[Spridgets] Spridget heads and CR

[Ron Soave]

--- On Sun, 11/30/08, Billy Zoom <billyzoom at billyzoom.com> wrote:
> with a displacement of 21.4 cc on a 1275? Wouldn't that
> give you nearly a
> 15/1 ratio?  318.75 divided by 21.4 =14.895

Short answer is in real life with flat tops it's closer to 12.5:1. To paraphrase a reply to a similar question last year or so:

To get a decent idea of CR, you need 5 key volumes which add the unswept volume, plus cylinder swept volume:

1) Distance from top of piston crown to top of block;

2) Ring land volume.  Typical for my J&E stuff is approx .75 cc

3) Compressed head gasket volume. Usually our stuff is stamped with thickness or the supplier can tell you the volume directly (the holes aren't exactly round). The compressed volume for the TAM1521 syle that I favour is 2.8CC.

4)Piston dish.

5) Head chamber volume.  My cheapo $800 Longman head (often accused of being a cheater big valve or Bauchman $3000 head, much to my delight) was stamped and advertised as 21.4 cc; when I cc'd it, it was closer to 23! Have it cc'd no matter what the sales literature says.

So....
Add the unswept volumes.

Swept bore volume is bore x stroke.

Compression ratio is the sum of (swept + unswept) divided by unswept.

Ron