Iam to believe that a 1969 440 has a compression ratio of 10:1 to 10:5:1 ........how is that when the piston is down in the whole and the heads are open chamber ????......wouldn't the compression be like 9:2:1 ??

How would these motors run on pump gas with no quench ?? Wouldn't it be like 120-150 ??

They were in the 9.5 to 9.7 range

IIRC JohnRR priorly posted that a 68-69 piston has a 2.030"CH & is ~.050 in the hole (as we know block deck heights vary) but you might plug it all into united machines' calculator. You know the std bore and stroke & the gasket dimention is available along with a good guesstimate on the 906's CC's & I'm assuming the piston has reliefs (find that info too) then let your fingers do the walking. also see if there are differences in 6pak pistons (not my area).

Not the four barrel magnum motors from the factory would be over 9.7to1, probably closer to 8.8 to 1
I've CC a lot of stock 906 heads that had 92.0+ CC per chamber and those stock magnum flat top pistons with no valve reliefs where down over -.030 in the hole
No where close to NHRA blue print specs.

9.2-9.4 is about where the math comes out........ Depends on the actual deck clearance (block height) and chamber volume.

The added c/h of the 6bbl piston is offset by the 4 valve pockets, so there is no gain in compression with those pistons over the 2.030 c/h 4bbl piston.

Stock '68-'69 Magnum with 90 cc heads, .050" deck and a .022" head gasket is around 9.4-1

Assumptions:
4.32 x 3.75 bore and stroke
Deck height 10.725
comp height of piston 2.030
Rod length 6.768
Head gasket 5cc
Head volume 92cc

Calculated
901.1 cc swept volume
deck clearance 0.052
deck clearance volume 12.5cc
volume above piston 109.5cc
compression ratio 9.229:1

Change head volume to 88cc
New CR is 9.54:1

This explains why my uncle could brag about running his high compression Plymouths on regular gas.

Buy a regular gasket set and add the composition head gasket and you're looking at 8.9:1 or thereabouts.

R.

Factory advertised and ACTUAL compression ratio are not the same when it comes to Ma Mopar .

they ran on pump gas in the day because the gas then is not the same as the swamp water being passed off a gasoline today ...

That would also explain why some ran better than others. A lot of variation between engines.

Did you guys really get 90 cc from original 906s back in the 1970s?

Back then, when I measured unmolested, uncut, no valve grind, and no sunk valves I got about 87 cc. I suspect that anything measured in the last 30 years would likely not meet this criteria. One valve grind is probably 2 cc.

That's a fair question Russ.

You're probably correct in that "new" they were likely closer to 87-88cc.

I have checked more than one 906 that was never rebuilt, and never cut, and they were 90+ , valve recession ?