*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. *

Feel compelled to add my 2 cents here. If HP is the ultimate goal, this machine work to raise compression is not good value for the dollar. Going from 8.2:1 to 9:1 will likely add 5 HP or less on a 318, according to readily available calculators. This – and a lot more – can be made up for by careful cam selection. And as this build will undoubtedly involve a cam change, net additional cost to make up that HP difference should be zero.

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I quote you ” 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.” Is this correct or vice versa ?