they basically change the tuning on the vehicle. They mess with the A/F ratio and usually lean them out a little and change the timing somewhat. That's where they get extra power. Then they mess with the transmission shift points, you can change the RPM limiter, etc..... helps with drag racing, etc... Most can be removed before taking them into service.... but some manufactures can still see where one has been installed.
But none have anything for a Tundra/Tacoma... There's one company "Unichip" that claims they do, but most everyone says it's not a good one and the truck will compensate and cancel it out. Some have even claimed damage from it
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