Trivia question: Why did the redesigned engine for 1963 stay at 390 cubes...

[C. Van Tune]

...and then the next year, in 1964, get enlarged to 429?

I would've thought that with Cadillac's going to the time, trouble and expense of re-engineering the old 331-390 engine into the next gen, they would've wanted to come right out of the box as a 429. 

Doing a one-year-only new gen 390 with the same hp and torque figures as the old engine, seems like a waste of effort. 

My only guess as to why it stayed at 390 cubes is that perhaps the Turbo 400 transmission wasn't ready yet in '63, and the 4-speed Hydra-matic couldn't handle the torque of the 429.

Any truth to this? 

Thoughts?  Information?  Guesses? 

[Kurt]

The 64 Series 62, 75 and commercial chassis used the Hydramatic, so that can't be the reason. Not sure what it is though.

[C. Van Tune]

Good point indeed!   I'd overlooked that.   

Deepens the mystery even more. 

[24ro8f5mbs2jc]

1962 & 1963 Cadillac Serviceman October issue might give a hint. At least partly.

[C. Van Tune]

I checked out the '63 and '64 Serviceman reports, and other than details of the new 390 engine for '63, I didn't see where it explained why the displacement, hp and torque were left the same as in the old-gen 390 instead of just going straight to the 429 they they knew was coming out in '64.  

As you know, engine programs call for planning years in advance of production, and so when the '63 390 was being designed, they already were aware of wanting to enlarge it to 429.

My question stands:  Why did Cadillac produce a one-year-only new-generation 390?  Seems like a lot of added cost and complexity, to do it that way. 

IMO: When engineering a new engine, don't hide that fact (and the advertising value) by using the same displacement and output as the engine it's replacing.

[Jason Edge]

For those that have read about the new for 63 390 you know there were very significant changes made from the 62 390 - 52 lbs lighter, shorter block, different crank, distributor and other components brought to front of engine, etc.  Also, while the 1962 and 1963 390 blocks were very different, they used the same cylinder heads.  With the 429 displacement and wider bore the heads had to be redesigned. You also have to consider that the Cadillac 390/429 is actually about the same size, maybe a tad bigger, than a small block Chevy, and they had to do some creative redesign to get that 4.13” bore. For example that reduction in head bolt size from 7/16” to the very odd 13/32” had to be to gain a bit more clearance for the thinner block walls. Also, keep in mind that by 1963/64 Cadillac was deep into development of the new V-12 that never made it to production. As I recall six V-12’s prototypes were built in 1963 and 1964.

So there was a lot going on and my guess is they decided to modernize the 390 as both a selling point and real improvement but also to buy time for continued development of the V-12.  As far as the 1964 and later 429, I’m also guessing there were external factors pushing Cadillac to produce bigger torque and horsepower numbers, until they could get the 472 in production in 1968.  For me the odd head bolt size has always made me think the 429 was a stopgap to keep up and ahead, but as we all know the 429 went on to be used for 4 years!  
A lot of things like mounting a Buick transmission on a 1964 429 with an adapter ring, selling 1965 Series 65’s that look just like a 1964, or introducing a new for 1963 390 with the same specs as the year before May all seem odd now, but it was usually because of timing… or the lack of!

For more on that V-12 that never was, here is one article https://macsmotorcitygarage.com/a-cadillac-v12-that-never-was/