Overdrive tranny in 1956 Cadillac?

Hey there,

I am planning on changing the transmission of my '56 Cadillac. And I am wondering, would a 700R4 work well with this type of car? It does weigh over 5000 pounds! I was told that the car would be too heavy for the overdrive. Any suggestions?
Advice would be greatly appreciated!

 

You can use a 700r 4,just upgrade some of the internal parts,they put them in 4x4 trucks,if your gearing is right you'll be fine.

 

Do you know what parts need upgrade? I have the original diff.
Thanks!

 

I would recomend a 3.73 gear ratio for the rearend. Thats what works good from gm in my in my suburban.

 

I believe my Cadillac has 3.07 final drive gear ratio. Too low?

 

I'll ask the question; with a 3.07 rear ratio, why overdrive? Better gas mileage, or lower highway rpm? 3.07 seems like a good final drive ratio, at least for top end.

 

Well...both! lower RPM = better gas mileage.
With my 3 speed Jetaway, I'm lucky if I get 15mpg/hwy. And the RPM runs higher than the Chevy Caprice I had, which got over 25mpg/hwy! The tranny is tired and showing signs of impending retirement, so I figure on putting in an overdrive tranny to lower the RPM more....especially with today's skyrocketing gas prices!

 

Thanks for the tip! Does the 4L80 run by computer? If so, are there kits for this?
I saw an ad for a 4L80 from a 95 GMC, in my area for $200. Very tempting...

 

Jetaways, used in primarily, Cadillac, Oldsmobile, and Pontiacs from 56-64 are really 4 speeds. You might not be able to feel the 3-4 shift cause these transmissions were smooth shifters. But go down the highway at 50-60 in Dr4, then shift to Dr3 and you will feel the lower gear.

 

Lots of 700R4's in one ton trucks at least as heavy as your Caddy. the RPM would drop about 1/3 in OD, so 3000 now will be 2000 in OD. It won't be a racer with 3.07 gears, but it should cruise easy enough.

 

Thanks. I also thought my transmission was a 4 speed. The selector has no "Dr3 -Dr4". Rather, under the Dr, there is a horizontal arrow. The owner's manual says that selecting on the right side of the arrow will lower the speed, and to the left for high. But I don't notice any change in shift or RPM. Makes no difference.
Been driving the car for 7 years now and never noticed a 4th gear. RPM stays the same. 4th speed broke,maybe?

 

308 gears both jetaway and 350-400 are a finel drive of 1 to 1 before you go to all the overdrive stuff look at a lock up converter first I did this in my 52 and got 23 mpg.wasnt a speed racer but good on gas. what are you useing for a tires ?

 

So you think the 700R4 is strong enough for my '56?

 

Yep, 4th gear is probably not existent due to a stuck 3-4 shift valve. Might be able to be repaired without a major rebuild.

The only component that changes from 3 to 4 is the controlled coupling, but that also happens on 1-2 shift, so obviously that is working. But a defective 3-4 shift valve will keep the controlled coupling from filling, and therefore be 3rd gear, where you think you are in 4th. BTW: 3rd gear is 1.45 to 1 reduction, meaning your engine will be running at a higher RPM than if you had 4th, which is 1 to 1 ratio. You might want to have someone do a fluid change and clean filter while there.

The new transmissions are great, but have you priced the adapter for your engine to the new transmissions. How 'bout 1k or thereabouts, not to mention the crossmember work, linkage, etc.

 

Not always. With your tall gearing and overdrive you might really be chugging the motor at certain speeds and might get worse mileage than you do in third.

 

id say even if you do get an 700-r4 and it bogs in overdrive, you can always keep it in direct drive to get the torque you need. id say get the 700r4 cause parts are easy to come by as well as a cheap rebuildable core from a junk yard. i picked one up at pick-your-part a while back 80 bucks. and having that 3.06 first gear is really nice with such a big car. good luck!

 

who makes the adapter to the long bellhousing cad?

 

The 700r4 is strong enough. generally the tranny needs to be strong enough for the motor not the weight of the car. the 4l80e is a good trans but to much money and trans for what you got. It needs a computer and a throttle postion sensor. i have one in a 7 ton bus.

 

you gonna use the car on the super slab? long trips? go with the od. hell my ot/burb came with 3:08 with a 700r4. come on guys i have forgotten what the upgrades are. use parts from a vette tranny ,don't you? shoot can't remember? the low 1st gear will get that heavy boat moving quicker, even with the 3:07 rear. like othesr i wonder about the adapter tho?

 

Wilcap will have an adapter to put that 700r4 in there.

 

The fluid coupling(s) in the dual range hydro doesn't (don't) contain a third element to multiply torque at stall (the reason for a four gear box with nearly a four to one low). It's because of that third element (stator) that torque converters loose efficiency as the RPM goes up and need a lockup feature to gain back that efficiency. Dual range hydro fluid coupling actually becomes more efficient as you turn it tighter.

Before you get all involved with the expense of adapting to a later powertrain that may or may not pay for the cost of all the changes in improved operating effiency over the time you own the vehicle, do consider rebuilding the existing transmission (probably wouldn't hurt to make sure the thing actually can get in fourth gear) while concentrating on improving engine efficiency by updating the cam to something with a modern profile (possibly a roller). Although it's back in the "will this thing ever be able to pay for it self" category, converting to one of those new aftermarket throttle body injections that seem to have quite an edge when it comes to operating economy could give a considerable boost and be hidden under a stock air clearner.

Just bear in mind that while you can probably get the mileage up considerably in steady state cruise conditions, accelerating a 5000 pound brick is going to require far more fuel than doing so with a 1500 pound econobox deathtrap.

Expensive or not, there is something to be said for driving around in something that wasn't styled in a studio in SoCal, and doesn't look like the pile of cow manure that fell off the barn wall just after someone carved a caracature of an Asian face laughing at you in one end.

 

My Tires are "American Classics" 235/75R15

 

Mostly highway driving.

I have a good friend who's offering me a deal for a 350 4 bolt main and a 700R4 or TH400.

 

If you get an od transmission you can swap out the rear with chevy 1/2 ton rear . get a 70's -mid 80's 5x5 bolt pattern 12 bolt truck . The rearends are a dime a dozen and dirt cheap . Straight up swap with lots of gearing options . I am using one in my 53 Cadillac . they are 64 inches wide same as the caddy rear

 

Thanks for the tip! So you have a 700R4 in your Cad?

 

Lol!! Well said!

 

The 4L80E is the OD version of the TH400 and comes in electronic form only.

The stock trans is 4 speeds already, just going to a more highway friendly gear, if it's not already like a 3.08, will accomplish the same thing. The Caddy has more than enough torque to handle that. But it's a 5000-lb car give or take a few hundred pounds, you're looking at 15 MPG average tops without going to all-out electronic engine management.

 

No, I am running a 454 with a turbo 375. the turbo 375 is a turbo 400 body and a turbo 350 long tailshaft. Its the same length as a hydromatic. The 375 were is late 60's Novas and early 70's 2wd pickups. I am running Coker Classics L-78. The rear is 3.42 and the tires are 29 inches tall. Last weekend I drove down to Maxton 660 miles round trip I got 15 mpg at 72mph. But my last 100 miles were driven very aggressively and got 9 mpg. At 72 mph I am turnnig about 2800 rpm.

 

Guys, I'm a little confused. On one hand, the 700R4 is good, on the other hand, it's not good for my car
I got in touch with a friend who owns a BadAss Kustom 1957 Cadillac which has a 350, 700R4 tranny and original diff. He says he loves it!
I also checked a 1992 Buick Roadmaster stock gear ratio and it is 3.08, which is almost the same as mine, which is 3.07.
Shouldn't the 700R4,mated to a 350, with my gear ratio work fine?
Unless I'm misunderstanding the way gear ratios work?

http://www.carsdirect.com/1992/buick/roadmaster/specs

 

700r4 work great with 3.73 or lower gearing . if you keep the caddy rear it would drop your engine rpm too much and wouldnt be much fun to drive . a turbo trans would be a better match if you keep the 3.08 . I would have kept the caddy rear with my current engine/trans combo but the rearend needed to be rebuilt, leaked everywhere and the drivers side drum was covered it axle grease. my gas mileage would be better with the caddy rear. The main difference in the two types of transmissions are that the 700r4 has overdrive and the turbo trans are direct 1-1 drive.