Mismatched heads and block on 40-72

[Doug Houston]

Lessee if I can explain all this mish-mosh.

The LaSalle engines had smaller bores, as everyone here seems to understand. The heads that were designed for them, had slightly smaller combustion chambers, to accomoomodate the smaller bore. The compression ratio (it should be in the shop manual), was probably the same as for the 346 cu. in engine, used in all lother Cadillacs.

BUT, if you were to install LaSalle heads on a 346 cu. in engine, you'd be putting a head on, with a smaller combustion chamber, and you'd get a better compression ratio.  It's almost like planing the heads.That's what Cadillac did on their pre-1941 engine for the 70,72, and 75 cars. That was that little bit of performance gained by the use of LaSalle heads. It was necessary to mark them for the engine displacement (322 for LaSalle), and 346 for Cadillac.  Beginning in 1941, LsSalle was gone, and all Cadillac engines had the 7.25:1 compression ratio through 1948.  If they're marked, they'd say: 346 725. It's confusing, because so many think that it's a part number, but it ain't.

[jbento]

So if I understand this right, basically these heads would work on either a Cadillac block with a larger bore, OR a LaSalle block with the smaller bore. The markings then are to inform the mechanic that if on the former 346 engine they should expect a compression ratio of 6.70. But if these heads were installed on the smaller bore LaSalle block, one should be looking for a compression ratio of 6.25.

Right?

[Steve Passmore]

Doug's post now explains things very well and clears up another query I had here, none of the 16 heads I have carry any of these numbers on them, that's because I have never have a head from a La Salle or from a bigger series car, the pre war heads are all from 60 series cars and the later ones with no La Salle in production would not need to differentiate with numbers as they are all  the same,   The only heads with number are therefor prewar La Salle/65/and up. that gos on the explain why on all the heads I have here the combustion chambers are the same.
jbentos heads are correct for his 1940 70 series if not for the later engine in his car. Looks like we should all be fitting La Salle heads on our pre war models and gaining 6.7 ratio.
                                     

[Doug Houston]

Well yes, but if you want to tke the best advantage of compression ratios, you'd use the '41 and later heads on all of the cars. Not sure what compression ratio you'd get on a LaSalle with 41-48 heads, but it would have to be higher than even the original LaSalle heads, or so it would seem. We don't have the numbers for swapping heads back to earlier cars. We do know about later heads, though. Retro-upgrading was never done, or at least publicised by Cadillac.

And too, if you wanted to, you could take the older heads to a machine shop, and have a couple thousandths or so milled off of the heads, you could get a better compression ratio. That was done all the time by hot roders in the old days, and I suppose, even today to some extent.

In some instances, a little meat has to be planed off of a head if there is warpage. Many cases of that are told. I would expect then, that you'd need to have the other head planed an equal amount. Warpage seems to be an ailment, found in longer heads from L-head angines, like Pontiac, Oldsmobile, Packard, and many others. Cadillac's short lemgth heads are not so prone to warpage.   

[Steve Passmore]

[quote . Looks like we should all be fitting La Salle heads on our pre war models and gaining 6.7 ratio.
                                     
[/quote]
Just kidding there Doug, I have never found these 346ci flat head engines wanting in any way for the use I put them to, quite the contrary, so machining unnecessarily just to gain a small percentage of compression ratio which I think would be hardly noticeable would not be for me.
Steve