COMPRESSION RATIO CALCULATOR

Calculate the compression ratio for any VW/Audi engine build. Select an engine code for factory specs, then adjust gasket thickness, piston dish volume, and chamber volume for your build configuration.

Select VW/Audi Engine Code

ENGINE DIMENSIONS

Bore 81.0 mm

Stroke 86.4 mm

Cylinders

COMBUSTION CHAMBER

Chamber Volume (cc'd) 42.0 cc

Measured by filling with fluid (cc'ing the head). Factory specs used if engine selected.

Head Gasket Thickness 1.60 mm

Compressed thickness. Stock MLS gaskets: ~1.2-1.6mm. Thicker gaskets lower CR.

Gasket Bore Diameter 82.0 mm

Inner opening of the head gasket. Usually 0.5-1.5mm larger than cylinder bore.

Piston Deck Height 0.00 mm

Positive = below deck (in bore). Negative = above deck (poking out). Zero = flush.

Piston Dish/Dome Volume 4.0 cc

Positive = dish (concave, lowers CR). Negative = dome (convex, raises CR). Zero = flat-top.

COMPRESSION RATIO

9.5

COMPRESSION RATIO 9.5 : 1

Swept Volume (1 cyl) 445.1 cc

Gasket Volume 8.4 cc

Deck Clearance Volume 0.0 cc

Piston Dish Volume 4.0 cc

Chamber Volume 42.0 cc

Total Clearance Volume 54.4 cc

Total Displacement 1,781 cc

FUEL OCTANE RECOMMENDATION

CR = (V_swept + V_clearance) ÷ V_clearance

COMPRESSION RATIO GUIDE FOR TURBO BUILDS

Recommended CR ranges by boost level and fuel type for VW/Audi turbocharged engines.

Build Type	CR Range	Boost (PSI)	Fuel	Notes
Stage 1 (ECU tune only)	9.0–10.5:1	18–22 PSI	91–93 AKI	Stock CR is fine. No internal changes needed.
Stage 2 (bolt-ons + tune)	9.0–10.0:1	22–28 PSI	93 AKI min	Stock CR works on most EA888/EA113 engines.
Big Turbo (500–600 WHP)	8.5–9.5:1	28–35 PSI	E85 or 100+ RON	Lower CR with forged internals. E85 allows higher CR.
High Power (700+ WHP)	8.0–9.0:1	35–45+ PSI	E85 or methanol	Forged pistons with specific dish to target CR.
NA Build (ported head)	11.0–13.0:1	N/A	93–100 AKI	Higher CR = more power on NA. Limited by fuel octane.

WHAT IS COMPRESSION RATIO?

Compression ratio (CR) is the ratio of the total cylinder volume (when the piston is at the bottom) to the clearance volume (when the piston is at the top).

A higher CR means the air-fuel mixture is squeezed more before ignition, extracting more energy per cycle. But higher CR also increases cylinder pressure and temperature, which can cause detonation (knock) if the fuel can't resist auto-ignition.

CR AND FORCED INDUCTION

Turbocharging already increases cylinder pressure by forcing more air in. Adding high static CR on top of boost compounds the pressure, increasing knock risk.

Lower CR + more boost = safer, more predictable tuning window
Higher CR + less boost = better throttle response, more efficient at part throttle
Modern direct injection (EA888 Gen3) tolerates higher CR under boost due to charge cooling
E85's high octane (105 AKI equivalent) allows running higher CR safely under boost

HOW TO LOWER CR

Thicker head gasket — cheapest method. A 2mm gasket vs 1.2mm can drop CR by ~0.5 points
Dished pistons — forged pistons with a specific dish volume are the precision method
Deck height increase — machining the block deck (irreversible)
Larger chamber head — different cylinder head casting or CNC porting to enlarge chambers

HOW TO RAISE CR

Flat-top or domed pistons — replace dished pistons with flat-top for higher CR
Thinner head gasket — MLS gaskets available in various thicknesses
Deck the head — mill material off the head surface (reduces chamber volume)
Deck the block — reduce piston-to-deck clearance (brings piston closer to head)