Curious on a couple things. If a set of 906's run about 90cc's and and a set of Stealths runs about 80cc's, what is the compression bump by switching them out?
Would a current 9:1 engine still be able to run pump premium?
Thanks in advance, I appreciate all the help when my mind starts playing 'what if' scenarios!

If you have a measured 9-1 I'd plug the stealth cc's into Kieth Blacks very user friendly CR calculator (google it) & see what you'll actually end up with for a start.

Sadly it is a guesstimated compression. But thanks. Will see what I can figure out and then plug some numbers.

Well, that's not quite enough information. 361? 383? 400?413?426?440?451?etc.
So I will assume it's a 440.
Each cylinder swept volume is something like 902cc.
To see what volume above the piston for a given compression ratio r, divide swept volume by (r-1).
So to get 9:1 compression with a 440 you need 902 / 8 = 112.75cc above the piston.
So again an assumption: this is what you have.

Simple math, let's take 10cc out of the chamber. Now you have 102.75cc above the piston.

(902 + 102.75) / 102.75 = 9.78:1 compression.

Simple answer is yes, it'll run on premium, maybe even on mid-grade.

Now there are a million complications, such as head gasket thickness and the amount piston is in the hole. But the answer you got is at the same level of acuracy as the information you gave. Drive those new heads in good health.

R.

Sorry. 383. I put all the info in my last question post, forgot to add it to this one.
'69 383.TRW flat tops. (don't have a part #).. 505 Isky cam. 906 heads. I am still researching the rest. I bough it built and just thinking winter upgrades. Early.

Okay, more to go on.....
Another assumption, TRW flattops probably 30 over. Your 383 is now a 389. Swept volume is now 797.2.
797.2 / 8 = 99.65cc

At this point I must digress. As JohnRR will tell you, it is really difficult to get compression in a 383 with flattop pistons. Bigblock Mopar open chamber heads normally run 88 - 93cc, let's pick 90 as your guess. You need a head gasket in there, and I believe the steel shim gasket is around 5cc. So with the piston at zero deck, you are already really close to your magic number. And with the piston that high you need valve reliefs in the piston top, add another 5cc. So now you're at 100cc.
(797.2 + 100) / 100 = 8.97:1 That's the maximum of what you have now.

But your builder probably used a Fel-Pro composition gasket and that's something like 12cc.
And, the piston is probably at least 0.030 in the hole, adding another 7.1cc.

New math is 90 + 7.1 + 12 = 109.1 if there are no valve notches, 114.1 if there are.

(797.2 + 109.1) / 109.1 = 8.31:1 with no valve notches and
(797.2 + 114.1) / 114.1 = 7.99:1 with valve notches.
If I was betting that's what I'd bet your compression ratio is right now. Bummer, huh?

Let's take 10cc out for the Stealth heads.
(797.2 + 99.1) / 99.1 = 9.04
(797.2 + 194.1) / 104.1 = 8.66:1

You would be able to use unleaded regular, most likely.

R.

John has mentioned a couple times the difficulty in building compression in a 383.
I guess I need to pull a head and do some measuring. The edges do look like a composite head gaske. I have always run steel gaskets.
Anyway, thanks for the info and formulas.

# of Cyl. 8 Competition Wedge Engines
Bore 4.2500 383 with
Stroke 3.3800 Stealth Heads
Rod Len. 6.760 assumed flat top 5cc V/R
Gskt Bore 4.410
Comp. Gskt .0390
Block Deck Ht. 9.9980
Top ring down 0.300
Chamb. Vol. 81.0
Dome (-) Dish (+)5.00
Piston / deck 0.010

Total volume 885.31
Cylinder volume 785.75
Clearance volume99.56
Gasket volume 9.76
Top ring volume 1.47
Deck volume 2.32
Piston top land 4.205
1/2 Stroke 1.690
Comp. Ht. 1.538
Cubic Inches 383.60
Compression ratio 8.89

making some assupmtions here as well..

John, thanks for the numbers. Not a safe assumption at all! I'm bench racing wit myself right now and the engine going in is an already built Non-HP 383 that will get me moving. I'm shoveling money into getting the things I need to drive the car. When I do it 'right' I will want the engine you have to put in it. So I'm still headed that direction! Slowly.