For the tests Piper worked with Airworthy AutoGas of Phoenix to evaluate a new high-purity unleaded fuel due to go on sale this fall. The Piper Archer is powered by a single 180 horsepower Lycoming O-360-A4M engine that is approved for use of 93-octane unleaded fuel.
Source : www.flyingmag.com/news/piper-aircraft-evaluating-unleaded-auto-gasoline