What's my compression ratio? (head swap question)

I have 62cc heads currently and my CR is 9.62:1. If I switch to 64cc heads what would my CR be then? 58cc? What's the math?

Reply to
66 6F HCS
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This is oversimplified, but basically static CR = volume of combustion chamber divided by volume of combustion chamber + volume of cylinder.

In fact what you have to do, first, is calculate the displacement of one cylinder. That's bore x bore x stroke x 0.786. (0.786 = pi divided by 4.) Then you add the volume of the compressed head gasket, bore x bore x compressed thickness x 0.786. Then you add the volume of the combustion chamber. It's easiest to convert cc to ci. The conversion factor is cc x 0.061. (64 cc = 3.78 ci.)

That's the demonimator.

Now for the numerator, you have to take into account deck height, valve reliefs, and dish (assuming no pop-up pistons.) So the numerator includes the comustion chamber. It includes that part of the displacement that remains with the piston at TDC, which is bore x bore x deck height x 0.786. It includes the head gasket, bore x bore x compressed thickness x 0.786. And it includes the volume of any valve reliefs or dish in the top of the piston.

These calculations also leave out the factors, that the bore holes in your head gasket might be bigger than the bores in the block, and that there's also a tiny amount of volume between the piston and the bore, extending to the back of the compression ring land, that we're ignoring. If you're really going to get picky you have to account for these volumes too.

What you're probably realizing is that you need to know these other things -- deck height, compressed height of the head gasket, and volume of piston dish/reliefs -- to be able to recalculate your static CR when you change heads.

But here's a guesstimator tool, from a website I was checking out, speedomotive.com:

408 PUMP GAS STREET MASTER ASSEMBLED SHORT BLOCK $2400.00

ASSEMBLED SHORT BLOCK INCLUDES:

FULLY PREP'D 351W 2 BOLT BLOCK HI-NODULAR CAST 4.00" STROKER CRANK

5140 FORGED I-BEAM RODS - 6.200" LONG w/ARP 7/16 BOLTS KEITH BLACK HYPEREUTECTIC DISH TOP PISTONS COMP. RATIO: 47cc =10.94-1, 56cc =9.98-1, 60cc = 9.61-1, 64cc = 9.27-1, 70cc = 8.8-1

So there you go. For this particular 408 stroker (which is 4.030 x

4.000), which has dished pistons, a 60 cc head will give you a 9.61:1 and a 64 cc head will give you 9.27:1. Although the graph is not a straight line, a 62 cc head would be roughly at the midpoint, or 9.44:1. You can guesstimate that same kinds of variance for your 9.62:1 @ 64 cc engine, i.e., add 0.2 points, for each 2 cc.
Reply to
one80out

Addendum: I left this out in my original post, that you have to take into account valve reliefs and/or piston dish in the denominator, too. That is, you have to know the cc's of these items, and add them to the denominator. You don't need to worry about deck height, though, because that volume is already included in the bore x bore x stroke x

0.786 volume.

Again, for a Speed-O-Motive 408, with dished pistons of unspecified cc's:

Extrapolating between the 60 cc head and the 64 cc head, a 62 cc head like you've got now would give you 9.44:1. Even though your 62 cc head is giving you a 9.62:1 (you have a bigger overbore than 0.030, iirc), it would still be close enough to estimate an subtract 0.2 points of CR for a 64 cc head, and to add 0.4 for a 58 cc head.

180 Out
Reply to
one80out

wrote

I've got a .060" overbore with 12cc dish pistons. Everything else is stock. I don't remember what my engine builder told me on how far down the hole the pistons are, .04"? sound right? I don't remember. But I do know for a fact that it worked out to 9.62:1 CR on a 62 cc combustion chamber. 4.060" bore,

3.85" stroke, stock Windsor rod, 12cc dish 302 pistons.
Reply to
66 6F HCS

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