54cc + flattops = ? compression?

[steel1212]

I'm looking a set of rebuilt heads and they have 54cc chambers. I was wondering with my flattops what the compression would be.

 

[rbohm]

the factory rates this combo at 10:1 compression. i think it comes out to more like 9.6:1 in actual fact(it has been awhile since i ran the numbers through a compression ratio calculator, and it also depends on how far down the hole the pistons are).

 

[66P51GT]

Static compression is 10.6:1. This is considering .060 overbore and +0cc for flat tops. Dynamic compression will vary based on your cam specifics but it should be close to ~10.0:1.

 

[steel1212]

what about head gasket compressed volume and head gasket thickness?

Cylinder Diameter (inches) 4.060
Stroke Length (inches) 3.00 (302)
Number of cylinders 8
Chamber in cc's 58cc head plus 6.78 cc compressed volume on the gasket
Piston flat tops so 0
Gasket Thickness in inches 0.32

Comes out to 9.9
Does this sound about right?

 

[rbohm]

most head gaskets for the small block ford are between .043 and .058 in thickness, and the piston is usually between .002 and .010 down the hole. that is why i say more like 9.6:1 is closer to accurate.

 

[steel1212]

Yeah, but when I try to use the calculator do you factor in the compressed volume or not?

 

[SoCalCruising]

Most flat top pistons have valve reliefs of 4-6cc. If we assume 5cc, .040" compressed head gasket volume, .005" in the hole, and 54cc chambers, 4.060 bore, I get 10.1:1 static compression.

 

[steel1212]

How are you getting 10.1 to 1 when your adding 6cc more and useing a thicker gasket than I did in the calculator and I only get 9.9 without adding 6ccs and a thinner gasket.

 

[SoCalCruising]

Sorry:
5cc valve relief
.005" in the hole
.040" gasket thickness
54cc heads
4.060" bore
3.00" stroke
10.09:1 static compression

I built a speadsheet. It even accounts for piston to wall clearance at the top. I trust it. You don't have to.

 

[steel1212]

I'm not saying I don't trust it I'm just trying to figure things out. If I don't understand them then I don't know how to do it and it buggs me. I don't like asking questions I want to be able to figure it out by myself but some times you have to lol.

Could you email me a copy of that spreedsheet?

 

[SoCalCruising]

PM me your e-mail address and I'll send it.

 

[D.Hearne]

Depends also on the pin height of the pistons used. Most are either 1.585 or 1.605, a few are 1.610. The 1.585's will give around 9 to 1, the other two will get it to 10.0 to 10.5 to 1

 

[SoCalCruising]

Yeah, you have to know the C-H and deck height in order to estimate the piston-to-deck height, which plays a big role in CR. I ran across C-Hs of 1.165-1.175 for the 331. I guess the same must be true of stockers in order to accommodate different deck heights (from decking duing block machining). Good point.