perhaps the guy that wants to cut 1.5's on a fairly quick car but can't drop $2200 or more on a pretty much race unit?

8.5" are readily available and takes much less power to turn not to mention they are strong. I realize that a big holdup would be the TA mount - but what if someone could develop a mount that would hold up to say 1.50 60's - wouldn't this be a much better alternative than a 12bolt or 9inch rear?

Just throwing out some ideas - I know that nobody has tried to sell them - but I also know that at least one board member has one in his car.

If someone could fab one up - NOT for uber fast guys - would this be a good buy? You could always option up/down on pieces to either cut the cost or increase it depending on your desires.

Just thinking out loud here guys.

The 8.5" would be a nice replacement but myself would much rather see some of the big names offer a 8.8" bolt in rear for our cars. Parts are cheap and they have proven to be reliable and strong. I believe the problem is that even if a couple vendors open there eyes and offer what we are looking for I believe that they will still end up falling in to the 9" and 12 bolt category of pricing. I say this because as much as people complain about the price we always end up ponying up the cash for the 12 bolt or 9" when it comes down to it. So obviously we are willing to pay whatever they cost.

I'm not sure what costs would be on a project like that - if it worked well. I would love to see a more inexpensive alternative to the 12 and 9 especially since I really can't afford to lose the HP that those 2 suck up - and my car will probably never utilize the strength of those rears. Not too mention the cost of a new 12 would probably be around $2500 when all is said and done. I know we have to pay to play but damn we're getting hammered on a unit that some of us may never need.

I broke my first 10 bolt while running 14.20's - imagine having a strong replacement rear that you could get for $1000 less than a unit that you may never need. I put down 431 on the dyno - i'd probably drop to 380 or so just by installing a 12bolt - would cost me even more money to get the car back to the 430 mark - granted you could launch 'harder' but...

The idea is great IMO I just wish it would come true.

The 8.8 is another alternative - I would think there's a little more work involved in it tho?

I have thought about this as well. Especially with the recent magazine coverage of the 8.5 buildup.
You would think there would be a very good market for a much stronger rear (relative to the 7.5) that would cost half what a 12 bolt or 9 inch costs.
1200 dollars for a rear that could stand up to 5 or 600 horse? Sign me up.

There are companies out there already doing the 8.8". There are two that I know of, but cannot remember their names. A search would bring them up.

The problem they are having is that the fabbed torque arm mounts seem to be failing. This is unfortunate as I really would like to use one of these. They have a T2R for this diff, but not 9" or 12 bolts.:(

www.extremechassis.com was one of them but I thought they had issues and quit doing them - I could be wrong on this.

The TA mount seems to be the holdup - damnit I wish I had the tools to try this out.

I am aware of another individual on another board that made a jig for the 8.8" rear end conversion and it came out very well, he has had it in his car for a while now without any issues. The cost came out to be alot higher then he previously thought it would be. When i spoke with him about making more of these he seemed interested in doing so, but felt that it wasn't worth doing on an individual basis, he tried that before and didn't get enough interest from people. I was also informed that if I was able to get 10 people together he would make the rears and the price would be $850 for the housing only. What I find pretty cool about this conversion is there are factory Ford differentials that seem to be holding up very well in the high HP cars, and if I am not mistaken they come in 28 and 31 spline. This is cool because the guys that would rather spend less money have a strong differential where you could reuse your stock 7.5" axles(28 spline). This means there would be a wide variety of options for each rear ranging from Mild, for those with modest power, and wild for some high HP cars.

The idea is great IMO I just wish it would come true.

The 8.8 is another alternative - I would think there's a little more work involved in it tho?

It can't be much more work to do the same to a 8.8, you have to do the same exact things to it. Both would be strong but I would think they price would be around the same if someone did either one, and the 8.8 is a stronger lighter rear.

The problem is after market parts tend to cost about the same for an 8.8 or 10 bolt as the same parts for a 9inch or 12 bolt. Gears, axles, bearings, seals, labor for install is gonna cost the same for just about any rear end. The housing is about the only place where price could differ. A custom built rear end housing is going to cost as much or more as a mass produced rear end housing like ex a 9 inch from moser. The only key to a cheap rear end is a 9 inch with a used Factory Ford center section or to be able to fab and build a rear yourself.

I think some of you are missing the point - i'm not talking about building a race 8.5/8.8, i'm talking about building a mid level rearend that can handle 250-450hp

An 8.8 with 28 spline stock axles would be better than a 7.5" all day - no need to upgrade to the trick equipment. What i'd like to see is a rear designed to replace broken 7.5"'s but NOT designed for race cars. I broke my 7.5" while running a mere 14.0's - now, if I decided that I still wanted to race my car but not mod it - why should I be forced to buy a 12bolt or 9" which would gobble up the very little horsepower that I had?

word brother! i wish we had this too, for my simple bolt on car, it just isnt fun thinking that later on ill have to buy a new rear for <380rwhp :(

I would be interested in something like this as well. I don't ever plan on using slicks or drag racing my car, its daily driven and autox, but have already broke my stock 10-bolt launching on the street with street tires when I was at 263rwhp. I would love to see someone make some 8.8" rears for our cars, as they are light, strong, can use a T2R, and cost for parts are similar. That way when I go heads and cam, I wont have to worry about breaking the rear end.

I'm so glad the average 4th genner hasn't realized the 9-bolt is a direct bolt in for the 4 gen and even 2 inches narrower so you can stick wider wheels in there.
Leaves more cheap ones for me :)

I'm so glad the average 4th genner hasn't realized the 9-bolt is a direct bolt in for the 4 gen and even 2 inches narrower so you can stick wider wheels in there.
Same problem as a 9" and 12 bolts. No T2R available.

I'm so glad the average 4th genner hasn't realized the 9-bolt is a direct bolt in for the 4 gen and even 2 inches narrower so you can stick wider wheels in there.
Leaves more cheap ones for me :)

Thats not that great of a rear either, I have broken several. It is nice that its narrower but thats about it. Besides that the limited choices of gears and parts and the outrageous price of those parts is another downfall.

They aren't a 9" but I blew up my 10-bolt but my 9 has worked perfectly behind my big engine. The definalty hold more then a 10 bolt.
Common sense tells you 4 spiders are much better then 2. Plus some other things. Overall a very good rear for the price.
They're cheap from the junkyard :D

They aren't a 9" but I blew up my 10-bolt but my 9 has worked perfectly behind my big engine. The definalty hold more then a 10 bolt.
Common sense tells you 4 spiders are much better then 2. Plus some other things. Overall a very good rear for the price.
They're cheap from the junkyard :D

You must be one of the lucky ones who can pick them up cheap out of a junkyard. When i broke mine I always went looking and they were wanting like $250 for a 2.77 rear, and around $400+ for the 3.27 and 3.45 rears. I ended up just paying for it and I still needed to replace bearings. The brake mounting is also different on those rears compared to the newer ones as well.

If you want a cheap rear end that'll do what your asking just go buy a Moser 9 inch housing and 31 spline axle setup. They run about 9-950 shipped from many vendors. Then either go to ebay and buy a used 9 inch 31 spline 3.50+ posi center section out of a pick up. I've seen them as cheap as 150 buy it now on ebay. So with shipping its probably 250. You can either bolt it in and go or get the posi rebuilt. So for probably 12-1300 you can have a 9 inch that'll do what you want. For another 50-100 you can build your own 3 channel abs setup for it and go.

If you want a cheap rear end that'll do what your asking just go buy a Moser 9 inch housing and 31 spline axle setup. They run about 9-950 shipped from many vendors. Then either go to ebay and buy a used 9 inch 31 spline 3.50+ posi center section out of a pick up. I've seen them as cheap as 150 buy it now on ebay. So with shipping its probably 250. You can either bolt it in and go or get the posi rebuilt. So for probably 12-1300 you can have a 9 inch that'll do what you want. For another 50-100 you can build your own 3 channel abs setup for it and go.


Again, let's assume for a second here that we're talking about some guy that puts down 250rwhp - why should he sacrifice soaking up all of that HP that a 9" is going to rob him of? Put yourself in the shoes of the guys that put down between stock and say 450 - these guys don't necessarily NEED a 9" or 12bolt. In fact, it's going to hurt them more than anything (Speaking on the lower end of those numbers at least).

Again, let's assume for a second here that we're talking about some guy that puts down 250rwhp - why should he sacrifice soaking up all of that HP that a 9" is going to rob him of? Put yourself in the shoes of the guys that put down between stock and say 450 - these guys don't necessarily NEED a 9" or 12bolt. In fact, it's going to hurt them more than anything (Speaking on the lower end of those numbers at least).

Well you're too picky you can't have everything. You want a cheap but good rear that doesn't eat up power. Its kind of like having a cammed car. You want a big cam to make power but you don't want to give up drivability and gas mileage. You must make some concessions sometimes with your car unless you have the ability to make it on your own. I also think you overrate the amount of power lost. You mention droping approx. 50 rwhp by adding a 12 bolt. I think at most a person would lose would be 20 and thats pushing it. Also what you tend to lose in hp you gain in ET because you are able to hook so much better and have it survive.

Well you're too picky you can't have everything. You want a cheap but good rear that doesn't eat up power. Its kind of like having a cammed car. You want a big cam to make power but you don't want to give up drivability and gas mileage. You must make some concessions sometimes with your car unless you have the ability to make it on your own. I also think you overrate the amount of power lost. You mention droping approx. 50 rwhp by adding a 12 bolt. I think at most a person would lose would be 20 and thats pushing it. Also what you tend to lose in hp you gain in ET because you are able to hook so much better and have it survive.


How am I being too picky? $1500 for a rearend is cheap? I guess everyones perception of "cheap" is different as well.

I also think you overrate the amount of power lost. You mention droping approx. 50 rwhp by adding a 12 bolt. I think at most a person would lose would be 20 and thats pushing it.

EDIT: I must say that I am grossly mistaken here with the following comments. After searching for our dyno results - it was the switch from the T56 to the A4 that had the drop in HP for Grease. I'm leaving my post here - to show that I can eat crow with the best. :D

I'll talk to him tomorrow and find out what the drop was with the 12 bolt install as obviously - I can't remember.

You "think" and I know - we put GREASE's car on the dyno after installing his 12bolt dropped from the 412 listed on AI's website to 360ish - after tweaking we got it back up to 390 I believe. Had we played with PCMforless's mail order tune before we installed the 12bolt we would have seen a pretty big increase in power but we didn't - so the drop is relative. Does every car lose that much? No, probably not but that's what I personally saw on the dyno so that's what I'm going with.

Also, I agree that you can launch harder but that doesn't mean you're going to hook any better - a rearend is not a traction device. A stronger rearend will survive a harder launch but that doesn't mean squat if you can't get it to hook with proper suspension and traction pieces.

Bottom line is this - there is a genuine desire for people that don't plan on flogging their cars at the track all the time to have a relatively inexpensive (or cheap) replacement rearend that is stronger than the crappy 7.5" that comes stock in our cars. I would think that these same people would like to install said rearend without fear of losing any of the little power that they may currently have - whether that be 20, 30, 50, 100 - whatever.

I'm just surprised that no company has stepped up and addressed this - the market would be there I think.

My first post, just recently bought a '94 a4 Z28 I remember these cars when they were new in HS, when I worked at the dealer. Anyway.....I have alot of experience with the GM 8.5,12 bolt, 7.5.Also Ford 8.8 in friends and brothers stangs. Did all the rear work on a friends Buick regal with a 400 P, also my old 80 bird went 9.50's w/an 8.5 w/GM posi. I also have 2 Chevelle 12 bolts in my old GTO's and an old 8.2 10 bolt from my old Chevelle. I know how crappy those 7.5 rears are, the original "vega" rear. We tried to be on the cheap with my friends buick and used a jy s-10 3.73 non posi, ended up blowing the pin out. Then an 89 TA posi and jy 28 spline axles and 4.10's from a s10 4wd, ended up breaking the side gears. The car only ran high 12's low 13's. I am afraid to run the Z28 hard for fear of the rear. Why did GM use those crap rears??? What is considered junk the 8.2 would be better! I even thought about that since it is just laying there, of course 'cept for the ta mnt. BTW: the 8.8, 8.5 and 12 bolt are real close internally 'cept for ring gear size,the 12 has an 8.875 ring gear.The 9 in uses the most power. Sorry about the long post for my first one.

Thanks for the post, and welcome to the board.:)

Who knows why GM would put a piece of garbage at one of the most abused and vulnerable parts of the car. Let's give the car rods that will withstand almost twice the power, but give a rear that isn't good enough for stock. Yeah, that sounds like the ticket!:confused:

Why would it be any cheaper than a 12-bolt or 9"?

Rich

I'm getting ready to start my 8.5 inch ten bolt project. I think with a eaton posi and what ever gear set I would be able to get the rear end out the door using stock brakes and rear girdle for around 1600-1700 no problem. Now that would be for a 4 channel and I would have to work on a 3 channel but I dont see that being too much of an issue. Of course the torque arm mount is the biggest issue but I wont weld it too the cast iron. The tubes would be welded thou too the diff itself. I'll keep the board notified as too the progress and let you know how it turns out. I will take my time with it thou so that I get it done right. will post photos when done.

I'm with Rich... I don't see if any of the major companies offered a drop-in 8.5" for the F-bodies how it could be any cheaper than a 9" or 12-bolt. Just adding up the cost of gearsets, gear install kits, axles, differentials, etc., etc.... the $2000-$2500 cost of brand new, ready-to-run 9", 12-bolt, and now Dana 60 rears really doesn't seem that expensive to me. Heck, people are paying $1000 for a set of headers - looking at the huge difference in materials, machine costs, etc. a $2000 rear seems like a lot better deal.

The 8.5" 10-bolt truly is the rear end that GM should have put in the car at the factory, but it doesn't seem like a logical upgrade path to me. Tbyrne is running a GP on the new Strange F-body Dana 60 rear... $2049. This rear is the closest thing to indestructable for our cars, and is theoretically more efficient than the stock 10-bolt due to the pinion shaft / axle centerline relationship.

All this talk of losing big HP with a 12-bolt just doesn't make sense. The 12-bolt and 10-bolt have very similar geometry... and similar efficiency. Now, a 9" is less efficient, but the new Dana 60 is actually more efficient.

just added up all the parts for the conversion and the most expensive part or it is the posi. All the other stuff can be had for under 250 bucks. And thats including the backing plates for the axles. all said and done the conversion can be done for around 1200-1300 dollars depending on gear set and extras that some might want. and for that you have a rearend that is damn strong and will handle just about all the abuse you can throw at it. now for those cars that are running in the low tens and faster then ya start looking for the dana 60 and the 9inch fords.

Don't forget that you need to fab the housing with proper mounts for the LCA's, shocks, PHR, anti-roll bar. And don't underestimate how difficult it will be to fab a TA mount that won't end up breaking. Time is money, and Strange already has all that stuff done for you with their Dana 60.

The 8.5" 10-bolt is a good rear... that's what's in my Blazer, and I like it. The idea of a strong F-body rear for under $1500 is certainly appealing, but if/when my 10-bolt breaks, I'm going overkill with the Dana 60.

For what it's worth, there are a number of 2-ton 10-second B-bodies running 8.5" 10 bolts. http://dan.esteban.com/impalass/ReturnTopET.asp?Sort=60&SelectedClass=

There are many different meanings of the word "cheap". Time has a value. The $/h value of your time varies tremendously with whatever else is going on in your life. Piecing together a rear end and fabbing it up takes a lot of time. Also, parts bought on eBay are a crapshoot. If we are comparing costs, using a low price seen on eBay as the typical price for a part is the most optimistic scenario.

My point is simply that for comparable quality parts and convenience of installation, putting together a 12-bolt or 9" is not going to much more expensive than an 8.5" 10-bolt when the same assumptions are made about how much fab is going into it and how much scrounging for used parts you are willing to do. I admire the old "hot-rodding" mentality that is represented by scrounging up the stuff you need to do the job at hand. If you have the time, interest, and skills needed that's cool! Just pointing out that to some extent this discussion is "apples to oranges". The answer to the original question as I understood it is still the same - for someone to come up with a bolt-in 8.5" 10-bolt conversion wouldn't be significantly less expensive than a 12-bolt or 9". This is not to say that a skilled/motivated person couldn't do it on the cheap. But that isn't what most people mean by a "conversion".

Rich

i totally agree with everything DarkHorse has said. and to people who dont think the big rear ends rob power, a couple of my friends cars have lost two tenths in the quarter after swapping to a 9"

The 8.5" is what should have been in these cars from the factory. Theyre light axles that can hold a lot of torque and take a beating.

I guess what im not understanding is what is so difficult and expensive for a company to make a mold for an 8.5" housing with torque arm accomodations. Just seems to me like someone could easily turn a profit designing one, then mass producing and selling them for like $250. If someone does that... bingo: instant cheap conversion kit for someone who is inclined enough to fab the rest of his cheaply aquired 8.5" 10 bolt to fbody specifications.

Im no ASE mechanic by any means, but I would much rather attempt fabbing my own 8.5" ten bolt for ~$750. As hobbyists, we have a way of getting through the tough stuff to save money and get the job done. Im a college student with no money and less time, and id still rather spend the time when it comes to my car.

I guess what im not understanding is what is so difficult and expensive for a company to make a mold for an 8.5" housing with torque arm accomodations. Just seems to me like someone could easily turn a profit designing one, then mass producing and selling them for like $250. If someone does that... bingo: instant cheap conversion kit for someone who is inclined enough to fab the rest of his cheaply aquired 8.5" 10 bolt to fbody specifications.


just because you get the housing done for a few $$$ doesn't mean you still don't have to buy new gears, bearings, posi, better axles, and pay someone to set it up for you. That is where all the cost is at.

just because you get the housing done for a few $$$ doesn't mean you still don't have to buy new gears, bearings, posi, better axles, and pay someone to set it up for you. That is where all the cost is at.
I would use the gears and axles (that i would fab) from the "donor" 8.5" that i would be swapping the housing on. most 8.5" seem to have 3.42s /3.73s/4.10s already in them. I would also attempt to set it up myself. i figure everythings worth a try, if i dont succeed then i can pay somebody :shrug:

... and to people who dont think the big rear ends rob power, a couple of my friends cars have lost two tenths in the quarter after swapping to a 9" ...

No one has questioned that the 9" is less efficient than the stock 10-bolt... that's a well-known fact. But you will not see that kind of drop in efficiency with a 12-bolt or Dana 60.

All these rears are a little heavier than the stocker, so that will slow you down a tiny bit.

just adding here, my buddy had a 9" rear installed at extreme and they dyno'd it before and after. it lost 7RWHP. which equates to roughly 10-11HP from the flywheel. no track times but Im sure it would slow him a bit with the added weight and less RWHP but he upgraded the ring/pinion to 3:89's from 3:23's. so that wouldnt be an apple's to apple's comparo anyway. does anyone know if the dana 60 rear's are heavier then stock? lighter then 12bolt?

does anyone know if the dana 60 rear's are heavier then stock? lighter then 12bolt?
Weight is the biggest drawback to the Dana 60... According to http://www.tbyrnemotorsports.com/ the Dana 60 weighs 215lbs, compared to 189lbs for the 12-bolt, and around 200lbs for the 9". I can't remember what a stock 7 5/8 10-bolt weighs, but its definitely a lot less than 215.

at least its weight that is added to the best possible spot to help traction.