Ever get frustrated by the poor performance of stock engines on Eve? This is because the stock engines are design to provide optimum performance on Kerbin, not Eve. But why would you use on Eve an engine designed for Kerbin? The simple answer, you wouldn't.
Improving engine performance in the thick lower atmosphere of Eve is no mystery – simply adapt the nozzle's expansion ratio to the high ambient pressure environment. We don't have to settle for stock engines when legitimate engineering practices can produce engines designed specifically for Eve.
Eve Optimized Engines provides low expansion ratio variants of six of the game's most popular engines – Reliant, Swivel, Spark, Thud, Skipper and Mainsail. The six new engines – Adam, Abel, Asp, Boa, Viper and Cobra – have greatly improved high ambient pressure performance, which comes by sacrificing low-pressure performance. When operating on Kerbin or in space, stick with the stock engines. But when launching from the surface of Eve, the modified engines provide good first stage power and performance.