Transmission Decisions Decisions

[James_Douglas]

Well, I need to decide what "modern" automatic to use with the flathead six for the rebuilding of the big Desoto for get it ready in 36 months for a Grand Tour of the USA.

 

I thought I would get other folks opinions on this.

 

The big Desoto is very heavy. I cannot impress on everyone enough what the difference of a 1000 pounds makes on driving this car versus a standard coupe or sedan.  The same engine in the '49 CV and the car flies up a hill.  In the big Desoto it necessitates down shifting going over big mountain passes.

 

I have the weight of the reciprocating mass of the fluid-coupling and all the associated gears through the rear brake drum.  However, I have not been able to get the reciprocating weights of the usual GM automatic's one would consider to be able to do a apple to apple comparison of the parasitic drag differences. 

 

The 200R4 has less loss as the internal parts are lighter than the 700R4. But it may or may not take the load.  Also the gearing my be better with the 700R4 as drop at freeway speed may push the engine too far down on its torque curve and would require coming out of OD a lot more often.

 

I have asked a couple of "hot rod" transmission houses about using a 200R4 versus a 700R4 with a car that when loaded will be in the 5500 pound range.  I of course received different answers.  One answer was that the OD unit of the 200R4 may not take the weight and would likely require a "bullet OD unit" which is not inexpensive. 

 

Attached are the gear choices.  I could also look into the electronic transmissions like the six speed, but I really do not want to be out on the bleeding edge that far.

 

Thoughts?  James

 

Red is what is in the car now:

 

 

 

Edited April 17, 2019 by James_Douglas

[kencombs]

Can't help with the selection from those choices.  But, I would suggest adding the A500 and A518 to the list of contenders.  There low gear ratios aren't as steep but a good converter will offset that.  IMO the Mopar trans of that era are superior to the GM versions.

[Adam H P15 D30]

Hello James,

 

GM used the 2004R transmission in full sized Caddys back in the 80's so weight shouldn't be an issue.  Also, the 200 has a fairly steep OD .67, so I would recommend 3.9-4.1 gears at a minimum, probably 4.3 depending on tire size. I don't care for the 700's low 3.06 first gear then a 1.62 second, that kind of RPM drop might be a little much for our low revving flatties.  Another issue is the parasitic loss of the 700 is much higher than the 200.  Our 35 Ford has a 2004R, 3.89 gears and 27" tall tires and the OD is pretty tall for the LS, if I were to do it again, I would've put 4.10's in the rear.

I would also second the idea above for going to a A500 or 518 as it is a much better transmission in stock form to the GM's but they are a BIG BEAST and you would have to cut your floors to fit.

For the 200, I went with Bowtie Overdrives out of LA, price was good, service excellent and they will build what you want / need, give them a call and talk to them.

If you do go with the GM overdrive, setting the TV pressure is a must.  I have the necessary gauges to do so and you live near me so let me know.....

 

You can see in this picture how the A518/500 transmissions are big in all the wrong places.

 

Adam

[James_Douglas]

I have been also thinking of a 4L60E as I can set all the shift points by computer...

 

The parasitic loss is not bothering me too much, like I said, the fluid coupling, mini flywheel, clutch and pressure plate come in at 75-80 pounds.  A modern converter, even a lock up, tops out at 45 pounds.  Even if the trans parts were 30 pounds more than the stock 3-speed, I would net out even and with the torque multiplication on the launch it would be better I would think.

 

If I use a 4L60, I am thinking that I can program it to shift quick out of 1st gear, which I hardly use, into second. The "fast" second gear I am using now allows me with the 3.91 to hit 25 MPH for city stop and go without changing gears.  The 200R4 second is very close. The 700R is a little numerically higher, but still far below the "stock" second gear. 

 

I worry on the freeway, with the heavy car, that the 200R4 will have to come out of overdrive too often with the .67 OD.

 

You may be right about the 1-2 gear spread of the 700R4. When I had the M5/M6 in the car here in SF I would often start out in low range (1st) then manually shift to high range (3rd). The 700R4 would be in the same ball park as doing that.  My hope would be with the computer controller, I could have the car start out in low and then do a fast shift into second...

 

As to using a GM versus a MOPAR trans...there is a much better after market for assessorial like the controllers and the shifters. I have been using a stock transmissions in this car for daily driving of 16 years and 70K miles. I have earned my stripes and do not feel the need to be all MOPAR on this.

 

Keep the thoughts coming. 

 

James

[Adam H P15 D30]

James,

 

Does your DeSoto frame have an X-member?  One reason I went with the 200 in the 35 is because the 700 would've required major X-member surgery.

I would avoid the 4L60 simply because of all the electronics, but that's personal reasons, just never needed or wanted all that "stuff."

The tall OD in the 200 is why I would use 4.30 gears or lower.  Plus you get the benefit of better get up and go for those SF hills....

Throttle pressure points (for downshifting out of OD) can be built into the 200/700 without using a computer by the builder.

Your flathead doesn't make enough torque to bust a decently built 200 no matter how much you flog it.

Thinking about this you might consider an A904 or TH350 and a set of 3.0 or 3.23 gears.  You will have the torque multiplication that the fluid drive doesn't have, 1/2 the cost and a much smaller unit.  Would move your beast out much better than the M6 .

Just thinking out loud.....

 

Adam

[kencombs]

If I were building without a budget, I'd use a 904 and a Hone OD on a 9" Ford differential.  But the OD and 9" would kill my entire truck budget. 

[YukonJack]

If I remember correctly, the 200-4R was used in full size sedans, and 80"s SS Monte Carlos and Buick Grand Nationals. I would think it would be stout enough.  

[derbydad276]

go electric !!!

https://www.summitracing.com/parts/tci-302820?seid=srese1&cm_mmc=pla-google-_-shopping-_-srese1-_-tci-auto&gclid=EAIaIQobChMImsfv4c3Y4QI

[James_Douglas]

All very good points.

 

Finding rear end gears for the LWB cars is kind of tough. I could swap it out for a ford 9 inch, but I would like to avoid that as I have rebuilt the rear end with all new bearings and have a lot of spare parts for it. I also have a rear, custom made, disc brake set up for it.

 

I have talked with a few of the large transmission houses.  They only way to "match" the sift points to the torque curve of a flathead is to play trial and error with the transmission governor weights.  That would get old quick and it would take lot of time. not to mention a lot of machining of weights. I was told in no uncertain terms to not use a TV cable or rod to try and change the shift points as that would kill the transmission particularly in a heavy car.

 

That is why I am leaning toward an electronic version.  The shift points can be changed without touching the transmission.

 

The issue is that it will cost a fair amount.

 

 

[greg g]

I may have already given this info/opinion on another transmission related thread but I have a 700 r4 behind my 259 v8 (175hp) in my Studebaker half ton pickup'  the rear end is a ford 9 inch out of a T Bird, the trans was out of an 87 caprice. the rear gears are in the 3.23/3.55 neighborhood.  The truck weighs 3200 plus or minus.  with no load and 1/4 throttle, first/second shift occurs at 18 to 20 mph,2nd third shift occurs at 35/40, third to fourth at about 50 or earlier if you lift.  lock up occurs under slack throttle shortly after top gear is engaged.  With 235 75r15 tires, 60 mph is in the mid 2000 rpm range seat of the pants tach.  Don't know what hills or mountains you will be climbing but on interstate grades it will probably unlock in top gear and for steeper stuff it will probably spend most of its time in third.  I would imagine if your are touring you will probably have at least 500 pounds of junk in the trunk plus the weight of you and your passengers.  with both the 200 and 700 r's, auxiliary transmission cooling is a must.  I have mine running through an good sized heater core mounted in front of the radiator.  I know a guy who has a GMC pickup with a 700 r, with what looks like a baseboard heater element running the length of his passenger side frame rail in addition to the radiator cooler.

[55 Fargo]

10 hours ago, James_Douglas said:

All very good points.

 

Finding rear end gears for the LWB cars is kind of tough. I could swap it out for a ford 9 inch, but I would like to avoid that as I have rebuilt the rear end with all new bearings and have a lot of spare parts for it. I also have a rear, custom made, disc brake set up for it.

 

I have talked with a few of the large transmission houses.  They only way to "match" the sift points to the torque curve of a flathead is to play trial and error with the transmission governor weights.  That would get old quick and it would take lot of time. not to mention a lot of machining of weights. I was told in no uncertain terms to not use a TV cable or rod to try and change the shift points as that would kill the transmission particularly in a heavy car.

 

That is why I am leaning toward an electronic version.  The shift points can be changed without touching the transmission.

 

The issue is that it will cost a fair amount.

 

 

Kool on the rear disc conversion.

How bout some pics or did you post it on here in the past?

[James_Douglas]

Here is a couple of photos of the '49 conversion.  I did the same for the '47 although I have not installed it yet. The BIG issue on the rear is the tapered axel.  You MUST take an axel and a hub flange and put it into a big lathe and turn the face of the hub flat.  The you have to make sure you use the same key, axle and hub as a set.  The owner at ECI told me he does not do rear disc conversions due to this issue. He said people did not "get it" and would then complain that the disk did not run true and the brakes pulsed.  I even went so far as to place the actual disc onto the hub and axle and made sure it all ran true.  On the '49 it has worked just fine. 

 

Note in the photo that I made a rim mounting stud. It is threaded and goes through one of the old rivet holes. It is threaded and holds the disc in place and helps when mounting the rear wheel.  Slick I thought.

 

James

 

PS. My friend George in the photo.

Edited April 19, 2019 by James_Douglas

[55 Fargo]

Thanx work looks very good...