Studies have shown that low temperature combustion (LTC), has the ability to decrease both soot and NOx emissions while maintaining high efficiency. LTC relies on long ignition delay, which is feasible through the use of high exhaust gas recirculation (EGR) in combination with high octane fuels. High efficiency is possible due to the rapid heat release at autoignition of the premixed or partially premixed fuel and air charge. This rapid combustion limits the load range of LTC, thereby diffusion like combustion is needed at higher loads. Earlier studies have shown that gasoline has the ability to reduce smoke emissions even at high load. This study aims to optically investigate the possible mechanisms behind the reduction of soot in comparison to diesel diffusion combustion, at high load operation (22 bar IMEP$_g$) in a heavy-duty truck engine. Primary reference fuels (PRF) were used as pump fuel surrogates. The experiment was performed by injection pressure and inlet temperature sweeps in order to vary the ignition delay and lift-off length (LoL). CA50 was kept constant throughout the study. Results show that stable quasi-steady LoL is equal for all tested conditions and fuels. This implies that the ambient condition is the dominant factor of the quasi-steady LoL at high load.Results also shows how ignition delay varies with the different fuels and set initial parameters, therefore the ignition delay can be the factor that is dominant to the soot reduction with gasoline as fuel.