Mitchell Over/Under Drive

[keeper]

I was hoping to get some opinions and thought on using a Mitchell Over/Under Drive on a build. I have access to one from my Uncle who was an old Tea Timer. It really seems like it would be a fun way to set the gears for a little drag car and still be able to drive the thing around.

One down side I see is the size. It is really going to shorten up the drive shaft, and it is real tall so it is going to eat up some floor space.

Here is a parts drawing for refrence:


Thanks for the info.

[Flat Ernie]

Have heard decent things of them over on Fordbarn, but have no experience with them myself - might ask over there on both the Model-A & early V8 sites - Ahooga.com might also be a good place to ask as they seem to be popular with the Model-A guys.

[keeper]

The ones over at the fordbarn are a bit different, the are the Mitchell 1000 and are used mainly for overdrive. The one I have access to is a Mitchell 2000, and the gearing is very flexable.

What I want to know is could you use this as an UNDERdrive at the strip with a big C.I. and low RPM engine to jump off the line, like a 346 Caddy Flatty?

[Wesley]

lots of things to consider
a) how much torque is the Mitchell rated for, or known to take before failing.

b) what ratios are available for the rear end that are going to run.

c) do you want the most efficient transmission of power to be on the strip or on the street.
the general rule of thumb is that an over/under unit is the strongest and most
efficient in direct drive.

d) how much torque is the engine realistically going to make

e) how much of that torque will be able to be applied to the pavement.


The up side to running in direct drive on the street and underdrive on the strip is you will have the most efficient power transmission where you will spend the most time driving. The down side is that on the strip you will be running in underdrive which is not ideal for efficient power transmission, is the most likely scenario for breaking your over/under unit.
If you choose to go the other way and run in overdrive on the street and gear the car to run in direct drive at the strip you will have to deal with less efficient power transmission on the street, more heat generated in your over/under unit during most of your driving and the possibility of some pretty high drive shaft speeds into the rear end. High pinion/drive shaft speeds generally equate to high lube temps in the rear end. However you will have the most efficient power transmission at the track, and less of a chance of breaking your over/under unit during racing conditions.

Most of this comes down to picking nits for the average rodder. As the horsepower/torque values go up, and the ability to actually hook that power up is realized these considerations become more important. My guess is that alot of your decisions are going to be based on what gear ratios are available for the rear end you are going to run.



"there is no real reason to listen to me, there are way too many medical professionals and legal documents that say I am nuts"

[keeper]

Well I only have a few answers to the questions you asked, and let me say thank you for the response.

The Mitchell 2000 is rated for 20,000 GVW

The gear set up I have available in the Mitchell is 8% over and 28% under and I am thinking about using a 4.10, or 4.11 rearend. So that will put me at 3.78 in overdrive and 4.85 in underdrive if I use a 4.11.

So my next question is what years and makes can I find a beefy, open drive, with a 4.10 or 4.11 gear set in it.

[carsguitars]

Gear the rear end deep, then find a three speed electric overdrive from a 50's or 60's ford police car. It has four planetarys (instead of three) and is almost industructible. Trust me, put a lot of power from diesel motors into them. Put a button on the sifter so you can split gears if you want, (they an shift fast if set up right) and you will be all set.

[38FLATTIE]

They are junk!

Send it to me, and i'll dispose of it for you! Lol!