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This calculator will calculate both Static and Dynamic compression ratio

Compression Ratio is the ratio of an engine's cylinder volume vs. its combustion chamber size. Static Compression Ratio numbers are the ones you hear thrown around the most ("10:1 compression"), and it takes into consideration the full sweep volume of the cylinder in regard to its range of crankshaft stroke. Unlike Static Compression Ratio, Dynamic Compression Ratio takes into account camshaft timing by considering the intake valve closing point in relation to the piston position.

To give an example, lets say you're considering only pump gas for your engine, and it has a static compression ratio of 11:1. If you put a very mild camshaft (194/204 duration @.050"), this cam will have an "early" IVC (intake valve closing point), and will "bleed off" less compression than a radical camshaft with 259/269 duration @.050" and a considerably "later" IVC. Therefore with the mild cam it will have a high dynamic compression ratio, probably 9.5:1+ which would be way too high to run safely on 91 octane gas. However, that same 11:1 static compression ratio engine with the radical 259/269 duration camshaft would have a dynamic compression ratio in the neighborhood of 7.5:1, totally acceptable to run on pump gas. General rule of thumb for acceptable dynamic compression ratio to run safely on pump gas is 8:1 maximum for engines with cast iron cylinder heads and 8.5:1 with aluminum cylinder heads.

(enter negative # for dome)
(.038 is a typical value)
(.020 is a typical value)

Optional for Dynamic CR

Intake valve closing point (IVC) is available on most cam spec cards which can be found on sites like summitracing.com or your cam manufacturer's website. e.g. the Summit cam SUM-2802 has an IVC @ .050" of 41)

Most camshaft manufacturers provide you with valve timing numbers '@ .050" lift'. However, Comp Cams will typically provide valve timing numbers '@ .006" lift'. If you only have an IVC @ .006" number, then that IVC number will need to be converted to IVC "@ .050 lift" in order to use this calculator correctly. You can calculate your @ .050" valve timing numbers here based on intake/exhaust duration @ .050", lobe separation (LSA), and intake centerline. Intake/exhaust duration @ .050" and LSA are commonly available on most camshaft sellers' websites (e.g. Summit Racing or Jegs). You can find out your intake centerline for your Comp Cams camshaft by searching its part # or grind # on the Comp Cams website and then finding your cam's specs page.

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