Fits 1991 Jeep Wrangler YJ with 4.0 Liter 6 Cylinder engine Jet Performance Chip Stage 2 with automatic transmissions Computer controlled engines on all new cars and trucks are run by programmed chips that tellthe engine what to do. Factory chips compromise between fuel economy and performance. The Jet Powertech Performance Chip replaces the stock computer chip and maximizes various performance characteristics. New programming adjusts spark timing & the air/fuel ratio to tune the engine for maximum power when you need it. Provides improved performance under demanding conditions such as towing & wide open throttle acceleration. Installs in less than 30 minutes and is legal in all 50 states.