71supreme

OK, I know the 442's were available with optional differential ratios, but were other models also able to be ordered with specific ratios? Were they geared a certain way based on their standard equipment? Did this process change through the years?

My reason for asking; The csm for my 71 cutlass supreme lists many possible ratios, and mine has the absolutely lowest ratio available for that year, the 2.56:1 It's an original A/C car. Even with this crazy low number, the car is still able to get out of it's own way pretty nicely, and due in part to a shift kit installed by the PO, will actually break traction in 3rd gear, so I was amazed to find out this was the gear my rear was blessed with.

Any insights on this? Thoughts? Were A/C cars given lower ratios for fuel economy in 71?

BlackGold

I don't think there's any way to summarize all the "rules" used for rear ends over the years. But the answer to your qustions is, Yes, different ratios were standard based on a car's model, engine, transmission type, and options (such as A/C). Those same factors also determined which ratios were available as options.

Bluevista

I always thought the numerically lower gear ratios had to be used in the A/C cars to keep the rpm's down to prevent overheating?
Nobody really cared that much about mileage in '71, I know I didn't.

Oldsmaniac

I would guess that ratios that were not ordered by the customer were chosen by the area where the car would be sold/driven. More flat land the lower number and hilly area a steeper gear. Maybe I presume too much?

stevengerard

2.56 was considered a "highway" gear. In part these were chosen depending on the usage. And yes typically 3.23 was the highest numerical number you can get with AC (in 70 anyway), though my car and a few other AC cars with a W27 have the 3.42 gear in it which supposedly was not an option.

MDchanic

I believe that the reason for not hooking up A/C cars to numerically high ratios was that the A/C compressors couldn't take the prolonged high RPM's these engines might see on the highway. Same basic reason why A/C wasn't permitted at all in the highest performance models - a motor with a redline over 6,000 would have been too much for the compressor, and re-gearing the pulleys to lower the compressor speed would have meant poor cooling in low-rpm situations, inviting unending customer complaints.

- Eric

507OLDS

The 3:42 ratio was the highest numerical ratio that you could get with a AC-equipped car.Yes,you could special order a higher gear,but certain warranties would be voided.A lot of this was in relation the the AC compressor and the higher rpms.
The majority of your typical Cutlasses came with 2:56's or 2:73's.If they were daring,they might check the big 3:08,and that usually came in a car with a bunch of options.
Having a set of 2:56's is like being chained to a brick wall,but yes,you can break the tires loose easier with those gears than if you have a set of 4:11's. It doesn't make sense to some,but true.

Jamesbo

Brian,

Would you mind elaborating on this some? I don't understand how/why?

defiant1

Also, if anyone can elaborate on A/C compressor not handling higher RPMs I would appreciate it. Is there a big difference if I am running the A/C at 2800 RPM vs. 3500 RPM?
Was it just a longevity issue with compressors of that time period?

d1

Rickman48

Having owned a '69 442 w/ A/C and 3:42 gears from the factory, I can atttest to GM's policy about ratios.
Also having been a Olds, Cadillac, Pontiac, and Chevy partsman 'back in the day', don't remember ANY warrantee problems with A/C compressors!
I do know that Chevy came out with a switch for carburetored Corvettes that would 'kick out' the compressor during 'full throttle' events!
There were 'bulletins' put out about this switch and bracket, and installed on some cars under warrantee, but I don't recall under what conditions.
I'm sure it coud be adapted to an Olds, and look factory, if still around.
IMO, it was just a 'safeguard' implemented by GM on their Hi-Perf cars based on horsepower, not RPM's, unnecessarily.
Olds and Pontiacs are 'done' by 5000 RPM - SB and BB Chevs can go 6000 all day, no matter what horsepower designation, so I don't think the limiting factor was RPM's, but advertised horsepower.

71supreme

Thank you all for this great info! I am learning more than I planned here.

Rickman, in your experience, were their fears justified, or irrational caution?

Brian, I too am curious and confused about the whole breaking traction being easier thing.

In my case it was probably a blessing in disguise. When I bought her at 19, I was a dumb kid who'd never owned anything more powerful than a straight six, and I was lucky enough to have gullible, and or (innocently)ignorant friends.

alek72us

Hey mike who did u buy your car from? my dad used to have a cream cutlass supreme and it had a shift kit and i love making the tire bark when changing gears.

71supreme

LOL, I bought it from a guy named Mike in Easton MA with 137,000 miles on it in march 1989. He said he was the son of the original owner, and it had been in the family it's whole life, and spent the majority of that life in Alabama.

It was pretty fun, I didn't have to brag, anyone who rode in the car did that for me. How was I to know it was chirping the tires for all the wrong reasons.

All of this is making me curious though. I know the rear gear has some influence on cam selection. With no other mods to the car but a 3.42 gear, how would that change the ideal cam specs?

alek72us

Wasnt my dad then i was only 5 in 1989 lol.

71supreme

HA! Yep, I think the registry would have had a problem with a bill of sale signed by a five year old. I once sold my dad's metal shed without permission and caught holy hell.(loooong story) I can't believe the beating ida taken from selling his car!

BlackGold

There's an awful lot of factory literature from 1970 that says that 3.23 was the highest (numerical) ratio available with A/C. But I'm sure there were ways around that through your dealer, especially if you wanted to drop some coin on a W-27. I dno't know off-hand what the rules were for other years, and I know that other GM makes set their own rules.

Personally, I think the overheating issue was the primary reason for the restriction with A/C. But it would also make sense that they were concerned about waranty repairs on A/C compressors. There doesn't have to be just one reason.

As for breaking the tires loose, I've got two theories why a 2.56 rear will do it easier than a 4.10. First, because the 4.10 gears will in fact accelerate the car quicker, they cause a quicker weight transfer. So whatever small amount of spin they start with quickly gets mitigated by there being more weight on the rear.

Second, the 2.56 gears cause the tires to spin much faster for a given engine RPM than the 4.10 gears do. The faster the tires spin, the less traction they have and the less chance they will ever gain traction again. As an example, at 4000 engine RPM, the 4.10 gears spin the tires an equivalent of about 30 mph, while the 2.56 gears spin the tires an equivalent of about 50 mph.

MDchanic

It's easier to spin a wheel with a taller rear if your engine has the low-end torque for it because a given amount of engine rotation will create a larger amount of tire rotation with a taller rear than with a higher-geared rear. So long as the engine can create an instantaneous torque at the wheels greater than their static coefficient of friction, then their kinetic coefficient of friction, being much lower, will not prevent them from continuing to turn (it's harder to start an object moving against friction than it is to keep it moving once it has "broken free" and begun to move), and a taller gear will provide more turning than a higher one.

All of this assumes enough torque at low RPMs to break a wheel free at a given axle ratio in the first place - an engine may not have enough torque to do it through a tall gear, but have enough torque through a (torque-multiplying) higher gear. However, in the case of '60's and '70's Oldsmobiles, most engines do have enough torque to do the job.

I hope that made sense.

- Eric

ps: note that the coefficient of friction of a rubber tire will change with temperature, so that, up to a point, spinning the tire and heating it up will increase its coefficient of friction, and it may finally bite even though it is spinning.

71supreme

Wow, so from all of the info you've given me, it sounds like my 60 series tires on 14 inch rims only magnified the problem.

OK, engine = air/fuel pump, more air/fuel flow = more power, so the point is to crank up the rpm's increasing the amount of air & fuel through the pump. lower numerical gear = lower rpms = less air/fuel through the pump.

So, would the high torque motor then benefit from the numerically lower gear? At what point, when it comes to gear ratio, does the need for engine RPM eclipse the need for torque, say, in terms of a 1/4 mile?

MDchanic

Smaller diameter wheels travel less distance per revolution, so are the same as raising your numerical rear end ratio.

Right.

Power = Work per unit time.

Work = Force x Distance.

So, your engine does work by applying a force to push a piston a certain distance down the cylinder, and, ultimately, to push the car forward a certain distance.

Power is the amount of that work it can do in a given period of time.

More work per minute = more power, so more revolutions per minute = more power.

You can also increase power by increasing the amount of work, such as by increasing the size of the cylinder, or increasing the amount of fuel-air mixture crammed into the cylinder (more fuel burned makes more energy to do more work).

I have no experience with drag racing, but torque measures instantaneous force, while power measures force over distance per unit time.
So torque measures the "push" on your wheels, but acceleration is really measured by how quickly you can increase the speed of the wheels over time, not by how much force you can put on them in any one instant.
Therefore, ¼ mile times (after controlling for vehicle weight and aerodynamic drag) are actually no more than measurements of power.

In general, you notice torque as a nice brisk feeling in the gas pedal, but you notice power when accelerating from an on-ramp, or passing on the highway.
With good torque, you feel a solid kick in the pants when you drop the hammer.
With good power, you find you're past that guy before you realized you would be, even if it didn't "feel" that fast.

- Eric

71supreme

Thank you Eric & Brian, I think I finally get it!

One last thing. The original reason for my post was to try to figure out If you could tell based on the model of car and or options on that model car etc what rear gear was likely to be in it. I suppose it's a moot point, as I have a good basic starting point for a rebuild (GM corporate 8.5 10 bolt) But I figured it would be good for folks to know who are just trying to swap in a higher gear from another olds.

Thanks again all, this has been fun!

507OLDS

You have a better chance of finding a higher-numbered gear in a rearend that came out of a 442,since they came standard with the more desireable ratios.If I counted all the times I found a 3:08 or 3:23 rear under a plain Cutlass,I could do it with 10 fingers or less,and were talking well over a thousand rears.The Cutlass wagon & Vista Cruiser is a little better chance,as I have found a fair share of 3:08's & 3:23's in some of those.

anthonyP

Hi Steve,

Do you have the build sheet or window sticker showing the 3.42 axle ratio with the A/C? According to all of the 1970 literature, 3.23 was highest numerical axle ratio with A/C, including with the W-27 option.

Though, in 1970, A/C was not suppose to be available with a 4-speed 442, as only the close-ratio M-21 was available, which was only available with 3.42 or higher numerical ratios, which were not available with A/C. Oldsmobile did build 4-speed 442's with A/C by installing the HD wide-ratio M-20 with 3.23 gears. This was documented by factory build sheets.

These '70 442's are very rare. In 1971, the problem was mute since the M-20 was the standard 4-speed that year, and was compatible with the 3.23 A/C gears. Also, in 1972, A/C was not available with any W-30, as the only available axle ratios were 3.42 & 3.73.

Anthony

stevengerard

nope, only a broadcast card for the car with the W27. That rear end could have come from the factory, the dealer or put on in the next decade from a junkyard find.

507OLDS

I spoke to a guy,some years back,that was looking for a single F-head,for his original 70 W30 convertible,automatic,with AC. One of his original heads had cracked from overheating. After some discussion,I found that his car came from the factory with 3:91's.It was specially ordered that way,and he had all the original paperwork to prove it,including the voided warranties that are mentioned with the 3:91's and AC.
It just goes to show that anything is possible.If you were standing there with money to buy a car,you could order it with what you want.