Overdrive VS. Differential...Another Discussion

[Frank Elder]

5 hours ago, 48ply1stcar said:

Wrong quote

 

5 hours ago, 47 dodge 1.5 ton said:

Why not use the 8 3/4” mopar rear? I always wondered why many on here go 8 1/4” or 9 1/4”? 
With a hundred horse or less at the crank a 7 inch would be more than enough....the stock rearend is overkill for the flattie.

[Young Ed]

10 hours ago, Bob Riding said:

Very interesting about the '40 gearset. I have the stock 'P10 (1940) third member with a George Asche OD that works great in my 1940 Suburban. I can easily spend a significant amount of time in 2nd and 2nd-over around town. In my current '52 Suburban project, I installed a Ford Ranger rear end (for the parking brake setup) so  I have the spare 3:73 pumpkin that came out of the '52. I was wondering if it would be an improvement to transplant it into the '40 - obviously final gear would allow lower rpm cruising on the highway, but I wasn't sure about whether it would be worth the trouble, especially after your comment about the one year only gearset. I thought the the '40 and '41s were pretty much the same under the sheet metal...

Believe they are talking trans gear sets not diff

[derbydad276]

My 53 Coronet has 7.10 x 15 tires ....I want to switch to 215/75/15 radials ... Last year I swapped in a axle from a Cherokee last year ... 3.55 ratio .....car had 3.90 gears.. the drivability change was night and day ...

I live in SE Michigan which is relatively flat... with the 3.90 gear  set 60 mph sounded (wound up )...now it cruises with ease at 60 Perfect for Michigan's 2 lane highways .

In town ... with the tall gears  15- 20 mph second gear was done ....now drives 20-25 in second fine ....

I also picked up a OD trans ... looking to be able to drive 70+ on the interstate 

[ccudahy]

I don't want to stir the pot here, but someone is going to have to spill the beans! My interest is peaked with hearing about this mysterious 1940 transmission gear set and overdrive combo. I tried searching on the web and nothing comes up. Never heard of it!

 

The Mopar historical folklore and tweaks made back in the day are not as common as, say Ford practices, and needs to be shared. The new changeout stuff is great and easy to do. Sometimes it is nice to know what was done before disk brake conversions kits and Jeep rear end swaps. No offense intended.

 

I know this not a Ford Forum but for example. The Ford folklore was the '39-41 Lincoln Zephyr transmission gear sets were magic and add that to the Colombia 2-speed rear end, the Ford flathead became pretty quick instantly. Brake upgrades use the '46-48 Lincoln Bendix units. As soon as someone finds out you own a Ford Flathead, that recipe card is handed to you.  

 

Mopars it been 50/50, embrace the old or rip all that old stuff out and put new in. There have been a couple old tricks I have picked up from here that I didn't know. Drums can be upgraded to 12'' ones off Chryslers and DeSotos. Third members can be swapped out for 3.73:1 or 3.54:1 ratios on pre-1953 rear ends. Cool stuff, IMO! 

Edited February 18, 2021 by ccudahy

[Sniper]

Effective rear gear ratios need to be considered.  Ideally, the OD swap will let you keep your stock transmission gear ratios but when in OD lower the engine RPM.  The original OD setup basically does this.  Just about any other OD swap, the A833 for example, has different 1-2 gear ratios and can alter the driving characteristics in an unacceptable manner.  The A833 OD variant has a much lower first gear ratio than my stock trans which would make being in first gear less useful as the engine would wind out much quicker.  To some extent you can crutch this issue with tire size changes.  Bu there is a limit and out stock tires are already on the tall end of the chart.

 

Having driven a stock Mopar, 87 Diplomat, that used the rear gear change method from the factory to lower engine rpm, it's the worse choice available.  It had 2.26 rear gears and a three speed auto, slug came to mind.  I put in a cop spec axle that was stronger and had 2.94 rear gears.  I would have preferred 3.23's but that would have cost me a lot more than the $100 I spent on the axle.  Make a real noticeable difference too.

Edited February 18, 2021 by Sniper

[Loren]

Three speed low gear is meant to get you across an intersection 2nd gear is meant for in town driving and third gear was meant for the highways of the time.

 

My Dad told the story of Mrs. Heddington who's first car was a 1919 Center Door Model T. She drove this car for 20 years and finally had to give it up for a new 39 Ford. Not being familiar with gear shifting and a clutch that didn't work at all like her Model T she found driving a little confusing. My Dad who was born the same year as her Model T took great pains to teach her how to drive a stick shift. In the Model T the left pedal controlled low, neutral and high. Put the hand brake in the middle and it depressed the left pedal enough for neutral, step on the left pedal and the car moves forward in low. Put the hand brake all the way forward and take your foot off the left pedal and your in high. He said she got the clutch figured out pretty fast but gear lever position was a mystery. You would see her driving along with the engine racing (low gear) or lugging horribly (high gear). He patiently explained the "H" pattern numerous times and finally got her to start in low gear and shift to second. She never drove fast enough to select high gear.

Her comment was "Why did they have to change it?"

How many times have we said that when getting a new phone or some other device we refuse to read the manual for?

The dealer wouldn't take the Model T in on trade so Mrs Heddington offered it to my Dad for helping her.

 

At this part in the story I asked the burning question, "Why didn't you take it?" I got the answer "Oh I would have had to fix it and there were more free Model Ts around than takers." It seemed obvious to me but I had to bring it up anyway, "If you had fixed the Model T, she might have traded you the 39 for it."

He didn't have an answer for that.

 

One of the most appealing things about post-war Plymouths is that they simple basic cars. How much can possibly go wrong with them? When you start changing things like transmissions its a cascade of re-engineering. Change the transmission and you lose the eBrake so you have to change the rear end to get an eBrake back. Change the rear end and its brakes then you need to change the front brakes so the service brakes work right. Ugh!

In racing I found out proof positive I am a much better mechanic than I am an engineer. I don't like to change the design of anything unless there is no alternative. If something performs less than desired, try the next bigger version of the same design family. In Mopar brakes, 10 inch Plymouth to 11 inch DeSoto to 12 inch Chrysler. In rear ends if the ratio doesn't work for you, change the pumpkin. They made a lot of ratios and if you get it wrong it's easy to change back or find another ratio. I've never heard of anyone breaking the 8 1/4 rear end but if it happens then you could get a 8 3/4 with even more ratios and limited slip differentials. If a 218 Plymouth engine is not fast enough maybe a 230 Dodge might be and if your speed crazy a 251 or 265 Chrysler might be more your style. The Plymouth Overdrive transmission does nothing but add to the charm of the car. It is not a "Speed shift" racer gear box but neither is the rest of the car. It was good reliable transportation not a racer (apologies to Lee Petty). If you want a racer get something later because Plymouth certainly made them. The 1940 gear set is just an alternative. A fun thing you could add or try if you want to.

Just my humble opinion your mileage may vary.

[Sniper]

1 hour ago, Loren said:

Three speed low gear is meant to get you across an intersection 2nd gear is meant for in town driving and third gear was meant for the highways of the time.

 

Very true, the A833OD swap I mentioned makes 1st more of a halfway across the intersection gear.

[Loren]

FYI

There is a Mopar pumpkin on eBay right now that has the four spider differential that is billed as a 3.54 ratio.

I am thinking it had to come out of the bigger Chrysler because of the 4 spider diff. The seller has the housing casting number in the listing.

This might be what someone is looking for.

[plyroadking]

13 hours ago, Loren said:

FYI

There is a Mopar pumpkin on eBay right now that has the four spider differential that is billed as a 3.54 ratio.

I am thinking it had to come out of the bigger Chrysler because of the 4 spider diff. The seller has the housing casting number in the listing.

This might be what someone is looking for.

Hard to tell but looks like it has 16 count spline for the axle shafts. I'm not sure when they changed from 10 spline but that 3rd won't just slap into at least a 1940 though 53 Plymouth. 

[Booger]

theres a R10 OD on EBay starting at $1000 looks to be all there

[Dartgame]

For 833 4 speeds, there were close ratio boxes used in B body applications, perfect spacing - RPM change of 1500 on each gear. I'm not sure if you can adapt that gearing to an overdrive 833. Maybe just the 1-2 gears.  If I recall correctly mopar flipped the shifter lever on the 3 -4 gears and made what was 3rd gear (now 4th) into the .7 ratio overdrive, and 3rd gear what was 4th is 1:1. A body/aspen 833's used a steep 1st gear of around 3.09 and b bodies used 2.66 as I recall.

Edited February 19, 2021 by Dartgame

[Plymouthy Adams]

and you just answered your own question of not being able to use the other gear set for OD as they were not OD by design....a significant difference in the two arrangements...no under drive gearing in the stock 4 speed boxes..no flipping would gain that...

[Frank Elder]

I believe when my left knee has had enough abuse a 518 is in my future,  I’m sure that @Wayfarer will be able to whip up an adapter for me.

[Sniper]

On the A833's the gears on the main shaft are possibly swappable, but I think there are also main shaft diameter differences.  However the cluster gears are all one piece and not individually swappable.  So you can't mix and match gears, wish you could.  Now there is a source for an aftermarket OD gearset that does have a good gear spread, 2.65 first gear and a .7 OD IIRC.  Not cheap though, probably indestructible.

[Plymouthy Adams]

could be they are....I have never taken the A833 down.....I guess I was thinking more of the stationary lay gears not being changed....

[Frank Elder]

For the wealthy you could also order a brand new tremec with whatever gears you want in it.