Replacing your Rear
SUBJECT: REPLACING YOUR REAR END (pictures below)
My rear axle seems to be making many unpleasant noises, has a lot of mileage on it and probably needs work. What are my options and what should I be concerned with?
Before doing anything, you should make sure that the noises are really
coming from the axle. Noises in the drive line are very tricky and hairy to solve as all the metal parts transmit noise all up and down it’s
length. What sounds like a drive line noise, may, in fact, be coming from the brakes, transmission, U joints, et; If all is well in these
areas, then these are the usual options:
1. Get a used axle with same ratio and posi. The problem is, there are very few axles available with less than 75Kmiles for under $500.00 and
they are mostly 2.73:1 ratios. This is also a very ‘high wear’ part. These things are usually abused by kids doing 4,000 rpm dropped shifts.
These are also NOT high strength Dana or Ford 9 inchers or even a GM 8.5
incher, that could take the guff. This axle was a ‘point price’ unit
that the bean counters - Sorry Jeff G. ;-) - helped with the engineering. We all know what that means.... Also, the axle was made for
the cars’ space limitations as the axle has a lot of vertical travel to absorb bumps. Unlike a more space saving independent system, a live axle
requires a lot of room and can’t be too large.
2. Have the axle rebuilt by someone who knows what they are doing. Around Connecticut, there is: Bill Mitchell=Special Vehicle Development
in Cheshire CT. He’s an ex GM Engineer who also races and he’d be happy
to rebuild it to a 3.73:1, but, it was going to cost me about $2,250.00 with taxes. The new parts are real pricey and the labor was also
expensive, but, it’d be done perfectly. There is also Reeves Calloway, but, I’ve not personally
dealt with him.
3. Get a 4rth gen axle. Unfortunately, all the mounting brackets are different and the whole unit is 4” wider than the 3rd gen unit. A
proportioning valve also needs to be installed in line with the master cylinder to make the rear brakes function properly. This is now looking
like complex and expensive surgery if you want the car to have a stock track width. The axle may only be a few hundred, but, the time and
expense to get it in correctly may not be worth while. This site does have a procedure from Jeff Cooper on replacing the stock axle with a
4rth gen unit, but, there is a warning along with it. Its up to you ....
4. Have the dealer rebuild the axle. This is expensive and not all dealerships have people well enough versed in axle rebuilding as most
cars today are FWD. The entire transaxle is usually replaced as a unit. Also, the ring and pinion gears aren’t normally on a ‘bi-level’ relationship to each other. This is done with rear axles to keep the
driveshaft low to reduce driveshaft tunnel height. This makes it very tricky to align the gears properly. It’s almost a lost art, and very few
mechanics these days have the experience or the patience to do this kind of work.
5. Get a rear axle as a complete assembly with all new parts from some dedicated service. This would include a completely refurbished axle
housing and all new, stronger than OEM innards for a really good price. All Timken bearings, Federal Mogul seals, 7 5/8” OEM ring & pinion -
good for up to about 350 HP - and Moser Eng. high strength race quality axle and hubs. Total cost, $1370.00 plus differential choice and
shipping for a ‘bolt in’ turn key assembly. Should I change my ratio from stock?
If you have 2.73s, most definitely, yes. What to go to? If you want tremendous acceleration but still be streetable and have a manual
transmission with overdrive, 3.73:1 should be the maximum, whether for a 305 or 350 engine. Just make sure you have the 5,500 RPM camshaft
because you’ll be using all of your available rpms. 1rst will be a creeper, and all other gears will be moved down about 1 gear range.
Like, 5th will be where 4rth was, 4rth will be where 3rd was, et. The engine will run about 600 rpm higher in all gear ranges. 5,000 rpm will
still get you to 150 MPH in 5th. Gas mileage may suffer or stay the same depending on driving stile. Starting up from a dead stop in 2nd will be
a breeze. Especially with a 5.7l TPI.
If you have an automatic 4 speed, go with a max of 3.42:1 as the torque converter in the auto multiplies torque and you’ll still be able to keep
some semblance of gas mileage.
What kind of differential should I go with?
This depends on your application. If you’re drag racing all the time, you probably want the Auburn posi. They go for about $280-$400 depending
on application and where you buy it. If you buy from the dealership, a differential will already be bolted to the ring gear. This pinion, ring
gear and attached differential carrier is called a ‘chuck.’ This is the
only way these parts are purchased from the dealer. This parts are not sold individually. The differential carrier can be either be a posi or
an open carrier. The dealership can supply any ratio chuck you desire. But, you’ll pay through the nose. Make sure you ask for your bar of soap
and your kiss before hand, as they always seem to forget that after they take the money.
If you want maximum traction, especially in the wet, and want the best traction device possible, the Zexel Torsen is superb. It has all the
best of both worlds. It has the smoothness of an ‘open carrier’ with
more ‘locking’ ability than a standard posi, with completely variable torque division between the back wheels. Instead of over coming
‘stiction’ of the clutches, which are not linear in torque transfer
between the rear wheels, the Torsen provides smooth, variable torque to the wheel that still has traction. It works with the ‘worm gear’
principal, in that, you can turn the worm gear itself but the worm gear can not be driven because the gear pitch is too perpendicular. This is
the same action that goes on inside this type of diff. The torque is instantly transferred because the wheel that’s loose is now working
against the worm connected to the wheel with traction; frictional resistance of pitch. Bias transfer ratio is adjustable by varying the amount of gear tooth pitch of the worm gears. It’s a very complicated
mathematical issue. Lots of Cos ß’s and Cos <theta>‘s. There is a link
provided on the site to explain the Torsen operation. It’s quite unique.
The Torsen is used in such cars as BMW, Lexus, Nissan, HummVee, Subaru, Audi
Quatro, et. The Torsen is used in many of these cars. In 4 wheel
drive applications, it’s mounted between the front and rear axles. It’s
also used as a front wheel drive posi traction device.
Its main failing is that the Torsen is torque limited and care must be made when selecting for high HP applications - 400 HP and above. When a
Torsen breaks, in simply turns into an ‘open’ differential. At least if
you have to replace it, its pretty simple for a competent mechanic to do, as the ring an pinion gear lashes don’t have to be reset.
If I change my ratio, will it affect my speedometer and odometer? What can I do about it?
Yes, and your speedo can be re calibrated. It’s expensive to do a direct
conversion because the speedo drive gear is mounted on the output shaft of the transmission. This requires taking apart the tranny to change the ‘drive’ gear. There is a much easier solution. A company in California
makes a mechanical ratio re calibration box that mounts in line with the speedo cable. It’s mountable at either the transmission or on the back
of the speedo.
You have to remember that if you go from a 2.73:1 to a 3.73:1, that is a 36.6% change. This will make your speedo read about 10mph faster than
actual road speed. The best and easiest way to determine this is to go out onto the highway, look for the mileage markers on the side of the
road. Note your odo reading as the first marker passes by the front of your hood. Then note it again as the following marker passes your hood. Deduct one reading from the other. In the case of this kind of ratio
change, one mile will be about 1.15 miles on the difference. This is
about a 22-23% change. You’ll need to do this with your car to get an accurate re calibration. The name of the company is:
http://www.speedometershop.com
The cost of the unit is a reasonable $61.00 + shipping, and they take major credit cards. Ask for Carl.
This is pretty important if your speedo is an Carter Admin. 85 mph unit. It’ll loose about 10 mph of usefulness without re calibration.
Also, if your car is equipped with cruise control, it probably makes the most sense to connect the ratio converter to the transmission side of
the speedo cable, as there may be two cables, one going to the cruise control controller and a cable running from the cruise controller to the
speedometer gage head.
If I do go with a whole axle assembly, where can I get one?
There is really only one place; Art Houser’s Rear End Services, or AHRES
for short, or simply, the Rearman. One problem is getting a good rear
axle housing that’s been fully inspected, as GM probably doesn’t stock
these anymore, and if they do, they probably cost a small fortune.
Advantages:
• The Axle Housing:
You can get a fully inspected, degreased, sandblasted axle housing that is painted with rust resistant primer and a black epoxy gloss type
paint. It is magnifluxed for cracks and reinforced welded in known weak
areas. The connection mounting brackets and plates appear to be from a type of carbon steel, rather than cast.
• The innards:
All the best. For standard high performance street applications under 400 HP: OEM ring and pinion, Auburn,
Torsen, et. differential,
Timken bearings, Federal Mogul seals and Moser Engineering high strength steel axles and Hub spools. Also included is a quality lubricant, all
the wheel lug studs - high strength - and a high strength driveshaft
collar yoke.
AHRES has been lashing and patterning rear axles since 1973 and does about 4,000 per year. If you do that many, how can you not be good at
it?
• Pricing:
A typical 3rd gen axle with the above specifications will run about $1770.00 plus shipping to your area. It will take about 3-5 hours to
install at your local garage if you don’t do it your self. It does weigh
in at about 200-250 lbs, so its’ major surgery. Your brake callipers
will need to be transferred. So, total cost should be in the $2020.00 area. Again, a lot of money, but, if you can install it your self,
you’ll save a lot.
• Available Kinds:
Ford 9”, GM 8.5”, Dana, GM 75/8”, all kinds and custom. Just ask. They are exceptionally friendly and Arts helper, Yvonne, can help set
you up. Just see their link on the link’s portion of the site.
• Disadvantages:
Cost. Working with this major part of your car ain’t cheap and can be more than buying a new
transmission when all is said and done; if not
abused, it should last the life of the car.
• Time:
For now, AHRES is moving into a new location. There may be delays due to this. It might take up to six weeks to get the axle delivered at
the present time. AHRES also has contracts with local police, fire and ambulance services. If their vehicles break, Art’s got to fix them first, so your project might have to be temporarily shunted to the side.
Also, summer time is car show time and the racing season. So, a delay might occur due to racers getting their cars ready for the season. They do a lot of that, and they can get really loaded in the summer.
Recommend that the winter might be the best time as they might not be
quite so jammed.
• Payment:
AHRES does do layaways and they accept checks but no credit cards. Fortunately, they are reasonably honest and provide a very high quality product. They are very well known in the industry.
What kind of unexpected things can happen when changing the rear axle in this way?
Some of the brake calliper holes might not have threads. Your local shop can easily helicoil the holes as it’s not that big a deal. Your brake
lines might be corroded and have to be replaced. Your callipers or drum
brakes might not be in the best shape and need R/R. It’s also a good idea to check out your universals and motor/tranny mounts, just in case.
It’s always a good idea to catch problems before they occur down the road. Also, there may be the usual damaged rubber mounts, bushings, et.
What if I notice a lot more vibration at high speeds? What can I do about it? Its' possible that your driveshaft is going out of balance at the new higher rotational speeds. Since it's spinning about 37% faster from a 2.73:1 gear set to 3.73:1, it may not be balanced for those kind of RPMs with the 3.73s. You might want to try to contact a shop near you that can rebalance the driveshaft, or check out an aluminum one.
What if I have a problem?
One of the biggest problems is shipping damage. Be sure to inspect the axle or have your mechanic inspect the axle thoroughly for shipping
damage. Damage can occur internally or externally. Any damage should be photographed and the shipper needs to be informed immediately.
I some ways, you’re kind of on your own, unless you can get the axle back to it’s builder. If it’s something like a leaky seal, you can
either bring/send the axle back to AHRES or they will send you new parts under a 1 year warranty. If you can do the work yourself, that’s fine.
But, a lot of us would have to have this professionally installed. Art
will gladly send the new part gratis, but, if the shop you take it to messes up, its now your problem. Unfortunately, I’m in the process of
getting a leaky pinion seal resolved. The original seal was leaking and Art and Yvonne were kind enough to not only send me a new seal, but also
pay for the first labor to have it installed. Unfortunately, the new seal is now leaking and its looking like improper installation. This is
an area of the car I seem to have problems with. AHRES is sending me another new seal gratis, and I will have to see if the shop who
installed the last seal will reinstall the new one under their warranty.
If not, I will probably just go to the dealer and have them install one of their seals so I can at least get warranty on it. Bringing in someone
else’s part to a dealership allows them to finger any blame for repair induced problems elsewhere. Unfortunately, this is one of the risks of
buying a large, complex to assemble subsystem which carries fluids. At some point in the future, I hope tohave this problem resolved.
What about getting an aluminum driveshaft with 3.73:1 gearing?
If you stay with 3.42:1 or lower gearing, an aluminum driveshaft can be very beneficial for acceleration. However, if you select 3.73:1 gearing,
you may run into severe vibration problems. Racers are generally unconcerned about vibration just as long as they win the race, or all
the parts last the race. But, when you’re pleasure cruising on the highway, the level of vibration might be too intense. I called Driveline
Systems, and they said my steel driveshaft will have a critical balance/speed at 5,000 RPM shaft rotation at 130 MPH. I do get some
‘roughness’ in the drivetrain now at highway speeds when going from 2.73
ot 3.73:1s. In the F-body, driveline vibrations are a real problem. Going to a light weight shaft will only make things worse as the
drivetrain depends on the ‘mass’ of the shaft to help quell vibrations.
A light shaft will not be able to do this as effectively. The vibration stems from the front of the driveshaft and newer steel
shafts now come with harmonic balancers on the front for helping reduce drivetrain
vibrations. I was told by a GM Engineer that this is why 3.73s were rarely installed on the F-body due to customer complaints. Gas mileage
could always be compensated for with juggling transmission gear ratios. And 4.10s would be totally unliveable.
Another point to consider, is that this ratio is too short for cars equipped with a automatic transmission. 3.73s are really only for use in
manual transmission cars with a minimum of 5 speeds. Otherwise, your fuel mileage will really suffer.
Additional information for those wanting 12 bolt rear axles:
Art Houser writes:
“Some F-body guys have been pursuing a 12-bolt option. Everyone we know who tried it had a problem, and they waited at least four months to
receive an unsatisfactory rear. Even if it would have no problem, it is no match for a 9-inch conversion. F-body owners, don't waste your money.
Art won't even pursue this option, since we've seen the heartaches.”
