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Gear Ratio Terminology
Gear Ratio Terminology
The terms higher and lower can get a little confusing when reffering to
rearend ratios. If someone changes to a higher gear ratio they are actually
changing to a numerically lower ratio. If you were to swap a 2.93:1 gearset
with a 3.42:1 gearset you would be switching to a lower gear even though the
ratio of driveshaft turns to turns of the wheel is higher. If you swap a
3.42:1 with a 2.93:1 you are switching to a higher ratio. I don't how
high ended up meaning low numerically or low ended up meaning high
numerically, but they do.
A higher gear ratio would improve mileage and lessen wear on
the engine somewhat but at the cost of less acceleration. A car is also
capable of a higher top speed with a higher gear. Cars that are meant to do
a lot of interstate cruising have higher gears, generally in the
2.48 - 3.08:1 range. Cars meant for acceleration have lower gears, usually
something numerically above 3.08:1.
A numerically higher ratio will cause the same tire travel but at higher
RPM. For example, a 3.42:1 ratio will cause the same tire travel but
at higher RPM than a 2.93.
A lower gear (3.42:1) will require a lesser amount of torque than a higher
gear (2.93:1) to accelerate at a given rate. A higher gear will require
more torque to accelerate at a given rate.
An engine that puts out a small amount of torque will generally be better
off with a lower gear ratio. An engine that puts out a lot of torque will
accelerate quite well with a higher gear ratio. That's why the 4500lb 1970
Delta 88 with the police package could run 0-60 in something like 6.9
seconds despite having a 2.93:1 rearend ratio. It had the 455 with 500
ft/lb of torque and didn't need a very low gear to get up to speed quickly.
(it also had an estimated top speed of 140). Those of us who just want
insane acceleration put a low gear behind a big 'ol 455 in a light car and
then spend the rest of their days trying to figure out how to eliminate tire
spin!
[ Thanks to Greg Pruett for this information ]
Determining Gear Ratio
You will have to put the rear up on jackstands, put the transmission in
neutral. Make sure that the car is quite stable, get under the car, and
TURN THE DRIVESHAFT. Count the number of turns of the driveshaft it takes
to turn one wheel one time. If you turn a wheel, instead of turning the
driveshaft, you will only turn the other wheel (not the driveshaft).
It also works better (more accurate) if you turn the driveshaft enough
times to turn one wheel ten times, and count the number of times you turn
the driveshaft. This is because it may be difficult to tell if the driveshaft
goes around, say, 2.56 or 2.78 times for one turn of a tire. But if you
turn the driveshaft enough times to turn a wheel ten times, you will have
25.6, or 27.8, or 41.1 (or whatever) turns of the driveshaft. Then just
divide by ten to get the ratio of the rear.
Note also that there are two different ring gear carriers, one for 3-series
ratio gears and one for 4-series gears. The difference is the dimension
from the ring gear mounting flange to the pinion centerline, due to the
larger diameter pinion required for the lower numerical gear ratios. A
set of 4-series gears can be installed in the 3-series carrier with a spacer
and longer bolts (not desireable due to reduced strength), but the reverse
is not true.
Formulas
These formulas are for giving you a good idea of what to expect not 100%
accurate, but pretty close.
Some things you need to remember is that if your tire is 25" standing
still it will be taller at speed. How much depends on tire construction, rubber
compound, inflaltion. It will be shorter on acceleration depending on
engine power, wheel spin. Then you have tranny slippage and probably other
small variables.
| To figure rpm: | 168 * gear ratio * mph / tire radius |
| To figure mph: | tire radius / 168 * rpm / gear ratio |
| To figure gear ratio: | tire radius * rpm / 168 / mph |
| To figure tire radius: | 168 * mph * gear ratio / rpm |
[ Thanks to Daren for this information ]
N-bolt Terminology
The tern "n-bolt", as pertaining to the rear end of the car, refers
to the number of bolts retaining the ring gear to the carrier inside the
differential. Fortunately (or unfortunately), this number usually corresponds
to the number of bolts holding the differential cover, so you can usually
tell whether you have a "10-bolt" or "12-bolt" rear
from the number of bolts on the outside of the casing. This is independent
of the rear end gear ratio.
The Type O (Oldsmobile) 12-bolt rear ends use a smaller ring gear than
the Type C (Chevy) 12-bolt, resulting in lower strength. The good news
it that the Type O units retain the axle shafts at their outboard ends,
while the Type C units use the infamous c-clips at the imboard end. This
is why you see C-Clip Eliminator kits available for the Chevy axles. Also
the Chevy rear has a "scalloped" cover where the Olds is smooth.
N-bolt Differences
An Olds 12 bolt rear end is not a "true" 12-bolt. It may have
12 bolts on the outside cover, but it is a 10 bolt gear on the inside.
The more common type "C" is very much different than the type
"O".
A "true 12-bolt" has come to mean a Chevy c-clip 12-bolt rear.
Oldsmobile 12 bolt rear ends differ from Chevys internally. In other words,
the guts of a Chevy rear end will not fit in an Olds rear end housing,
and vice versa. The phrase, "more like a 10-bolt", is usually
talking about the size of the ring gear and/or strength of the carrier.
The guts of a 10 bolt would not work or fit in a 12 bolt, and vice versa.
There are differences between the 10 bolt and the 12 bolt both internally
and externally.
Note also that there are two different ring gear carriers, one for 3-series
ratio gears and one for 4-series gears. 3 series and 4 series carriers are for
the 12 bolt Chevys. The difference is the dimension
from the ring gear mounting flange to the pinion centerline, due to the
larger diameter pinion required for the lower numerical gear ratios. A
set of 4-series gears can be installed in the 3-series carrier with a spacer
and longer bolts (not desireable due to reduced strength), but the reverse
is not true.
The ring gear size is the size of the largest gear, parallel to the wheels.
A 12 bolt Olds has a 10 bolt carrier that measures 8.3", whereas a
12 bolt Chevy has a 12 bolt carrier that measures 8.8". A 10 bolt
Olds out of a 1971 has a 10 bolt that measures 8.5". 1968 and 1969 10
bolts might have an 8.2" ring gear. Starting in 1970, 10 bolts might
be 8.5". Newer 10 bolts found under the last RWD Cutlass and Chevy
are little 7.5".
Oldsmobile bolted the axles from the inside of the housing, whereas
Chevy used c-clips. As for Buick and Pontiac, they used different types
of posi actuating items called "cones" and "spools",
whereas 12 bolt Chevys
and Olds used what are called "clutch packs".
Type "O" Oldsmobile Rear Ends
The biggest weak spots in Olds rears are the wheel bearings and the availability
of parts. Positive axle retention is good, but put in the new "updated"
style wheel bearings for more strength.
Gears for the Olds 12 bolt rear are hard to find. The "O" was only
manufactured for 2-3 years. So there isn't enough call for someone to
start casting and machining new cases. While posi cases are not available
(as far as I know) rebuild kits are. Unless the case you have is outright
cracked or otherwise junk you should be able to rebuild it.
Type "C" Chevy Rear Ends
This rear end retains the axle shafts with "C"
clips. These do break from time to time, and the result is the axle shaft
works its way out of the axle tube. A hint that a clip might be broken
is a faint moan from the rear axle when backing up (mine made no noise
going forward). Moral of the story: stick with your Olds rear.
The "C" rear can be modified with "a C-clip eliminator
kit". That solves that problem of relatively fragile broken "C"
clips.
If you want/need a stronger more available rear, the "C" 12
bolt, will bolt in with no changes to your car. It came in the C*evelle
after all. The rear U-joint will need to be changed to a hybrid one.
NAPA and others carry such a U-joint.
Given the "C" 12-bolt is almost a 9" ring gear, parts
are as common as sand on the beach, and almost as cheap. It has some redeeming
values once modified for performance. NHRA won't allow the faster cars
to run one without the eliminator.
Type "B" Buick Rear Ends
Type "P" Pontiac Rear Ends
Ford 9" Rear End
The 9 inch Ford is the setup these days for many of the same reasons
as the Chevy; plus you can change gears by yanking the axels
and swapping in the "pumpkin". But getting one setup to bolt
into the "A" or "G" bodies is much more expensive
at the outset.
The best will have a large "N" cast into the front of the case,
between the webs. This indicates it is made of nodular iron, as opposed
to slightly weaker regular iron.
Get a 9" out of the boneyard for the axles and the center section,
and buy a custom housing from Moser Engineering or Currie Enterprises
with your exact suspension system mounts. You could also use the boneyard
housing and narrow it, but, the two places I mentioned do it all for you.
Pay particular attention to the brakes and wheel bolt pattern on the 9"
you pull out. You could need a redrilled bolt pattern to match your wheels.
You can weld brackets onto the narrowed rear yourself, but I think this is
something worth farming out, unless you are/know a good welder.
[ Thanks to Cliff Feiler, Brian Kennedy for this information ]
Differential Usage
Actually, 1968 to 1970 Olds 442's (and I believe all Olds for that matter)
built in the US plants all had the Type O rear axle. All of the 442's built
in the Oshawa, Ontario plant had the Type C (Chevy) axle. In 1971 they went
to a GM corporate 10 bolt.
The ring gear size in the 1985 to 1987 442's is the 8.5". Very
desireable. The same is used in the Grand National. The Monte SS
has the 7.5/7.626" rear. Not nearly as hot as the 8.5" rear.
There were a number of differing 10 bolt differentials made during the
1970's by GM. And all the internals are different - nothing swaps.
Posi or Open Carrier
With both rear wheels off the ground (or front wheels for you Toro-heads!),
turn one wheel. If both wheels turn in
the same direction, then it is a posi. If they turn in opposite directions,
then it is an open carrier or blown posi.
A posi differential will have a metal tag attached to a lower right
hand cover bolt warning about the type of diff. oil to use. Open diffs
usually have no tags, except Chevy diffs usually have a code and ratio
tag attached to a lower left cover bolt.
Posi (short for posi-traction) is an option that is not specific to
amount of bolts (ie. 10 bolt or 12 bolt), but was a factory option. You
can change a single track car to posi by putting in a positraction differential.
In other words you can put a posi unit in a 10 or 12 bolt, as long as you
have the right posi unit. There are posi units for 10 bolts, and posi units
for 12 bolts (not interchangeable).
Clutch and Cone Anti-Spin
The cone units have advantages and disadvantages versus the clutch type
units. The most immediate advantage is that they seem to work fine with most
any gear lube. Friction discs and clutch plates are not used. Resistance is
achieved by the steel cone (which has a series of raised spiral groves)
rubbing on the smooth steel case. Under acceleration the cone is forced into
the case which results in limited slip. I have never had to use lubes
specifically formulated for posi units in my cone posi's. Even though the car
came with a limited slip lube tag on the housing. Mobil synthetic works
fine, as is. But doesn't work fine in the C rear without additive.
When a cone posi wears out it is because the cone has bottomed out in the
case. While you can't make it good as new, you can restore it to like new
or better limited slip action by grinding the lip off the bottom of the cones
and deepening the recess in the case. Olds/Pontiac cone posi's use a split
case. Open it up by removing the eight (I think) grade 8 bolts which hold
the two sections together. Grinding a ¼ total separation between the case
and cone is more than enough. I then use side bearing shims to take up the
slack in the gears. These are the same shims you need to use to set backlash.
You put them between the splined cone and splined driven sections on each
side. Installation is a lot easier if you use an axel shaft to keep the
splines aligned. If you over pack the shims, the cone unit will actually act
like a locker style rear. Been there, done that and changed it to a more
reasonable preload.
You can also shim, modify or replace the triple coil springs which serve as
preload. I've used an old exhaust valve to increase the preload once.
Increased it a LOT. The case halves should go together with some preload,
just don't over do it.
I don't know how long one restored this way will last, I'm still using one
after several years, lots of street use and some drag racing. It still works
great.
All the ruined cone type posi's I have seen were severly abused. They
should last quite a while if not abused. I still have the _original_ cone
posi that shipped with my '67 in the car. It has never worn out. The
restored unit cost me $10 bucks from a Pontiac guy in Hemmings.
If I had access to either cone or friction plate posi's I would go with
the cone, first because the original poster of this question stated he was
told they are cheaper. Next, in my experience, the cone works as well or
better than the more common design. And, given the smaller ring gear, they
are stonger than the other designs (four pinion gears is much more than
double the strength of two). The case is also almost full circle, there is no
need for a large opening through which the plates and frictions are
installed. Only enough space to ensure full lubrication is needed.
FWIW, this is the type of posi Olds used for the 66-67 W-30's. As noted
above, I've never broken it.
In the final analysis, both designs work and will last a long time if not
severely abused.
There are also other designs such as locking, Torsen (sp?), air lockers and
others I am sure. I have no experience with these units but they receive
rave reviews in the 4X4 mags. Just remember that magazines have two main
functions. To show you good stuff so you'll buy the mag again and to sell
the goods their advertises pay them to push. The latter usually prevails.
The 3.55 posi is almost certainly a two pinion posi, while good, it is not
extremely strong. It is also a cone type posi unit. If that is shot it
cannot be rebuilt in the same fashion as the plate/friction type. They can
however be made functional with a little work. The 4-pinion posi case is
pretty good, and a lot stronger. You can not, without swapping parts
around, install anything lower
than 3.55 gears. The case changes for the 3.90 and lower gears (higher
numerically).
[ Thanks to Bob Handren for this information ]
Locker Differentials
So you want a locker. Don't do it, you don't need it. Just buy the
Ford Motorsport 4 pinion traction-lok diff. Its about $100 cheaper,
and is much better suited for a mostly street and some strip car. A
"Detroit Locker" is streetable but only for true weekend
warrior street/strip cars. The are noisy, and tend to cause the car
to plow in corners (cause the axles are "locked" most of
the time) unless you are taking a real sharp turn.
Axles
The 1965 thru early 1967 axels are weaker than later axles.
In the late '70s and early '80s, GM converted axle shaft retention from bolted
in, to being held in with c-clips in the differential housing. There was a
problem with some axles. The groove in the axle shaft that the c-clip held on
to was machined too wide. This allowed excess endplay in the axle shaft.
Over time, the clip and the groove would wear due to the excessive side
movement and the clip would fall out. Then the axle shaft comes right out
of the axle carrier.
[ Thanks to Bob Handren, Steve Ochs for this information ]
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