ok i am lost Help on compression ratio goal PLEASE

[fe66comet]

At one point I had all this figured out and lost my notes, I decided to go with a higher compression ratio but the numbers do not add up? So here it goes I have a basic 390 block with a 10.17 deck height. Stroke is 4.25,rod 6.7. Piston is a SRP 271155 compression height 1.325 inverted dome -26 dish. I thought originally with a Fel Pro 4.060/ .040 gasket I would be at 10:1 with the Edelbrock head, it looks more like 9.5 which is dismally low? I would like to be at about 11:1,  the hard has a max mill of .060 and I could get a copper gasket I suppose. What do I need to do to acomplish my goal?

[fe66comet]

PS the ring to top of piston is .25.

[afret]

I think your comp ratio with a piston dish that big would be somewhere around 9-9.5 to 1 depending on your other numbers and flat top pistons with 5-6 cc valve reliefs would have gotten you closer to your target ratio. 

[fe66comet]

It was all that was available at the time, and I had to wait several weeks for those. If I mill and use a .010 copper Cometic gasket where would I be then and how much would I need to mill?.

[fe66comet]

It appears if I go 4.40 bore .16 compressed thickness on the head gasket I would be right at 11:1 could someone verify that? I hate to be wrong.

[TimeWarpF100]

At one point I had all this figured out and lost my notes, I decided to go with a higher compression ratio but the numbers do not add up? So here it goes I have a basic 390 block with a 10.17 deck height. Stroke is 4.25,rod 6.7. Piston is a SRP 271155 compression height 1.325 inverted dome -26 dish. I thought originally with a Fel Pro 4.060/ .040 gasket I would be at 10:1 with the Edelbrock head, it looks more like 9.5 which is dismally low? I would like to be at about 11:1,  the hard has a max mill of .060 and I could get a copper gasket I suppose. What do I need to do to acomplish my goal?

What is your deck height with current setup?

What are the CC's of heads? I have seen them all over the map

72cc head

.010 deck height

26 cc dish in piston

4.080 bore
4.25 stroke
head gasket bore of 4.100 and .040" thick
You would be at 9.37 to 1

Same combo zero deck you would be at 9.54
4.400 x .040 gasket would give you 9.43

same setup with a 80cc head would be 8.85 comp

setup as above with stated 72 cc head you would need a dish of only 9.1 cc's to get you 11 to 1

if as above and .010 deck the comp would be 10.77

.020 deck comp 10.55

Just not going to get to 11 to 1 with a 26cc dish

64cc head
zero deck
4.080 bore
4.25 stroke
26 cc dish piston
.020 head gasket
then you are at 10.65

Realistic with a 4.080 bore
4.25 stroke
72 cc head
zero deck
.040 gasket x 4.100
10 cc dish
that would get you 11.04 to 1 comp

[My427stang]

You have the wrong pistons for your goal, too much dish and not enough compression height.

If your numbers are where you say, you have a stack (rod, CH, 1/2 stroke) of 10.150, if your deck height is 10.170, which makes them .020 below deck.

Also, most Edelbrock heads run a little large unless one of the FE guys preps them for you, what I have seen is 74+cc from the 72cc CJ head, sometimes even bigger.

Also, not sure which gasket you are using because the standard blue Felpro for a 390 I believe is closer to .050 and the 1020 is a 4.4 inch bore and .041. 

Using the stuff we usually build with combined with what you gave us:

So static compression with 4.08 bore, 4.25 stroke, .020 below deck, 1020 gasket, 26 cc dish, 73 cc (remember it could be much bigger) = SCR = 9.02:1

If you used 72 cc = 9.09, and if you used 75ccs for the head (very well could be if out of the box) SCR = 8.88:1

So you probably need to do something, but boring to 4.40 won't get you there, the block likely wont even get you to 4.13, and even if it does, you are potentially building a grenade

If it were me, I would cc the heads, see where you are, then look at a new set of pistons, accounting for a deck height of your choosing (my guess would be 10.155-ish) and potentially cutting the heads if you needed to get the chambers to fine tune the compression. 

You won't get where you want with the parts you have though

or..............see if you can open those chambers up MORE and put a centrifugal blower on it 

[fe66comet]

I went through the numbers again, the deck height I took from the standard engine specs. @10.17. The total of the rod length 6.7 + .5 stroke 2.125 + compression height 1.325 + top ring land distance .25 gave me 10.15 so with that I came up with .02 deck to piston. The bore is 4.080 and piston dome is -26. I did not check the chambers but I can do that myself. Near as I can figure if I use a .010 compressed, 4.165 bore gasket with a 65cc chamber it would work out. According the the Edelbrock instructions it takes .005 for one cc, so if they are above spec. it looks like about .040 would need to be milled. I will match the chambers while I am at it and break the edges along with removing the burrs.

[BH107]

I went through the numbers again, the deck height I took from the standard engine specs. @10.17. The total of the rod length 6.7 + .5 stroke 2.125 + compression height 1.325 + top ring land distance .25 gave me 10.15 so with that I came up with .02 deck to piston. The bore is 4.080 and piston dome is -26. I did not check the chambers but I can do that myself. Near as I can figure if I use a .010 compressed, 4.165 bore gasket with a 65cc chamber it would work out. According the the Edelbrock instructions it takes .005 for one cc, so if they are above spec. it looks like about .040 would need to be milled. I will match the chambers while I am at it and break the edges along with removing the burrs.

You will not get there with those pistons. It would take zero deck and no head gasket w/ 65cc chambers to get you to your target 11:1, which we know is not possible.

[turbohunter]

Maybe I'm a moron but I'm having trouble with how you got 10.15 with the numbers you provided.
For rod length you gave 6.7 + .5.
For the ch you gave 1.325 + .25.
When added with everything else I get 10.9.
Should that be 6.7 minus .5, and 1.325 minus .25?
When I subtract those numbers from 10.9 I get your 10.15.
I guess my question is why the .5 added (or subtracted) to the rod length?
I kinda get subtracting .25 from the ch.
Thought I knew how to get cr but now I'm not so sure.

[My427stang]

Maybe I'm a moron but I'm having trouble with how you got 10.15 with the numbers you provided.
For rod length you gave 6.7 + .5.
For the ch you gave 1.325 + .25.
When added with everything else I get 10.9.
Should that be 6.7 minus .5, and 1.325 minus .25?
When I subtract those numbers from 10.9 I get your 10.15.
I guess my question is why the .5 added (or subtracted) to the rod length?
I kinda get subtracting .25 from the ch.
Thought I knew how to get cr but now I'm not so sure.

1/2 of stroke = 2.125
CH = 1.325
rod = 6.7
Total = 10.15

Subtract from 10.17 = .020 deck clearance

Here are the real problems to the original poster.  Blunt for for clarity only, I actually feel bad for you and want to help

1 - Have not measured chambers and do not know what your deck height really is
2 - Have the decks been squared to the mains, if not, you could have 8 cylinders all with different compression
3 - The pistons are likely wrong for the application before you considered #1 and 2. 
4 - You are assuming you can cut the block or heads to make them work.  You are too far off to do that.  FYI My truck 445 is 10:1 with a 17cc dish and 72cc chambers, 10.155 deck.
5 - You are focusing on chambers, it is smarter to get it closer to zero deck to gain some quench.  That's why I said expect closer to 10.155 later
6 - The head gaskets you are referencing likely do not exist, recommend you just plan with 1020s.  The data is there and they are affordable
7 - Your solution puts you with a .010 head gasket (tin foil?) and cutting the head to the point that you will have intake fitment problems, which I estimate as closer to .055 to get you there,  and only .030 quench distance (.020 deck clearance +.010 gasket).  If you could get there, it would be a mess

[turbohunter]

1/2 of stroke = 2.125
CH = 1.325
rod = 6.7
Total = 10.15

Yup, get that. I guess my question was where are those other numbers coming from in the equation. The +.25 and + .5.

[ScotiaFE]

Your may to have to chew on the lead projectile and get some small dish or flat tops.
To even get close to 65 with the Ed's your going to have to whack off close to .100" and then do some creative milling
on the intake to get it on.
It may be cheaper to buy new pistons then do all the machining required.
Unless you have a shop on standby, than of course never mind.

Also the quench is very important when trying to make power. .040" is where you want to be so when doing your
calculations you want to take that into consideration.
Flat Tops it is.

[cammerfe]

Last time I looked, Ross could supply pistons on very short notice. They've dons so for me on several occasions.

KS

[fe66comet]

Well let me explain a little, when I went into this project my health was much better and I could solve equations. Now it is more difficult and I make mistakes, making it even hard at times to do mathematic equations. I used to be able to do this type of thing off the top of my head , now at times it is hard to grasp.