.21
I have 2 gallons <25%>through my on road sirio .21and It still hauls some serious azz! Compression is still tight. It is as fast as my brothers' rody tuned rb .21 for almost half the price. Also, rc planes have used aac for a long time and those engines last and last. The metals expand and cool together, making the seal and wear more consistent...my 2 cents...for 319.00, it is off the hook. U can't go wrong.