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I have no idea what rear end I have in my 1972 Cutlass 350. I only can see it is a 10-bolt, but not sure if there are differences between GM 10-bolts/years. I'm asking what I need to know as I want to change the 2.73 open diff gears that are in there with a Posi-Unit with 3.73s.
Is there anything I need to know before buying expensive parts?
You have an 8.5" "corporate" 10-bolt axle, which was the only one installed in the Cutlass line that year. It is identical to the Chevy 10 bolt except for the fact that the Olds axle uses bolt-in axle shaft retainers at the outer ends instead of the stupid C-clips used by Chevy. All the center section parts (differential, gears, and bearings) are identical to those on a Chevy 8.5. There is a lot of mis-information about these axles, and many vendors will try to tell you that they are 8.2" or that the Chevy parts won't fit. Ignore this. The good news is that the posi and gears for these axles pretty much grow on trees.
Are you drag racing? I ask because 3.73 is a lot of gear with a 3 speed automatic. Or perhaps you have an OD trans?
Might want to back off the gear a little to a 3.23 or 3.42 or the recently available 3.55. You will get tired of 3.73s quickly if you still have the T350 or T400 or Muncie.
Might want to back off the gear a little to a 3.23 or 3.42 or the recently available 3.55. You will get tired of 3.73s quickly if you still have the T350 or T400 or Muncie.
Going from 3.73 to 3.55 will only make a 5% difference in engine RPM at a given speed - not enough to matter. Even going from 3.73 to 3.23 only makes a 13% difference. I'll take the steeper gears, especially for a weekend fun car.
This illustrates the benefit of an OD trans, however. The 0.68 fourth in the 200-4R is a 30% difference in engine RPM - equivalent to going from 3.73 to 2.53 gears.
Going from 3.73 to 3.55 will only make a 5% difference in engine RPM at a given speed - not enough to matter. Even going from 3.73 to 3.23 only makes a 13% difference. I'll take the steeper gears, especially for a weekend fun car.
This illustrates the benefit of an OD trans, however. The 0.68 fourth in the 200-4R is a 30% difference in engine RPM - equivalent to going from 3.73 to 2.53 gears.
3.73 down to 3.23 is certainly a worthwhile rpm drop despite your dismissing it. No one is suggesting going from 3.73 gears down to a 3.55 gear. The OP is stating that he has 2.73s and is looking to move up. I merely suggested some alternatives.
Why not consider numerically lower gear ratios if they will work for how he uses the car? I am not a mind reader & that is why I asked if he was drag racing the car. Your idea of a "weekend fun car" might not be the same as his.
No smart person would pick a gear without looking at all the other components that make up the driveline and ensure that they will work well together. 3.23 is a great compromise gear with a 455 but for a 350 engine 3.55 option might be best. Many don't even know that gear is now available for the 8.5" diff.
An OD transmission swap can be a very expensive once you consider ALL of the costs. It can take years of driving to recover the investment for some users.
Why not consider numerically lower gear ratios if they will work for how he uses the car? I am not a mind reader & that is why I asked if he was drag racing the car. Your idea of a "weekend fun car" might not be the same as his.
No smart person would pick a gear without looking at all the other components that make up the driveline and ensure that they will work well together. 3.23 is a great compromise gear with a 455 but for a 350 engine 3.55 option might be best. Many don't even know that gear is now available for the 8.5" diff.
An OD transmission swap can be a very expensive once you consider ALL of the costs. It can take years of driving to recover the investment for some users.
I don't disagree with any of this except your comment about "no one suggested 3.55", which is exactly how I read this:
Originally Posted by oldsmobiledave
... 3.73 is a lot of gear with a 3 speed automatic...
Might want to back off the gear a little to a 3.23 or 3.42 or the recently available 3.55.
You are completely correct that the total package needs to be assessed - intended use, tire diameter, etc. The reality is that people loose way too much sleep over this for a car that's often only driven to cruise nights. A lower numerical rear axle is not a panacea - one is obviously giving up low speed acceleration for lower highway RPMs. How does the owner feel when his shiny restored "musclecar" gets dusted by a new Camry at a traffic light? Sorry, that's not why I own musclecars.
While it can be a expensive option, the combination of the 2.74 first and 0.68 fourth in the 200-4R fixes both ends of the problem. Better off the line acceleration AND 30% lower highway RPMs.
Everyone assumes the rear end gear is the end all be all of getting a seat of the pants feel. Really the band aid of a bad running motor.
If a engine is old and tired it just will not perform good period.
Most of these Muscle car era cars came with 2.73 ratio and was great with the smaller camshaft profiles installed as the engines had very good low end quick torque it was the 4000 and up where the trouble came in. They broke the tires loose very easy.
BUT for most of us a car only gets used up to maybe 4500 at the most :even in good spirited driving on the street.
BUT we still add these cams that have power bands from 3000-7000 RPM range thinking were getting all this HP etc when really you just killed all the low end fun on the car and never get the car up to the RPM where it actually makes that power. 75% of rear diff business is because customer install WAY too big of cams and have many other miss match parts and to BIGGER is BETTER syndrome with intake heads exhaust .... Then we get the car on the road, mash the pedal and it goes NO WHERE until the RPMs hit 4000 then it starts to breath and kick out torque and power as the engine finally starts turning at an RPM that move the air through all that BIG stuff you picked.
BAND AIDE ....
SO we go and get a monster steep set of gears and a monster torque converter to compensate for the camshaft, heads, intake that were really RACE stuff not street stuff.
Getting hooked up in the HP number is a BAD way to make a motor for the street. TORQUE is what gives you the MUSCLE car feel that is why most go to a BIG cube 455 motor...instant torque.
Posi: If your going to just drive from place to place and not really tear the road or track up a posi will be an over priced item.... So if you want to lay out 2 tire patch and get the grip at the track and take advantage of the unit spend it... You will have a bit more maintenance on the oil change and could have some noise factors on HOT hard driving days ...
So, I ask why are you going to the 3.73 gears? How long have you had the car? Does the car do a good burn out now with the way it is set up?
What are your intentions for the car? (in other words...what do you want this car to do for you and or your family?) Lets get some info from you personally.
Be honest with your wants... This is your car not the guys on the forum... Everyone can tell you what they did but they may have totally different thrill seeking ideas.
You have an 8.5" "corporate" 10-bolt axle, which was the only one installed in the Cutlass line that year. It is identical to the Chevy 10 bolt except for the fact that the Olds axle uses bolt-in axle shaft retainers at the outer ends instead of the stupid C-clips used by Chevy. All the center section parts (differential, gears, and bearings) are identical to those on a Chevy 8.5. There is a lot of mis-information about these axles, and many vendors will try to tell you that they are 8.2" or that the Chevy parts won't fit. Ignore this. The good news is that the posi and gears for these axles pretty much grow on trees.
Great to hear, thank you!
Originally Posted by oldsmobiledave
Are you drag racing? I ask because 3.73 is a lot of gear with a 3 speed automatic. Or perhaps you have an OD trans?
Might want to back off the gear a little to a 3.23 or 3.42 or the recently available 3.55. You will get tired of 3.73s quickly if you still have the T350 or T400 or Muncie.
I will eventually be installing an OD automatic transmission, hence the desire for 3.73s.
Originally Posted by joe_padavano
Going from 3.73 to 3.55 will only make a 5% difference in engine RPM at a given speed - not enough to matter. Even going from 3.73 to 3.23 only makes a 13% difference. I'll take the steeper gears, especially for a weekend fun car.
This illustrates the benefit of an OD trans, however. The 0.68 fourth in the 200-4R is a 30% difference in engine RPM - equivalent to going from 3.73 to 2.53 gears.
Agreed, this is the transmission I'm getting built to replace the stock TH350.
Originally Posted by oldsmobiledave
3.73 down to 3.23 is certainly a worthwhile rpm drop despite your dismissing it. No one is suggesting going from 3.73 gears down to a 3.55 gear. The OP is stating that he has 2.73s and is looking to move up. I merely suggested some alternatives.
Why not consider numerically lower gear ratios if they will work for how he uses the car? I am not a mind reader & that is why I asked if he was drag racing the car. Your idea of a "weekend fun car" might not be the same as his.
No smart person would pick a gear without looking at all the other components that make up the driveline and ensure that they will work well together. 3.23 is a great compromise gear with a 455 but for a 350 engine 3.55 option might be best. Many don't even know that gear is now available for the 8.5" diff.
An OD transmission swap can be a very expensive once you consider ALL of the costs. It can take years of driving to recover the investment for some users.
An OD transmisson will be installed within the year, hence the need for 3.73s and a POSI rear end. Sorry, I could have been more clear on my original post.
Originally Posted by joe_padavano
I don't disagree with any of this except your comment about "no one suggested 3.55", which is exactly how I read this:
You are completely correct that the total package needs to be assessed - intended use, tire diameter, etc. The reality is that people loose way too much sleep over this for a car that's often only driven to cruise nights. A lower numerical rear axle is not a panacea - one is obviously giving up low speed acceleration for lower highway RPMs. How does the owner feel when his shiny restored "musclecar" gets dusted by a new Camry at a traffic light? Sorry, that's not why I own musclecars.
While it can be a expensive option, the combination of the 2.74 first and 0.68 fourth in the 200-4R fixes both ends of the problem. Better off the line acceleration AND 30% lower highway RPMs.
Overdrive will definitely be a good deal that first gear will help a heavy duration camshaft. As long as you do not cut corners on the trans parts, you will love that.
Overdrive will definitely be a good deal that first gear will help a heavy duration camshaft. As long as you do not cut corners on the trans parts, you will love that.
Great to hear! This will for sure be the last time I heavily modify a car. But this one has a lot of meaning to me.
This is is why I’m building a full driveline package. No corners will be cut!
I say Detroit Trutrac for a posi, no clutches to wear out and they seem to last much better than an Auburn. I would think most of gear brands are close to equal, go with the best price.
I say Detroit Trutrac for a posi, no clutches to wear out and they seem to last much better than an Auburn. I would think most of gear brands are close to equal, go with the best price.
X2. I put a Trutrac in my truck to replace the P.O.S. GovLoc unit. Much better.
I say Detroit Trutrac for a posi, no clutches to wear out and they seem to last much better than an Auburn. I would think most of gear brands are close to equal, go with the best price.
Which model number will fit my rear end? I do not know the difference between these two model numbers...
Last edited by 72GoldOlds350; March 28th, 2018 at 07:53 PM.
It is not designed for hole shots at the race track. It is the best unit for DUMB drivers which most are these days...lol.
If used like a normal driver they are the BEST at traction and driving and last forever as they are not being used under normal driving like a performance unit every turn you take wear the clutches and cones. Even true trac in the snow is very tedious for the normal everyday driver slide left and right.
Auto locker (gov lock) only engages when the wheel needs the traction. at low speeds... anything over 30-40 MPH it does not even engage. safer for the dumb driver as the car will not get loose like the performance unit if it looses traction for an instant.
Just some thought before you condemn the unit. IT WAS never designed to be a race or performance unit. THAT is why only small truck and big trucks had them. Cars received the auburn and torsen posi (late camaro and birds in the 99 2000 2001 2002 ls motor cars) units
Just not a muscle car burn out unit...LOL.
all cars and trucks with 8.5 were 28 spline from 1971-1988 then in 89 ish the 8.5 was upgraded to 30 spline 1990 -1999 ish again ... it all depends on plants and when each one ran out of the 8.5 and switched to 8.6
10 bolt 8.6 rears had the carrier bearing hubs increase and the bearing saddle diameter increased to 12 bolt Chevy diameter 3.062
REASON for the 8.6 rear was because of the failure of the 90-99 8.5 bearing hubs were too small OD and the ID was hogged out too far from the 30 spline axle upgrade.... JUST a engineering blunder. GM GOOFED. took them 10 years to correct.
Reason for the LM102949 and LM102911 conversion carrier bearing when using the eaton and true trac 28 or 30 spline units...AFTERMARKET corrected it before GM did...lol. then GM follow suite.
all cars and trucks with 8.5 were 28 spline from 1971-1988 then in 89 ish the 8.5 was upgraded to 30 spline 1990 -1999 ish again ... it all depends on plants and when each one ran out of the 8.5 and switched to 8.6
10 bolt 8.6 rears had the carrier bearing hubs increase and the bearing saddle diameter increased to 12 bolt Chevy diameter 3.062
REASON for the 8.6 rear was because of the failure of the 90-99 8.5 bearing hubs were too small OD and the ID was hogged out too far from the 30 spline axle upgrade.... JUST a engineering blunder. GM GOOFED. took them 10 years to correct.
Reason for the LM102949 and LM102911 conversion carrier bearing when using the eaton and true trac 28 or 30 spline units...AFTERMARKET corrected it before GM did...lol. then GM follow suite.
little history on that 8.5 evolution 101
Jim
J D
Thank you!
I want to get 3.73 gears as my stock gears are no good. Which gears would fit this unit the best? I'll be placing my order this weekend!
Thanks for the help, folks!
Last edited by 72GoldOlds350; March 30th, 2018 at 06:39 PM.
March 24th, 2018, 10:57 AM
