You’ve got questions, we’ve got answers. We work with the Summit Racing tech department to help you tackle your auto-related conundrums. In this week’s Mailbag, we’re calculating compression ratios and cylinder head volume on a modified 318 Chrysler small block

Q: I am planning to rebuild a Chrysler 318 engine. I want to keep the compression to around 9.0:1, but I want to replace the stock heads with Edelbrock Performer RPM aluminum heads. The stock heads have 57.3cc combustion chambers, while the Edelbrock heads have 65cc combustion chambers.

Someone told me that the Edelbrock heads will lower the compression to 8.5:1 with the stock 3.91-inch cylinder bores. Can I bore-out the cylinders to raise the compression up to 9.0:1? If so, how much should I overbore them? Is there a formula I can use to figure this out before I start machining?

A: Our calculations show the Edelbrock heads will lower the compression ratio more than you’re initially thinking—closer to 8.2:1 with the 65cc chambers.

To pick up the extra compression, you will have to bore the cylinders about 0.030-inch over and take about 0.033-inch off the head decks, or leave the bore stock and shave about 0.038-inch off the heads.

You can gain more compression with the head work than by overboring the cylinders. Since we don’t know how much piston-to-deck clearance you have, you will have to measure that clearance before doing any machine work.

Here is the formula for compression ratio.

Compression Ratio =

(Swept Cyl. Head Volume + Clearance Volume) ÷ Clearance Volume

Here is the formula for swept cylinder volume.

Swept Cylinder Volume =

Cylinder Radius x Cylinder Radius x Stroke x 3.145

To convert that to cubic centimeters, simply multiply your answer by 0.061

Here is the formula for clearance volume.

Clearance Volume =

Head CC – Piston CC + Deck CC + Gasket CC

Deck CC will be zero if the block is zero-decked (no material removed). Piston volume will be a negative number if the piston is dished, and a positive number if the piston is domed. 

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