Mitchell OD on Buick Eight Special 1950

Hello,

I have a Buick Eight Speciual 1950 with 248 and Dynaflow. (Believe I have the 3.9:1 rear end)

This car runs great at up to 55 Mph, then the rpm gets high and it feels wrong to push more. I live in the Midwest where driving on long stretches of Highway is common just to get to nicer and more scenic areas. It is annoying to go 55.

The car is quite and rides very smooth at 55 and there is still a lot of power so it feels like 70Mph should be no problem with a higher gearing.

It has a closed drive shaft and I have seen the Mitchell OD units (26% and 36%) at 2KUSD, I can fit one right in front of the differential and shorten the torque tube. Then make an "uppward" 3" box under the back seat to accomodate for the suspension movements since the box is bolted to and with the differential (that makes for a good acces hatch for the OD). There is also the Lycock de Normanville "Volvo OD's" at 22%, but with all the pumps and stuff in them they seem complicated.

Do you think this is possible from a engine power and gearing point of view? If so 26% or 36% OD?

The 248 has it greatest torque at 2000 rpm and max power at 3500 rpm.

BR

Nick

Just a comment about bolting the unit directly to the rear end. That will dramatically affect the unsprung weight of the rear axle which will have a detrimental affect on ride quality. The rear springs and shock absorbers would need to be upgraded to be capable of handling the added weight.

Terry

Terry,

Thank you for the concern, but do you really think a aluminum OD box with a few gears will be heavy enough to significantly change the unsprung weight if a large rear axle assembly of cast iron and steel parts with large bias ply tires at the ends?!

Also we are not talking about a sports car driven hard on uneven surfaces but about a 4000lbs sled that is cruised and driven gently.

What about gear ratios?

NB

With a 1950 3.9 you can bolt in a better ratio (3.6 ~ 3.4 ) from a mid 50's dynaflow car for a fraction of the cost and engineering complications

Look at the links here at the bottom of post 5 http://forums.aaca.org/showthread.php?t=342274&p=1113356&viewfull=1#post1113356

Nick, congratulations on having a 1950 Buick!! A man of superb taste.

I had the convert in my signature when I was 18 and 19 years old . I promise I did not baby it. On the hiway, she seldom saw less than 70 mph. Often more. Much more. We have become accustomed to the low rpm of modern engines to the point 3500 rpm seems too fast, and loud. Don't be afraid of pushing the old girl to 70 or beyond.

Now to your ratio question. If my math is correct, a 26% will leave you at about a 2.9 final drive. Pretty steep, but probably ok. My '50 sedan came with a 4.11. I changed out the gears for a 3,36 from a '55. I figure this out to be a 20% reduction. I can get the figures later, as I recorded before and after speed/rpm's. At 70mph I now run in the neighborhood of 2300 rpm. A very comfortable rpm.

An advantage to the OD versus the gear change is, with OD locked out, you would not lose the low speed driveability I lost. I now have to shift to 2nd for any thing below about 10 mph, wheras before 3 mph was possible. Your Dynaflow will help here.

If I can be of any help, please ask. I like talking Buicks. Especially '50 Buicks.

Ben

Guys, Ben,

Thanks for your response.

I know we want low rpm nowadays, I can drive 65 mpa now but it just feels wrong to push that long stroke thing to more than 2500rpm continous.

I have tried to se if my Dynaflow has 3.9 or 3.6, need to clean the housing more to see the numbers. It is anyhow very slow in acceleration now but from about 25-30 mph the Dynaflow stops slipping too much and the faster I go the more "direct drive" it feels like, at 45mph it accelerates well. A ratio like 3.36 would probably worsen the getting away from 0 mph situation. Also I have read that going from low to high frequently so do that to compensate the tall gearing would hurt the Dynaflow.

Reading what you wrote Ben, Mitchell w 26% is probably the way to go, 36% too much. Kind of crazy if I would need to go at 80-85 mph to get the engine into the powerband (1900-2300). Guess the cops wouldn't beleive what they saw flying down the road.

Painfull is the cutting in the floor under the back seat, will not be seen if done carefully.

As a general note, very little engine power is needed in any car when on level ground at constant speed. (Air resistance is the main part then). My daily driver (2.4 liter Subaru Forester = small engine, 4WD, and a lot of air resistance ) turns at only 1800 rpm at 65 but immidiately gears down when I accelerate or a hill comes. That kind of tells me that a very steep ratio is possible even with low powered, low displacement engines.

I think the slippage (torque multiplication) decreases when the output shaft spinns fast enough, manual says until 47mph it multiplicates. Hence with "OD in OD" to make sense I would need to drive minimum 1.26 x 47= 60 mph or 1.36 x 47=64 mph respectively otherwise the Dynaflow slips (torque multiplicates = ratio greater than 1:1 ratio (1.5:1 or whatever depending on load, speed etc) in the gear box ) and then OD overdrives (0.74:1 or 0.64:1 respectively) = no sense, just heating oil.

Is this reasoning correct?

I'll guess next step is to check if it is 3.6 or 3.9 and start measuring wheel circumference to see rpms at different speeds.

26% and 3.6 means 2,66 in OD 36% and 3.6 means 2.3 in OD

26% and 3.9 means 2,89 in OD 36% and 3.9 means 2,5 in OD

Ben, anyone any comments?

Nick

Guys, Ben,

Thanks for your response.

I know we want low rpm nowadays, I can drive 65 mpa now but it just feels wrong to push that long stroke thing to more than 2500rpm continous.

I have tried to se if my Dynaflow has 3.9 or 3.6, need to clean the housing more to see the numbers. It is anyhow very slow in acceleration now but from about 25-30 mph the Dynaflow stops slipping too much and the faster I go the more "direct drive" it feels like, at 45mph it accelerates well. A ratio like 3.36 would probably worsen the getting away from 0 mph situation. Also I have read that going from low to high frequently so do that to compensate the tall gearing would hurt the Dynaflow.

Reading what you wrote Ben, Mitchell w 26% is probably the way to go, 36% too much. Kind of crazy if I would need to go at 80-85 mph to get the engine into the powerband (1900-2300). Guess the cops wouldn't beleive what they saw flying down the road.

Painfull is the cutting in the floor under the back seat, will not be seen if done carefully.

As a general note, very little engine power is needed in any car when on level ground at constant speed. (Air resistance is the main part then). My daily driver (2.4 liter Subaru Forester = small engine, 4WD, and a lot of air resistance ) turns at only 1800 rpm at 65 but immidiately gears down when I accelerate or a hill comes. That kind of tells me that a very steep ratio is possible even with low powered, low displacement engines.

I think the slippage (torque multiplication) decreases when the output shaft spinns fast enough, manual says until 47mph it multiplicates. Hence with "OD in OD" to make sense I would need to drive minimum 1.26 x 47= 60 mph or 1.36 x 47=64 mph respectively otherwise the Dynaflow slips (torque multiplicates = ratio greater than 1:1 ratio (1.5:1 or whatever depending on load, speed etc) in the gear box ) and then OD overdrives (0.74:1 or 0.64:1 respectively) = no sense, just heating oil.

Is this reasoning correct?

I'll guess next step is to check if it is 3.6 or 3.9 and start measuring wheel circumference to see rpms at different speeds.

26% and 3.6 means 2,66 in OD 36% and 3.6 means 2.3 in OD

26% and 3.9 means 2,89 in OD 36% and 3.9 means 2,5 in OD

Ben, anyone any comments?

Nick

Please keep us informed.

Ben

NB74, where are you??

Ben

Your biggest problem will be attaching the OD to the rear differential. You will have to figure out a connector from the output from the OD to the input of the pinion shaft of the differential. You will also have to make some kind of adapter to connect the OD case to the differential housing.

The 26% ratio will most likely work very well for your car.

I have a Mitchell in my 1933 Pierce Arrow 836 Club sedan. It has the original 4.28:1 differential. The Mitchell brings it down to 3.17:1. I do not have any problems with power here in the Midwest. With my setup, it is a simple operation to shift the OD back to Direct, so for parades or slow touring I run direct.

The Pierce's 366 cubic inch inline 8 cylinder has peak torque around 1600-1800, so the 'tall' gearing is not an issue. I run 65mph on the freeways.

In my installation, the OD is mounted to the chassis, and a short 8" driveshaft connects the trans to the OD, then a new rear driveshaft is used to connect the OD to the input of the rear differential. I have no geometry issues with this setup.

You will probably have to figure out the venting of the OD case, unless you end up with a threaded plug at the top of the OD's case when installed. You must vent the OD, it gets warm, and if sealed would push out it's gear lube. I'm not sure if it would mingle lubrication in an installation where it is replacing an enclosed torque-tube drive system.

You will enjoy the 'new car' you create when you have the OD installed.

GLong