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tinlizzy

rear end swaps

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I have a 48 Pilothouse with a 4.11 rear end.  Just bought a 1950 dodge 2 door with fluid drive in hopes of finding a 3.55 pumpkin.  I couldn't see it before I bought as the car body is pretty well roached with all 4 tires were down in the mud.  So I bought it,  drug it home and spent last 3 days cleaning and clearing to get a view at what the rear end ratio actually is.  Of course the guy I bought it from swore it was a 3.55.  Well turns out it is a 3.90.  So my question is, is it worth changing out the gears from a 4.11 to a 3.90 as far as any difference on the road speeds?   If it isn't worth changing I will just leave it alone so I can get truck back on the road before I go over the rainbow bridge.  I do have a 87 dakota waiting in the wings if needed.  I already had a 48 Chrysler 3.90 so I don't need 2.  If someone needs one they can come and get it.

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From my research of other axle swaps, the ideal for the flathead is 3.73, as 3.55 is great on the highway but the flathead struggles off the line at low speeds...3.90 should give ya better highway speeds, but around 65 the flathead may seem to start hollering a li'l bit tho it can handle it...I drove 65 my 4.1 on a few occasions and the flathead could do it, it just didn't get in any hurry to do it nor did it seem like it wanted to do it for long...ya might want to examine those rear axles closely before liquidating inventory, I've seen ring gears get cracked and them things do not grow on trees :cool:

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3.73 ratio is probably optimum. You could pretty much get there if you fitted the 3.90 and a set of taller tires.

I have a 3.55 in my 3/4 ton with a slightly later 230 in it. If I was building another I would most likely go for the 3.73.

There are trade offs with all of this but some fine tuning can be done using different tire diameters.

Just something to consider.

Jeff

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thanks for the info.  My truck has a 251 Chrysler with 3 speed on the floor.  Looks like the easiest way to improve go power is get larger wheels and tires, currently has 15 inchers.

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I installed a 54 Plymouth differential into my 55 with 3.73 gears. Night and day difference from the 4.10's. I regularly cruise the highway at 70+. Those pop up from time to time on craigslist and ebay.

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I had 4.10 originally in my truck. It was screaming at 60mph. Switched the pumpkin to 3.90 and it did help a bit for cruising. I struggled with the un-syncronized transmission, so I eventually installed a Borg Warner T5 gearbox and a 3.55 rear out of a Jeep. I'm very happy with how it drives now. Plenty of get up and go at lower speeds, no more double clutching and gear grinds. I've noticed however sometimes on a long gradual grade it doesn't pull great in 5th gear. I'm OK with it the way it is, but if I were to do it again, I would go with a 3.73. Perhaps I'll change it out at some point.

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25 minutes ago, Jj1981 said:

Was an adapter needed for the new transmission

No adapter plate! I removed the bell housing and drilled and tapped mounting holes. Might be able to that without removing bell housing?  It was a project for sure on my truck. Requires and adapter ring for the transmission's input bearing housing, custom floor board, fabrication of emergency brake adapter, custom brake cables, custom driveshaft. custom speedometer cable, and a few other little things I can't remember. Was a fun project for me, but like always took me way longer than I figured.

DSCN2015.JPG

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Okay if I want to switch the differential from 410 to 390 how hard is it to get the differential out and back in.  I've loosened the housing nuts on the 390 which is out of the car but can't budge the pinion nut.  Does it have to come off before the center section will come loose.  Never taken one apart before so don't won't to tear up anything.  Is there something I am missing that still needs to be removed before the differential will come out?  Thanks for the advice.

 

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In general, axle shafts need to be removed before removing differential from axle housing, but I generally like to break the pinion nut loose with the tires planted on the pavement using a high torque impact gun...generally, the differential preload and backlash adjustments need to be done with the diff back in the axle housing with the pinion yoke in place...

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Thanks JB, I was able to get nut off pinion the hard way 4ft pipe wrench on housing braced so it wouldn't move then breaker bar and socket with 4 foot pipe on that, came off with a bang.

So it is not just a swap of the pumpkin. 

You have to more or less disassemble to the rear end and if you pull axle shaft you might as well replace bearings and seals.  Got it. 

I watched a utube video today on redoing truck rear brakes, but mine were different.  Only the drums came off leaving the axle nut and housing; but once off I wondered why the drums had a spot of weld on them with matching one on the housing.  I guess last person to do brakes didn't get the axle nut and housing off they just broke drum off?  I may just leave the truck as is rather than go to that much work.  My age is catching me and I would like to drive it before too much longer.

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On 7/12/2020 at 4:04 PM, tinlizzy said:

So my question is, is it worth changing out the gears from a 4.11 to a 3.90 as far as any difference on the road speeds?   If it isn't worth changing I will just leave it

 You asked for opinions so here goes ;   No , it is not worth it . I have gone that exact route and it was a waste of time and money . Yes you will see some improvement  , but only a little . 

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4 minutes ago, Jerry Roberts said:

 You asked for opinions so here goes ;   No , it is not worth it . I have gone that exact route and it was a waste of time and money . Yes you will see some improvement  , but only a little . 

I've got a 3.90 in mine but I built it that way-wasn't a swap out. Somewhere way back I remember reading that of you aren't switching by at least .5 it's not worth it or very noticeable

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I picture in my mind, all 23" flatheads running 3:73 gears ... while 3:55 will keep you very busy shifting

Young Ed has 3:90 .... he never followed the pack anyway ... sooo

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16 minutes ago, Los_Control said:

I picture in my mind, all 23" flatheads running 3:73 gears ... while 3:55 will keep you very busy shifting

Young Ed has 3:90 .... he never followed the pack anyway ... sooo

Someday when I have spare time I'm likely to switch my truck 3.90 with my plymouth coupes 3.73. The car has overdrive so the 3.90 would be better in there

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I swapped out a 3.23 in my long bed Dakota to a 3.55 Posi-Trac back in '99, spent time + $$$ to get slightly better grunt off the line + trailer pulling power but lost 1 mpg and was a little slower passing on the highway....ultimately kinda regretted the whole exercise...since then I learned more about doing calculations on paper better, and about powertrains in general, and it's all a balancing act dependent upon engine torque curve, gear ratios and tire diameter as well as costs in time + $$$...if your 4.1 gears have cracks and the 3.9 is pristine, then it's a no-brainer :cool:

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On 7/13/2020 at 11:28 AM, tinlizzy said:

thanks for the info.  My truck has a 251 Chrysler with 3 speed on the floor.  Looks like the easiest way to improve go power is get larger wheels and tires, currently has 15 inchers.

Yes that can help, taller tires will kill some highway RPM.  215/85R16s is a good size on these trucks.  They have a tall/skinny proportion like vintage bias ply tires and are about 31" tall. 

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I swapped out my 4.1 for a 3.54 and I have never looked back. Sure I have to change down a gear going up some hills, but the gain in reducing the working of the engine on my highway runs is more than worth the effort 😁

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