Overdrive or Rear diff Crownwheel / Pinion swap

[Kit]

Knowledgeable people of this fine forum, I have been offered a B&W R10 that needs rebuilding, which digging into it it is looking quite an expensive job with proable parts having to be sent from the US to jere in the UK so wondering if I can just swap the rear internal crownwheel & pinion to give me some higher cruise speed.

 

What do we think? and are there any internal parts that I can just drop in? All in looking around £2500, pretty sure a rear diff upgrade be alot more affordable. I don't wish to swap the rear axel as thats more work & fabrication. Thanks All

Edited December 6 by Kit

[Young Ed]

The good part of the overdrive is it lowers your final ratio for a higher top speed without sacrificing any acceleration. I would certainly grab it and inspect it. If the rebuilding just comes in for the 3 speed stuff vs the OD area I would think that would lower the price of parts. 

[Loren]

My first choice always would be a Plymouth R10 G1 Overdrive.

I feel the stock rear axle is a good design with standard off the shelf Timkin Tapered Roller bearings.

However finding 3.73 or 3.54 ring and pinions new is difficult. Finding a 3.54 carrier from say a 55 or 56 DeSoto or Dodge V8 with automatic is much easier.

Back to the Overdrive

All the gearbox parts are the same as a 3 speed, except for the Overdrive parts and they are nearly all the same as the 11 other makes of cars that used the B-W R10.

So parts are not that hard to find and while a R10-G1 ( the MoPar version ) might be priced pretty high others are much cheaper and you can harvest them for parts.

So taking an Overdrive and rebuilding it is not out of the question. 
 

On the road the Overdrive transmission performs just like the 3 speed you already have, except when you get up to cruising speed it automatically shifts into another gear dropping engine rpm by 30%. A 55 mph car suddenly becomes a 70 mph car with no performance loss in the lower gears. If you change rear axle gears to get higher hiway speeds you will lose acceleration. It’s that extra gear that makes it worth the effort.

It should also be mentioned that if you’d like a closer ratio transmission you can install a 1940 second gear and cluster gear to raise second gear. Since the 3 speed Overdrive transmission is a Plymouth, it will fit with no modification. I’ve done it.

[moparfun]

Here's what I did.

 

[Loren]

For my 49 Bus Coupe I have acquired a 230 engine with an aluminum head and a two single barrel Thickstun manifold with a set of Red's tube headers.

So I expect some additional H.P.

The newly rebuilt R10 I put together has the 1940 gear set.

With some extra power under the hood I feel the engine can handle a 3.54 gear ratio, which I have on hand.

Remember a flathead Plymouth was good for a 22 sec 1/4 mile, so don't be expecting to out run too many modern cars.

They will cruise nicely as described above at 65/70 all day, however.

 

2500 pounds sterling is rather steep even for the other side of the pond!

Which is about $3,175 in U.S. dollars!!

I don't know what shipping and duties are but you could probably get a really good one from someone in the U.S. for a good deal less.

I've been collecting them and I like to pay around $250 for a used one of unknown shape.

[FarmerJon]

I am on the tail end of installing an R10 OD, so obviously that is my preferred route.

I also found that 3.73 gear sets were difficult to find for sale. Easier to get a whole rear end, but shipping would kill it for you, I suspect. 

My cost for rebuilding the trans myself was around $1200 US. This includes the price of the core transmission, a new clutch and pressure plate, nos input shaft, clutch gear, syncros, 2nd gear, seals and gaskets, USA made bearings, new rollers, snap rings, thrust washers, assorted hardware, cup plugs, spare Overdrive parts, and snap ring pliers. 

I did not factor in some install costs, like having the flywheel machined, ARP pressure plate bolts, and fluids. These costs were around $300, with flywheel work being right under $200.

I already had a spare standard transmission that i pillaged a few parts out of.

The second gear, input shaft, clutch gear and shift fork I stole from my spare trans were to convert my '55 pin syncro to the older, and more standard, blocker ring syncros, because they are stronger and I drive my car hard. Some R10s have the blocker ring syncros from the factory, but not all. As long as your gear and input shaft were in good shape, there would be nothing wrong just replacing the friction rings and keeping the pin syncro, and this would reduce cost by around $250-300.

 

As far as the "Fast" 1940 gear set, here is the math. 1.83 is standard, so ratio spread is  2.57, 1.83, 1.281, 1.00, .7

1.55 is fast, so the ratio spread is  2.57, 1.55, 1.085, 1.00, .7

This makes 2nd OD a duplicate for 3rd, so I stuck with the standard ratio spread.

If you were able to shift from second to 3rd without OD kicking in, the fast second would be better, but in the few test drives I have taken, when you let up and press the clutch, the OD engages, so you would go from standard 2nd to OD 3rd. 

I have actually found a similar problem if I shift 1-2 at full tilt, I am above the cut in speed and it wants to kick in 1st OD, so I have been making a point to shift 1-2 at 25mph instead of my usual 30mph. 

The long term remedy for this is to duplicate what the factory did: taller rear gears. I have a 4.11 axle I plan on rebuilding and installing next. This will raise final drive rpm, by around 200rpm, which is still a huge improvement over my standard 3 speed and 3.90 gears, but will give more grunt in 1st and second, which is always appreciated with 96hp.

I have found 2nd OD to be very similar to 3rd, so I wouldn't want the ratios and closer.

 

Another point I feel the need to make is that these are NOT 55mph cars. With my standard 3 speed, I have driven it regularly at 65-70mph (based on GPS) for thousands of miles over the last 3 or 4 years. If you look at the non-OD gear calculator, 3600 rpm is 79mph. 

I have gone as fast as 83, but started loosing steam due to valve float or point float. Even with the OD, I likely won't push it that fast again untill I do some suspension work, as handling started getting uncomfortable. 

 

ETA: I see that you have a '50, have you checked what ratio your axle is? I believe that is around the time 3.73 gears became more common.

 

Edited December 7 by FarmerJon

[Loren]

Wow! You do drive your car a little hard!

The governor enables shifting into O.D. at approx 27 mph. With a 3.73 axle ratio it takes some practice and determination to shift into 2nd O.D.

1st O.D. Is theoretically possible but the engine doesn’t have enough useful rpm left to trigger the governor.

Seems to me your governor might be out of adjustment.

If you are working with a 4.1 final drive I would be changing that right away!

Although the mid year 1952s were spec’d with 4.3s.

I have several 3.9s I would donate for the shipping. It would be a small improvement but worth the effort.

 

As a rule cruise speed would be determined by 80% of peak horse power which is 3,600 rpm, thus  2,880 rpm.

If you’re wringing more than that from it you’re off the power curve needlessly because the engine’s pulling power drops off.

[greg g]

Get a hold of member Mark Duggan  (mark@rdusaclassics.com).  He has  sourced rebuild parts for a couple OD's he rebuilt, and a couple sets of parts for others.  He can also provide a list of part numbers if you would rather source common wear parts independently .  My car runs a 56 Plymouth 230 with 225 r 75 15, 4,11 rear and overdrive.  At 62mph it was turning ~3300. With OD its runs ~2400.  And gets 19/20 mpg on the highway.

Edited December 7 by greg g

[FarmerJon]

Loren,

I currently have the stock 3.9 gears and 28" tires. I will be swapping in the 4.1 gears (and matching speedo gear) to further improve in town performance some time early next year.

You can see in the ratio charts I posted that in first, 25 mph is 3000rpm. Fine shift point for leisurly, around town cruising.

Peak HP is at 3600, which is right around 30mph. When hot rodding around (not always, but often) I wring it out that high because that IS where the power is. Any higher and power does fall off. I would estimate that the "cliff" where power really drops off is around 3800 rpm. 

In my test drives, I have had no problem getting the 2-2OD shift. 30mph and up it will shift, if I lift. It is very controllable. I usually shift from second to third 35-40 mph, so that seems to be a plenty big window to actuate the OD.