4-speed option or Rear differential gearing question - 31 Commander

[slowjim]

My 31 Commander is close to "Hitting the Road"...finally.

My 19"wire wheels had spokes that were bent, broken, or were badly pitted.  I went with a custom option that was only available with 18" wheels.  Big Mistake...????

Max speed is 30 to 35 MPH on recent test drives.   

Does anyone know of a 4-speed transmission option, or different rear end that may help at least get up to 50 MPH..... Or should I just live with 30 MPH.

 

Looking for ideas....

 

Thanks

[Bloo]

30-35MPH is normal for the time. Roads were very different. I definitely would not have wanted to go smaller on wheel diameter on a car of that era, bigger if possible, But I know your options for tires can be very limited. I've not looked at the 19" situation lately. It might not be as big of a change as you think, and can be easily calculated. What is your original tire size and your current tire size?

 

A four speed transmission won't help, or probably won't. On almost all old transmissions high is direct. In the early 30s some manufacturers flirted with four speed transmissions, but declined to put in what would be more useful gearing, because it would have required more shifting. Ability to pull hills in "high" was a measure of a car's worth in those days because people did not want to shift. Thus, 4th gear was high and first was more like a granny gear in a truck, and generally not used at all. The idea quickly disappeared.

 

What you would need is an overdrive fourth (or fifth), and those transmissions almost all come from a much later era. Another thing that can be done is put a second gearbox in the driveline to add overdrive. People have converted Borg Warner or Laycock overdrives removed from the back of other transmissions. There is also the Mitchell if you do not have a torque tube. Torque tube versions are only for Ford. You could adapt but would be on your own for the engineering. Gear Vendors is the current owner of the Laycock design and can provide parts to install in either an open driveline or a torque tube. If you speed it up, I'd try to hold it to an extra 10MPH or so, make sure your steering and suspension is tight, and nitpick your brakes so they work as good as they possibly can.

 

Edited October 15, 2023 by Bloo (see edit history)

[nzcarnerd]

When you say 30-35 mph 'max', what do you mean?

 

Have you fitted a tachometer?  Peak power for that engine is developed at 3200 rpm so it should run up to 3000 easily and a healthy cruise would be at about 2500.

 

You should be able to calculate the figures from known parameters - diff ratio, and tire diameter etc.  Try putting figures in here - Speed versus RPM calculator (wahiduddin.net)

 

I put some figures in there and reckon at 2500 rpm you should get somewhere between 45 and 50 mph cruise depending on tire size. Btw one inch either way in wheel diameter does not make that much difference, but the tire section can.

 

There was a debate here in NZ recently about the tires fitted to the Morris 8hp cars in the 1930s. The standard size was a 4.50 x 17 but the export cars mostly got 5.25 x 16. There were those who thought the 16s would give lower gearing - perhaps for our bad roads of time. There is actually negligible difference between the two sizes.

 

One thing many forget is that without using a tachometer engine speed is hard to judge. Straight eights especially always sound busy, even at relatively low rpm. Also they are not like fuel injected moderns that will roll along at a highish cruise speed in overdrive top at 2000 or less.

[John DePrey]

I agree with nzcarnerd, one inch tire diameter difference at ~30 inches only changes overall gearing by about 3%. We just toured with several 20s and 30s Studes and Willys and for the most part 40-50 mph was not unusual on good flat roads, many of them lower end cars for their time than yours. I was especially impressed by the '23 Baby Overland and '26 Whippet, they kept right up. We were on the new car end in our '41 President. 

[slowjim]

Thans for the info guys.... it sort of headed where i thought that it would.

My car does have a torque tube drive shaft, so my options are limited.

I'm going to install a tach and see where that goes.

 

Thanks again...

 

[Henry Ford]

I had the same problem with my '30 Dictator with Straight 8. Rear axle is 4.73, engine was screaming at 40 MPH. I made an adapter plate to bolt to the original bellhousing to accept a T5 transmission from an S10. Much better driving now with a .72 O/D. Floor is still flat and no one would know unless they craw underneath. The six cylinder cars had much better rear axle ratios.

 

[nzcarnerd]

1 hour ago, Henry Ford said:

I had the same problem with my '30 Dictator with Straight 8. Rear axle is 4.73, engine was screaming at 40 MPH. I made an adapter plate to bolt to the original bellhousing to accept a T5 transmission from an S10. Much better driving now with a .72 O/D. Floor is still flat and no one would know unless they craw underneath. The six cylinder cars had much better rear axle ratios.

 

Your car maybe been 'screaming' at 40 mph, but it was likely doing only 2,000 rpm. As I noted in my earlier comment, straight eights can sound very busy, especially in an old car with little sound proofing. My guess is with the T5 that the slightest incline would mean dropping down to 4th gear. The six-cylinder Dictators of that era used the same short rear end ratios as the eights. What will give away that you have a modern box is the distinctive howl of second gear will be gone. That is one of the endearing characteristics of a pre-synchro box.