In response to the sensitivity to diesel aftertreatment costs in the medium duty market, a John Deere 4045 was converted to burn gasoline with high levels of EGR. This presented some unique challenges not seen in light duty gasoline engines as the flat head and diesel adapted ports do not provide optimum in-cylinder turbulence. As the bore size increases, there is more opportunity for knock or incomplete combustion to occur. Also, the high dilution used to reduce knock slows the burn rates.In order to speed up the burn rates, various levels of swirl were investigated. A four valve head with different levels of port masking showed that increasing the swirl ratio decreased the combustion duration, but ultimately ran into high pumping work required to generate the desired swirl. A two valve head was used to overcome the breathing issue seen in the four valve head with port masking. The low swirl port design in the two valve head offered a similar swirl ratio while the high swirl port design offered a swirl ratio double that of the low swirl port. The results from the experiments illustrate a trade-off between swirl ratio and heat transfer. While the highest swirl port slightly improved EGR tolerance and burn rates, it had a significant increase in heat transfer and pumping work causing a overall decrease in efficiency. An optimal swirl ratio exists such that the pumping and heat transfer losses are overcome by the improvement in burn rates and EGR tolerance.