Question about piston CC

[Slammy]

Arent flat tops with valve reliefs usually a positive CC because the reliefs add volume to the cylinder?

My pistons are listed as -4cc and that totally screws up my compression ratio i was going for. I was going for 10-10.5:1 yet with a -4cc that puts me over 11:1 which is too much.

[otrtruckpro]

-4cc would make your compression lower. If it was +4cc it would be higher. Positive is like a domed piston.

[Slammy]

That's what I thought. So when using a compression calculator, you would enter it as +4cc? Just threw me of since they list it as -4cc but I guess that means just -4 from 0. When figuring compression ratio, it becomes positive since it is adding volume?

[otrtruckpro]

I always just add it to the head chamber size when calculating C.R. on paper. If you have lets say 64cc heads the -4cc would make the chamber bigger. 64+4=69cc. Positive+ cc like a dome makes the chamber smaller. 64-4=60cc. If your using a compression calculator you need to enter it has Negative -4cc.

[Slammy]

Well when I enter it as -4cc I get over 11:1. +4cc puts me right where I wanted to be. Kinda freaking out because I may have bought the wrong heads if that is the case. My heads are 64cc

[Slammy]

My motor is

351w
Bore 4.03
Stroke 3.5
Rod 5.956
Pistons -4cc
Heads 64cc

[mk5.0]

http://www.coasthigh.com/Ford-351W-4-0cc-Flat-Top-SRS-Pistons-p/12355.htm

[mk5.0]

Compression Ratio:

Part Number Cylinder Head Volume
58.0cc 61.0cc 64.0cc 68.0cc
12355-030 11.09 10.69 10.32 9.86
12355-040 11.13 10.73 10.36 9.90
12355-060 11.22 10.81 10.44 9.98

[Slammy]

Yes I've seen that chart but I still don't understand why they list the piston cc as -4cc.

When you punch my numbers in on any compression calculator and you use -4 for the pistons, you get over 11:1.

Now if you use +4cc you get around 10:1.

It's a big deal because that's the difference between pump gas or race gas. I'm trying to keep it on 93 octane so it's crucial.

[Blue88Coupe]

use this calculator: http://wallaceracing.com/cr_test2.php

It states this at the bottom:
Input a positive number for valve reliefs.
Input a negative number if it has a dome.

[Slammy]

I hear ya man, that's what I've thought all along. Just had my wondering why they list them as -4cc on their site.

[racrguy]

-4cc is the amount of material removed for the valve reliefs, not the combustion area amount change. You're losing compression when you go with a -cc number.

[Slammy]

Thats the best explanation ive heard yet, thank you! It really confuses people with the way they do that!