VW Beetles
COMPRESSION RATIO CALCULATION
To calculate compression ratio, you must establish three things: combustion chamber volume (otherwise known as 'head cc'), deck height displacement (the volume of the area above the piston at top dead centre) and the individual cylinder displacement.
CYLINDER DISPLACEMENT= (Bore x Bore x Stroke x 0.003142) / 4
DECK HEIGHT DISPLACEMENT= (Bore x Bore x Deck Height x 0.003142) / 4
CR = (Cylinder Displacement + Deck Height Displacement + Head cc) / (Deck Height Displacement + Head cc)
FOR EXAMPLE:
Bore = 94mm; Stroke = 84mm; Deck height = 1 .5mm; Head cc = 48cc therefore:
COMPRESSION RATIO= (583.O2cc + 10.41 cc + 48cc) / ( 10.41 cc + 48cc) = 10.98:1
CARBURETTOR SIZE AND JETTING
CARBSIZE= (cc per cylinder x maximum rpm) 0.5/ 40
CHOKE DIAMETER= (Carb size x 40) / 50
MAIN JET= Choke diameter x 4
For example:
2007cc engine = 501 cc per cylinder
Max rpm = 6,8OOrpm, therefore:
Carburettor size = (540 x 7000) 0.5 = 45.1 mm (a 45DRLA Dell'Orto would be suitable in this instance) 40
Choke diameter = (45 x 40) / 50 = 36mm
Main jet = 36 x 4 = 144 (A 140 or 145 main jet would be a good starting point in this example)
These can only be considered approximate figures, necessary to get the engine running sufficiently well for break in. It is recommended that the vehicle be finished off on a dyno.