Rear Diff Swap

[moonshine runner]

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

[ActionYobbo]

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

[moonshine runner]

Originally Posted by ActionYobbo

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

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

[moonshine runner]

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

[tubman]

Originally Posted by moonshine runner

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

Which in itself is another big job.

[tubman]

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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.

[ford38v8]

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.

[The Art Doctor]

Taller tires could help too.

[hueyhoolihan]

Originally Posted by The Art Doctor

Taller tires could help too.

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.

[tubman]

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?

[moonshine runner]

Originally Posted by tubman

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?

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.

[alchemy]

I’d say install some 3.78 gears.

[Seth Swoboda]

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.

[34fordy]

[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.

[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.

[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.

[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.

[Kurt in NJ]

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

[34fordy]

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.

[moonshine runner]

Originally Posted by 34fordy

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.

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