The GT pipe is a great choice very good on bottom end torque. With an .18 that may not be needed so opt for the Drake pipe. I have no personal experience with the Ofna pipes(except for my 1/8 buugy) nor am I aware of anyone runnin one. The shape is significant to how it performs in what RPM range. Long pipes tend to be for high speed tracks short fat for low speed torquey tracks. This is not a firm rule cuz some engines wont run well with some pipes. Maybe you have some way of trying one from one of your fellow racers 1st, or contact OS and see what 2 pipes they would reccomend.