'70 Olds Cutlass Supreme - Rear Upgrade Options

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tgilligan
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'70 Olds Cutlass Supreme - Rear Upgrade Options

Post by tgilligan » Fri Apr 17, 2020 11:23 pm

Hi,
When it comes to differentials, I have very little knowledge so looking for some advice/input. My '70 Olds Cutlass Supreme is a mildly modified Olds 350 mated with the original TH350 and stock open rear. Looking for a little more out of the rear but it's a cruiser and a part-time highway car so I want to keep the RPM's within reason. To that end, should I be looking at a 3.08-3.23 range? I believe I'm in the 2.56-2.73 range stock range. It's the original 12 bolt cover/10 bolt carrier 8.5 with stock axles and drum brakes. To my knowledge, it's never been altered but I haven't done enough looking for any stampings or ID's to be sure.

Assuming it is the 10 bolt 8.5 original Olds rear, could I convert to a limited carrier with a 3.08/3.23 range and keep my existing axles, drive shaft, etc.? Looking to upgrade without having to completely tear it apart. It goes without saying I would have a shop do this as it's not in my comfort level to tackle but I'd like to be able to hand the parts to the shop and know what will be done and be sure it gets done correctly.

Any help/advice along with parts/pricing you could give would be greatly appreciated.

Thanks
Tom

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monzaz
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Re: '70 Olds Cutlass Supreme - Rear Upgrade Options

Post by monzaz » Sat Apr 18, 2020 12:05 am

Hello Tom,
Well you can put any gear ratio into any 12 bolt Olds rear. and Limited slip posi also. Housings are not posi only and are not limited to ratios etc. They are all relatively the same Meaning 12 bolt OLDS can have all these optional gears
2.56, 278 (2 series)
3.08 3.23 (3 series)
3.42 3.90 4.10 4.33 4.66 5.00 (4 series)
The tough part for most in the 3.08 to 3.23 range is FINDING the gears. NO aftermarket in those ratios.
3.42 yes 3.90 yes. (aftermarket fits the 3 series carrier)
Limited slip we have 3 series units (NEW Aftermarket Eaton units)
We have made custom 2 series units and if you can find factory 2 series 673 casting we can rebuild them for you)
Most all the 12 bolt olds came with 28 spline axles. 67-70
there were 67 68 models that had 31 spline axles HD application 3.42 - 5.00 ratios
Gear change will never mess with the fitment of your rear end in the car.... Same axle shafts can be used with our aftermarket carrier also.
Same yoke for aftermarket gears too.

Hope this answered some if not all your questions. Jim
JD Race
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tgilligan
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Re: '70 Olds Cutlass Supreme - Rear Upgrade Options

Post by tgilligan » Sat Apr 18, 2020 10:59 am

Thanks Jim, excellent information and very helpful. Avoiding the treasure hunt to find 3.08 or 3.23's, would the 3.42 make the car obnoxious in the RPM's on highway? Car has 15" aftermarket wheels with 205/70 tires and it's a combination of city streets and highways around me here in NJ.

Should I PM you for pricing info on the components I would need to upgrade to 3.42?

Thanks
Tom

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monzaz
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Re: '70 Olds Cutlass Supreme - Rear Upgrade Options

Post by monzaz » Sun Apr 19, 2020 9:48 am

You should first KNOW the exact ratio you have in the rear.
-open it up and rotate ring looking at the outer most diameter edge and get the stamped number finding out what the ratio is
- Usually 6-7 digit number is the part number, then the number from 8 -16 usually, another number from 37-43 usually date code etc.
(you can post all the number we can decipher it if you like. )
if you have freeway gears it will be a 16 and 41 or 14 and 39 (2.56 or 2.78 ratio)
You never know you may already have 3.08 in the rear...13 and 40 tooth count

3.42 rpm will depend on what you think is too much RPM...each customer is different and the amount of noise the RPM will make is dependent on the engine build exhaust how well the car structure can handle rpm with out rattling etc. :)

You say your looking for a bit more from the rear end...??? Explain more what you expect your Muscle car to do for you.
Accelerate faster? Do a nice 20-30 foot burn out patch? etc.

The best way to get the car to do a better burn out is to bump up the stall converter flash stall. that will usually make people feel better about their Muscle car...we really all expect the muscle car to just do a burn out quick and effortlessly. BUT over time we try to improve the cars HP it takes the low end torque from the lower RPM range and the car feel slow and unresponsive. Gears will help usually but at the cost of loosing top end comfort and freeway driving

Transmission over drive is the best way to get out of the OLD school funk of having to choose low gear punch and or freeway cruiser.

So if your looking for acceleration GEARS are usually the call to charge!... that is what make a car pull and accelerate quick through each gear.

Just remember the engine is still the main thing that makes the car move and perform...the transmission gearing and the rear end gearing tire height etc are all things that fine tune what the motor is putting out and tailor the car to improve the areas of driving you do with your car/truck

So if the motor does not produce the torque and or is not tuned up correctly or old and tired...All these changes will be minimal and just band aid the true trouble of your car....which is really the motor camshaft too big...lack of compression , carb not working correct, timing not functioning correct.
torque converter slipping....there are alot of causes to low performance in a vehicle.
Do the simple things first and see if that helps.
Ignition and carb are the BIGGEST culprits to power and torque loss.

Jim
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monzaz
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Re: '70 Olds Cutlass Supreme - Rear Upgrade Options

Post by monzaz » Sun Apr 19, 2020 9:54 am

run your RPM cruise speed equation

Gear ratio x 336 x MPH divide all that by tire size height

example

2.56 x 336 x 65 MPH divided by 27" tire = 2070 RPM roughly
(2.56 x 336 x 65 ÷ 27)

Remember Automatic torque converter slippage tire wear in exact size etc will affect the number BUT not by much.

Jim
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tgilligan
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Re: '70 Olds Cutlass Supreme - Rear Upgrade Options

Post by tgilligan » Sun Apr 19, 2020 8:40 pm

Thanks Jim. I'll crack the diff cover as the fluid hasn't been changed since I've owned the car so who knows how old that is. I'll get the numbers off it and see what I have.

To be honest, I don't have any desire to do many burnouts but I would like to get the car to accelerate quicker off a dead stop (traffic light). Trans and rear have not been touched on this car although the engine has been completely rebuilt. By that I mean that the block was decked/honed/bored .030 over, forged pistons, #6 Olds heads machined and worked, guts replaced with CompCams adjustable roller tip rockers, Comp camshaft (mild), aluminum intake, Holley carb, MSD ignition, etc. With those items completed this Olds 350 is 10.25:1 compression and a solid motor for a small block. It's not a racer nor do I street/strip it so it's a cruiser with some fun times on the highway. It's the slight sluggishness from a stop light that I wish was a bit different. Perhaps this is more of a stall converter issue rather than gearing?

Thank you for the advice. Hopefully when I get the numbers off the gears I can get a better understanding of what I have and see where to go from there.

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Re: '70 Olds Cutlass Supreme - Rear Upgrade Options

Post by monzaz » Mon Apr 20, 2020 9:37 am

Biggest PET PEEVE I HAVE!!!!!


When people call me up and say they have a MILD CAM....lol

WHAT is that? What reference point is a MILD CAM.

If your engine guy is a heavy drag racer mild is still too wild for street.


What I am trying to say is , that is where MOST trouble is connected to.The customer really has NO IDEA what they are tuning.
You need cam card specs to know the duration of the cam.
So you have a stock torque converter, 1350 - 1500 at the most. If the cam is .230 at .050 measuring point on the cam it will be a dog off the line.

You could be right with the stall being too stock under 2000 flash stall.
Maybe provide the carb size, cam specs and what you have the timing at, what was done on the heads? Bigger valves porting??? etc.
Jim
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Re: '70 Olds Cutlass Supreme - Rear Upgrade Options

Post by tgilligan » Wed Apr 22, 2020 10:50 pm

Sorry Jim. Here's a bit more detail:

The Olds #6 heads were machined, ported and new valves were placed (not sure of the actual size). The lifters, rods, springs and rockers were all upgraded based upon the upgraded cam. The cam is a Comp Cams XE268 which has 224/230 at .050 lift with 110 degree lobe. Yes, the engine builder is a drag racer so I get what you're saying.

The stock pistons were replaced with forged and the block was decked/honed and bored .030 over.

Stock intake was ditched for an Edelbrock Performer aluminum dual plane topped with a Holley 4150 750 DP with mechanical secondaries and electric choke. Stock jetting and power valve are being used. Carb is on a 1" spacer atop the intake mated with a 3" tall open element breather.

Points ignition was replaced for an MSD ready-to-run billet distributor with adjustable advance curve. The base timing is around 11 degrees BTDC with vacuum advance disconnected. Silver bushing is being used to provide 25 degrees of advance for a total timing of approx 36.

When I had the engine out, the engine shop mentioned I would want a different converter but my budget didn't have much room left in it, so the stock converter and trans went back in. There's a trans leak I'm trying to track down so if it has to come out, the converter will be something I will replace. While not completely on the same subject as what this thread started as, do you have any recommendations for a brand and/or stall speed?

Tom

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Re: '70 Olds Cutlass Supreme - Rear Upgrade Options

Post by monzaz » Thu Apr 23, 2020 9:24 pm

They are correct. You will need a converter to get that low end back.
Looks all good here. Bump up that converter first ...THEN see if you need more gear. That is what I would do.
2000 is minimum 2500 would be much better 3000 if you were a weekend warrior type guy.

Can is right at the cusp of performance for street driving normal range. anything over 230 duration at .050 we always worry customers will be disappointed with the low end take off.

The spacer likely will rob more low end again ...that is a top end rev trick. Take that out and see if you get better snappier throttle response.
IU do not think it will be enough to make the low end comeback....but will help idle smooth out

AGAIN all this is what you want too. NEVER let us push you from what you the customer wants. If you like the stuff you have definetely keep it. I am just trying to give pointers to where the troubles will lie in the torque output.

That is why we drive muscle cars snappy neck snapping torque... and style of the old car of course. :D

Hope this helps some.
Jim
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