500 Swap

[61caddy]

Hi
Any info will help a lot.
i have a 1961 Cadillac 4 door with a 390, i just picked up a 500 from 1975 cadillac with th400 trans.
Has anyone done the swap or have any instructions?
Also I was thinking about getting a set of heads from 1978 425 engine will they work and make more power?    

[Stampie]

Yes, not really, and kinda maybe.

Stampie

PS - In a rush will provide more info later if others haven't answered.

[STDog]

Putting the 425 heads on a 500 might raise the compression a little, but not a lot.
The heads are bigger than the common spec. Instead of 96cc they are ~110cc, even bigger when you make the mods you'd need to use them on a 500.

You need to open the chambers along the mating surface so you don't have the sharp edge in the cylinder, which would make it very prone to detonation (know/ping). You also have to modify the top of the block for some extra holes in the head, and the gasket to match.

Even then, the fire ring is dangerously close to a water passage and might not seal well.

That's a lot of work for little gain, especially when a 120cc headed 500 can make plenty of torque and power. Sure you'll loose a little due to the lower compression, but it's really not worthwhile.

[Cadillac Kid-CLC #15364]

You may have noticed a lot of negativity about your proposed swap.  A later engine looks (terminally) unique in an early Cad, but there is a tremendous amount of work involved in the transition, and if you are not able to weld, hammer sheet metal, run tubing and exhaust pipes it will be quite costly.
All that said, I am really fond of the 472/500 cubic inch family of Cadillac motors and when properly modified can produce gobs of Torque and enough horsepower to get out of their own way quite nicely.
A 1975 500 inch smog motor in all probability (gut feeling) probably puts out less actual usable power (in stock form of course) than a well built, high compression 390.
When your car was new it could keep up with most everything built, short of a 409 Chevy or a 406 Ford.
Remember you have 2-1/2 tons of steel to move, and the 390 was designed to do that quietly (you don't have to stay quiet) and without fuss.
If you are dead set on the conversion, there are quite a few on this forum that have done it or at least tried it and can give you a lot of useful, actual experience driven information.
Greg

[Geoff]

There is a '62 convertible in the Self-Starter for sale right now with a 472/500 in it.  But I'm with Greg, it has been/can be done, but a good 390 will be most satisfactory.
Geoff

[The Tassie Devil (Bruce Reynolds)]

I pulled a 454 Chev and TH400 out of my '60 CDV, stuck in a bog standard '61 390 and a '60 Trans.

Best thing I ever did.

Bruce.

[61caddy]

i know i love my 390 just wish there would be more aftermarket parts, I want to spin the tires at least .... 
 so you think the 1975 500 would be the same as my 390 as far as hp and trq?
I am planning to sell my 390 and trans after the swap is done and get some money back so the 500 swap should not be that bad maybe 2,500 out of poket.

My cad has no value in it seens its not original  its 4 door .... paint is blue and inside is custom... thats another reason why i am thinking about doing the swap 

[61caddy]

Bruce,

why did you take the 454 out u didnt like it?

[The Tassie Devil (Bruce Reynolds)]

There are a number of reasons, and I will try to remember them.

(1)   The weight difference affected handling.   454 and TH400 is about 300 pounds lighter.
(2)   The useable Torque of the 454 wasn't as good as the 390,
(3)   The HP of the 454 wasn't as high as a standard 390.
(4)   The Trans Selector of the TH400 is PRND21 whereas the Hydro was PNDLR, which affected the Reversing Light operation and Neutral Start Safety Switch.

The change back to the 390 and Hydro really turned driving the car back into a pleasure.

Bruce.

[STDog]

A 1975 500 inch smog motor in all probability (gut feeling) probably puts out less actual usable power (in stock form of course) than a well built, high compression 390.

I don't know the real output of any of the 390s (got any dyno info on one?), but
I have seen dyno numbers on a stock rebuild, 120cc headed 500.
http://www.cad500parts.com/catalog/page10.htm

493.5 Lb-Ft at 2600 RPM and 302.1 HP at 3600.

8:1 compression (big valve reliefs for later testing), 60 over.
Stock cam, intake and exhaust.

They also did a 10:1, 76cc build the same way, and

Bone stock it pulled 323 HP and 504 Ft-Lb of torque

Use those numbers and compare to the published spec for a 1970 500, and assume that the published 390 specs are just as far off.
MotorEra.com says the '59 -'63 390 spec was 325 hp @ 4800 rpm.
The '70 500 spec was 400 hp @ 4400 rpm

So, the 70 actual is 19.25% less than the spec.
For the 390 that would be 262 HP actual.

[Cadillac Kid-CLC #15364]

Thomas,
Not to be arguementative, but the horsepower numbers for the engines moving our big trucks are a lot less important than theTorque numbers and torque curve.
The issue with this post is the effort to convert from a 390 which, I believe has a torque rating of over 450 lb/ft. and a curve that starts at over 400 at just over idle, for a car that is just intended for driving.  The conversion although do able is, for someone not able to do all the work themselves, complex and costly.
This is the "Modified" forum, but sometimes the factory engineers get it right.
Greg

[61caddy]

well now i am lost...... 

Not sure if i am going to do the swap since it might bring a lot of other problems with it and stock 75' 500 might be slower then 390.

[mario]

... and stock 75' 500 might be slower then 390.

maybe in bizzaro world, but not in the real world...
ciao,
mario

[The Tassie Devil (Bruce Reynolds)]

The 390 will get the car up and moving pretty quickly, and keep it accelerating as well.

The big difference is the Compression Ratio of both engines.   10.25 to 1 for the 390, and 8.5 to 1 for the 500.   Power is made through compression, and the explosiveness of the combustible fuel during the combustion process.   Low Compression engines need huge areas of cylinders to produce power, and the 500 is one of those engines that show what was achieved with compression.   The 1970 engine with 10.5 to 1 was a powerhouse, and the 1975 engine with 8.5 to 1 was a whimp.   But, the high compression engine NEEDS higher octane fuel or it will destroy itself.

Bruce.

[MTS Marty]

The actual difference in HP between a properly tuned 70 500 cid and a 75 cid is approx. 25 whole HP
Both engines make well over 500 torque and the 390 never gets there. Maybe in "ratings" but not in the real world
Also the 61 can be made to handle better with the 500 if the proper front coil springs are installeddue to the lighter weight