Rear Diff Swap
 

Good evening all,

Guys,

I have some questions, please. I recently bought a 37 half ton, and to be honest, I am not real familiar with the banjo differential setup and the torque arms / suspension.

Top speed of this old truck is about 50, and I am not wanting a high speed vehicle but it would be nice to be able to run 55 or 60 with it.

I want to keep this truck stock, but wondering about changing the rear differential and what is entailed in doing something like that.

Can the banjo diff be re-geared, or can the complete rear housing be changed out and if so, what kind of challenges, and what all would need to be changed?

TIA - KT

Re: Rear Diff Swap
 

You should look at an add on overdrive or a T5 or T170 swap

Re: Rear Diff Swap
 

A friend of mine mentioned a T5 but can that be done with an enclosed driveline, or do I need an open driveline?

Re: Rear Diff Swap
 

It looks like Hot Rod Works has a conversion kit for the banjo rear end, to do an open driveline.

Re: Rear Diff Swap
 

A completely different transmission sounds a little extreme if all you're looking for is a little more top speed. Changing rear end gears would be cheaper and easier. From the sounds of it, you probably have 4.44 gears in it now. Perchance, is (or was) your truck equipped with V8-60? Available ratios are 4.11, 3.78. 3.54, or even 3.25, any one of which would increase your top speed. Depending on how hilly it is in your neck of the woods, a 3.78 or a 3.54 would work well. I had a '36 with a 4.11 and 6.00-16 tires and it would do 55-60 easily.

Changing rear end gears is a big job, requiring the removal and disassembly of the rear end and torque tube, but not nearly as big as replacing the transmission. It's still working with all stock components, with no special fabrication required. If you still have the stock brakes, you won't even have to bleed them. Gears are available. You might even get lucky and find some street rodder with a complete rear end assembly with a better ratio available he wants to get rid off.

Re: Rear Diff Swap
 

Moonshine, the advice above is expensive, not needed, and certainly not in keeping with your wanting to keep your Ford stock.
To answer your questions, yes, the gears within your banjo are changeable, with a wide variety of gear ratios available. Doing the job yourself will require a reasonable degree of mechanical ability, but is certainly doable, and at a very reasonable cost. …. I see that Tubman has posted just now before me, si I’ll just say he’s got the full story for you.

Re: Rear Diff Swap
 

Which in itself is another big job.

Re: Rear Diff Swap
 

Taller tires could help too.

Re: Rear Diff Swap
 

yes, The Art Doctor abides!

a tire with a diameter of 25", if increased to 26", will increase the speed at any give RPM by 4% (just a mathematical example), alone, without ANY other changes being made. simple arithmetic is all that's necessary to find the increase in speed one would experience at any give RPM in top gear.

and it's quite possible, considering the modest increase in top speed desired by the OP, that such a simple solution would find success, but i feel it should be noted, that there are practical limits, usually associated with hill climbing performance that are a consideration, whether the increase in speed per RPM be accomplished via transmission, differential, or tires.

when considering such a change myself, i simply bought a set of rear tires (used) with a larger diameter, mounted them, and headed for the nearby hills. for a final test i climbed the Sierra's with them and found that although i got the increased speed per RPM i was looking for on the flats, the hill climbing and increased frequency of downshifting on hills was impracticable for my purposes, and so reverted back to the tires i had.

Re: Rear Diff Swap
 

If the O/P is correct in stating that "Top speed of this old truck is about 50 (mph)", he either has very low (high numerically) rear end gears or a very weak engine. If the latter, none of these proposed solutions will help until the engine problem is corrected. If the former, all of them will work to some extent. As I said earlier, I had a '36 (3-window) with a stock engine, 6.00-16 tires, and 4.11 gears. Given my experience with that car, he must have the 4.44 gears that came with the V8-60 vehicles (yes, they put them in commercial vehicles) if the engine is in decent shape. We really should get to the root of the problem before offering solutions.

O/P, what sayest thou?

Re: Rear Diff Swap
 

I’d say install some 3.78 gears.

Re: Rear Diff Swap
 

Make life easy on yourself and your wallet. Put a 3.54 or 3.25 gear in the rear end. 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.


I did this to my '37 Tudor with a stock 221 engine and I can do 70 mph anytime. No problems.

Re: Rear Diff Swap
 

[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.

Re: Rear Diff Swap
 

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.

Re: Rear Diff Swap
 

perhaps you have a engine performance problem— I would drive the 60 hp car with the 4:44 rear at speeds up to 74 mph

Re: Rear Diff Swap
 

I have long wondered what is a "safe" RPM for an extended amount of time for the average 21 stud old Flathead V8 engine with decent oil pressure. With the 4.11 gear in my 36 pickup I rarely exceed 50 MPH. I tend to stay on "pleasure" roads where I can putt along about 40 MPH enjoying the sights and sounds.

Re: Rear Diff Swap
 

Thanks guys, for all this advice, and I just wanted some ideas to ponder on! I will read and digest it and understand.

Thanks!

KT

Re: Rear Diff Swap
 

No, the Flatty has good compression on all 8 cylinders so I am thinking it's the rear being low geared. The engine code starts with 18, so I belive that is an 85 hp.

Re: Rear Diff Swap
 

That is most of my driving, but there are times when we might drive 25 to 40 miles to a car show, and it would be nice to run 60 in a 65 speed zone

Re: Rear Diff Swap
 

I had a 4.44 dif, and could barely make 60 mph with it. I replace it with a 3.78, after which I could get up to 77 mph.
The Ford flathead with stock distributor will not redline. Point float will keep rpms down, safely enabling pedal-to-the-floor for extended periods.

Re: Rear Diff Swap
 

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.

Re: Rear Diff Swap
 

[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.

Re: Rear Diff Swap
 

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.

Re: Rear Diff Swap
 

[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.

Re: Rear Diff Swap
 

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.

Re: Rear Diff Swap
 

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.

https://www.fordbarn.com/forum/attac...8&d=1760536053

Re: Rear Diff Swap
 

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