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Message: <blockquote data-quote="ALBundy" data-source="post: 194615" data-attributes="member: 15">Half yes, half no.  When the ECU is in close loop (meaning to say, readings from the O2 sensor are used by the ECU --> iteration process), what you said is right. The iteration process will continue until the ECU finds the optimum settings, which is the pre-determined settings by the manufacturer. In other words, the "cheating" process by the piggyback will sort of wear off in due time, where it will be rendered useless. HOWEVER, when you floor your accelerator OR when you rev past 3000rpm (sometimes 3500rpm, depending on which model), the ECU will revert to an open loop system where there is no iteration process and the "cheating" process by the piggyback ECU is effective.  The other way is to run an open loop system, of which I am currently doing so. <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite1" alt=":)" title="Smile    :)" loading="lazy" data-shortname=":)" />  Both system will have its drawbacks. The better way would be to dyno tune the car and save the new mapping in an eprom and install it on the original ECU. Likewise for OBD2 cars, flash the new mapping (obtained through dyno) directly into the ECU. Heard this is possible now but not in Msia yet.</blockquote>