Cadillac 331 to 365 Compression Ratio Q

I know plenty of threads posted on the theoreticals of Cad 331 bored to 365, but who has actually done it? I have a 331 that needs a healthy bore to clean up, and thought about going out to 4" bore to 365. Does anyone know this has been done successfully, and what compression ratio changes can I expect?

I spoke to Marcus about his 354, he used custom pistons, I'm looking at stock 365 pistons swapped in.​

 

You probiably had 8.25 comp ratio with yhe 331. By boring to 4" you should have 9.25 ratio. Watch the sharp edge on your smaller combustion chamber as the bore will be larger. I had problems with that so I went to a 58 cylinder head. I use the shim steel head gaskets .

 

What condition are the heads in on your 331? Do they need to be reconditioned as well? I'm not sure what the combustion chamber size is of the 331 heads, but if this were my engine build and the heads needed to be reconditioned then I'd look for heads off of a '59 - '62 390 engine. The combustion chambers on those heads will support a 4" bore (no worries about valve shrouding or compression ratios), and they are supposedly better flowing heads than those that were on the 331 or 365 engines. Might as well bolt on the latest and greatest heads that Cadillac offered for these engines.

 

AH,,...sorry about not giving you the whole story! I've got a fresh set of 390 heads ready to bolt on...
my stock 331 rated at 7.5 to 1.

I don't know if the 365 CR (10.5 to 1 IIRC) is achieved by the piston or the head, or if my 390 heads will bump that ratio any more....

thanks for the responses!

 

You can port and polish and add bigger valves to the 58 head and flow better than the 390 stock heads.

 

And what would you get if you dumped that same amount of work into the 390 heads??

 

Thats not the point when I built my engine back in 65 it was hard to find 59 up Caddy heads we did with what we had.

 

So Hank, when you went to 4" bore did you use stock Caddy 365 pistons??

 

No I bought a Crankshaft Co. stroker kit and it came with 12:1 race pistons.

 

By the way you can use up to 58 365 pistons which are 10.25 ratio with 58 heads.

 

THANKS HANK! Exactly what I needed to know!

 

We bored these to 4" all the time back in the late 50's. Stroked them to 4" and got 404 CI's. Big for the day. We never had a cylinder issue.

 

How was the increase in stroke accomplished back then? Was the stock crank reworked by offset grinding or were there aftermarket cranks (and rods?) available? Or were you dropping in the cranks from the 390 cadillac engines?

Thanks!

 

This was a new steel crankshaft.

 

Great conversation...

I'm going to rebuild the 331 in my '51 caddy Convertible. Already got the '61 390 heads, and the edelbrock CD-693 3x2 intake...

Gonna be bored, stroked to 390, with stude adjustable rockers.