1960s TH400 transmission to 390 engine for 1961 Series 62

[Caddygranddaddy]

Hydramatic transmission is leaking and I'm thinking of swapping it for a TH400. I'm aware that there are adapter kits to allow the TH400 to mount correctly to the 390. Anyone do this swap? Any advice? Also, would be interested in having someone complete the job for me here is Austin.

[bcroe]

It may be relatively easy to swap engines and
transmissions of the BOP engine-trans era.  But
earlier stuff was not at all standardized, there
are a lot more obstacles. 

Aside from the engine interface, the shift pattern
is completely different.  Engine/trans to frame
mounts are affected, and no doubt a custom drive
shaft will be required.  To properly utilize the
400 torque converter, esp if it is switch pitch,
the axle ratio should be reduced.  Some wiring is
needed, check that your radiator can cool this
trans. good luck, Bruce (loves 400s) Roe

[Roger Zimmermann]

Having a car with the Hydramatic ('56 Eldo) and a car with  a THM 400, I prefer how the old Hydramatic is working. Another benefit from the older transmission: it's more economical than the THM 400. Both cars have about the same weight, the mileage from my '56 Eldo is better than the one from my '72 CdV.

[The Tassie Devil(le)]

When I purchased my '60 CDV, it already had a 454 and TH400 in the engine bay, and it ran well.

But, when I reverted to a '61 390 and a '60 Hydromatic, the car ran so much smoother because of the lower 1st gearing of the original transmission, plus the handling of the car improved with the heavier 390 and cast iron Hydro.   This weight brought the front down so the suspension worked as it was designed to.

The person who did the initial change made a new quadrant indicator to show P, R, N, D, 2 and 1, but didn't do anything to the NSS meaning that it wasn't correct, only allowing the engine to be started in Park and Drive, and the reversing lights only worked in low gear.

The other matter was that the TH400 in the car had the short extension.   The Cadillac TH400 with the long extension might not fit easily to insert the modified tailshaft which will be a lot closer to the X Member of the Chassis.

The only good thing about the swap to the TH400 is that the Kickdown switch is electric, and a simple micro switch can be adapted to work.

Bruce.

[bcroe]

Yes the axle gearing needs to be matched to the trans
type for optimum results.  The old hydo may be a bit
more efficient because of the torque converter in the
TH400.  BUT because the torque converter stretches the
useful speed range of each gear, it can produce the
same launch with a much lower axle number, which can
be seen by looking up examples.  That lower axle number
means lower engine cruising rpm and wear, with higher
engine efficiency.  I propose this pretty much cancels
any trans efficiency advantage. 

To take it another step, the 65-67 switch pitch
converters slip even less in economy while also
providing some advantage in brief performance mode
use.  Here a full sized 77 with stock engine (over
400 cu inch) running on crap 87 unleaded, with a
carefully calibrated speedometer/odometer, has over
and over provided 19 mpg at 70 mph cruising in good
conditions with a switch pitch TH400.  Good conditions
does not mean climbing mountains, zero degree temps,
or city stop and go.  Bruce Roe

[59-in-pieces]

As usual, I'm late to the party of switching a trans.

It seems that the TH400 (THM 400 - RZ)has just become the darling trans for discussion.
Just like in the 40's, everybody wanted and talked about switching out the gears for 411's.
What was 411's, and what did they do, don't know but had to have'm.

In 25 words or less, why would or should anybody switch out their trans, when there are so many "to dos" and stuff that needs to be changed to "make it work".
Not to mention, a shade tree mechanic is out of his depths - call in the expensive experts.

Have fun,
Steve B

[PHIL WHYTE CLC 14192]

Don't do it! Keep it standard.