Rear Diff Swap

[Flathead Fever]

I used to rebuild rearends at work, probably about one a month but I've never done an early Ford, there a little different. I have about a dozen '32-'40 rearends sitting out in the garage. I had a stroke, so I haven't felt like doing much of anything. I have the different thickness paper axle gaskets, new Ford bearings and a NOS set of Ford 3'54s for a late '32 to 34 rearend. Depending on your abilities you could rebuild one yourself, but I don't think most hobbyist's car guys would want to tackle it. You should have a hydraulic press and press plates. I have a 50-ton press here at home.

Hopefully one of these days I'll get back out in the 2400' garage, I have a lot of old Fords to work on out there. I bought them all over the years to have so I could work on them when I retired, but then I had the stroke and a back operation. Didn't drink or smoke, wasn't overweight (too much). I never counted on having the stroke.

[34fordy]

[QUOTE=ford38v8;

The Ford flathead with stock distributor will not redline. Point float will keep rpms down, safely enabling pedal-to-the-floor for extended periods.[/QUOTE]

Sounds great. I wonder how many rpms that would be. I have never had the nerve or the need to see what speed my 36 pickup would achieve but may be brave enough now LOL.

[Seth Swoboda]

Originally Posted by tubman

Third gear is still one to one., so changing the transmission gearing doesn't affect top speed.


If your going to higher (lower numerically) final drive ratio, I'd think that you would want to retain the wider ratio transmission gear set to retain drivability.

Correct, however with a 3.25 rear gear ratio you can drive 60-70 mph with lower engine RPM. With a 4.11 gear your engine is taxed harder with higher RPM's to achieve the same speeds. Put in a taller gear, you get lower engine RPM's at highway speeds, making the drive more enjoyable. It's my poor mans version of an overdrive. The first thing I tend to do is swap gears to a 3.54 or 3.25 in the rear and 15/29 tooth transmission gears. The ratio depends on the car. A heavier car I go with the 3.54. Also, I live in a relatively flat to hilly area. If you live in the mountains this will not work for you.


Why swap the transmission gear to a lower first gear? If I start out on a hill with a tall rear end ratio, I don't want the clutch to slip. I've found this combo to be very rewarding in my case.

[Seth Swoboda]

[QUOTE=34fordy;2417160][QUOTE=Seth Swoboda; Then change the transmission to a 15 tooth input shaft and a 29 tooth cluster. When you're done go drive your truck 70 mph.


Hey Seth, Please explain the results and experience of the above swap.[/QUOTE]


You say the top speed is 50 mph. Is this all it will give you or are you saying at 50 mph the engine RPM's are very high? I've had a '39 half ton pickup up to 70 mph with a 3.78 rear end gear and it had more.


I guess we need to know more. If 50 is it with the throttle wide open, you might have other problems.


My experience with the swap is pretty straight forward. Go fast with a lower engine RPM. Does the swap add MPH, no. My engine is capable of going "fast". My end goal was to lower the RPM's at 60-70 mph so that the drive was more comfortable. The swap for me is not complicated as I've done it many times. It is certainly less involved than going to a T5 transmission and an open drive line. Plus there is no butchery involved. It is simply swapping gears that Ford offered.

[Mart]

My 41 pickup came with 3.54's. I run 6.50:16s. I had to rebuild the trans and used the 15/29 gearset. It drives great and cruises nicely at 60. The reason for using the gearset with the lower first gear is to compensate for the taller rear gears when getting off the line. It's a good combo for a nice driving machine.
In my opinion top speed depends more on the smoothness of the engine. A factory stock engine that still has it's factory crank and matched set of rods will run smoothly. I had a badly out of balance motor in my truck and did not like driving over 45.
In my 33 I have a stockish 239 in good balance and that wants to go and go. That has 4.11s and 7.50:16s. I can do 70 plus in it but am happy to go about 60 on the motorway.
That has twin carbs and is happy to rev.
You have to consider these vehicles were not designed to run high speeds for long distances. The roads did not exist to allow it. Things like wind resistance, wind noise, road noise, and steering stability all can make driving at higher speeds very tiring.

It's an old car, take it easy.

Mart.

[glennpm]

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KT, it would be good to figure out what rearend gears you now have.

First check the bottom of the centersection. On most of the Flathead rearends from 1932 to 1948 there are 2 numbers stamped on the bottom. These represent the number of teeth on the pinion and the ring gear. Dividing one into the other gives the ratio. For example 37 and 9 represent 4:11 or 34 and 9 is 3:78. This assumes of course that the gears have not been changed.

With a closed driveshaft, i.e., torque tube design, you can't see driveshafts turns so you have to go to the crankshaft pulley. Make a chalk mark on the pulley any place convenient to view. Jack up one wheel, put the transmission in 3rd and put a chalk mark at 12:00 position. From the Techno document, page #707, which I edited below to fix it for 4.44

1) Jack up one rear wheel and chalk-mark the 12 o'clock position on the tire. (Make sure the slack is out of the drive train)
2) Hand crank engine 2 complete turns.
3) See where chalk mark on tire ends up.
A) Between 10 and 11 o'clock, ratio is 4.44
B) Between 11 and 12 o'clock, ratio is 4.11
C) Between 12 and 1 o'clock, ratio is 3.78


Once you know the ratio you could use this calculator to see how higher gears would do.
https://tremec.com/aftermarket/resou...io-calculator/

As far as the max RPM, about 3500 to 3800 RPM but the noise and vibration would be very annoying.

[Seth Swoboda]

Originally Posted by Mart

My 41 pickup came with 3.54's. I run 6.50:16s. I had to rebuild the trans and used the 15/29 gearset. It drives great and cruises nicely at 60. The reason for using the gearset with the lower first gear is to compensate for the taller rear gears when getting off the line. It's a good combo for a nice driving machine.
In my opinion top speed depends more on the smoothness of the engine. A factory stock engine that still has it's factory crank and matched set of rods will run smoothly. I had a badly out of balance motor in my truck and did not like driving over 45.
In my 33 I have a stockish 239 in good balance and that wants to go and go. That has 4.11s and 7.50:16s. I can do 70 plus in it but am happy to go about 60 on the motorway.
That has twin carbs and is happy to rev.
You have to consider these vehicles were not designed to run high speeds for long distances. The roads did not exist to allow it. Things like wind resistance, wind noise, road noise, and steering stability all can make driving at higher speeds very tiring.

It's an old car, take it easy.

Mart.

Whenever I rebuild a flathead V8 I have the entire rotating assembly balanced at the machine shop.