The most economical way to convert truck and bus DI-diesel engines to natural gas operation is to replace the injector with a spark plug and modify the combustion chamber in the piston crown for spark ignition operation. The modification of the piston crown should give a geometry well suited for spark ignition operation with the original swirling inlet port.Ten different geometries were tried on a converted VOLVO TD102 engine and a remarkably large difference in the rate of combustion was noted between the chambers. To find an explanation for this difference a cycle resolved measurement of the in-cylinder mean velocity and turbulence was performed with Laser Doppler Velocimetry (LDV).The results show a high correlation between in cylinder turbulence and rate of heat release in the main part of combustion. The very early part of combustion is more affected by other parameters but the intermediate part of combustion corresponding to 0.5-10% of the total heat released is influenced by both mean velocity and turbulence. There is a surprisingly good correlation between the average level of turbulence and the used squish area.