40 CFR 89.108 - Adjustable parameters, requirements.
(b) An operating parameter is not considered adjustable if it is permanently sealed or otherwise not normally accessible using ordinary tools.
(c) The Administrator may require that adjustable parameters be set to any specification within its adjustable range for certification, selective enforcement audit, or in-use testing to determine compliance with the requirements of this subpart.
(d) For engines that use noncommercial fuels significantly different than the specified test fuel of the same type, the manufacturer may ask to use the parameter-adjustment provisions of 40 CFR 1039.615 instead of those in this section. Engines certified under this paragraph (d) must be in a separate engine family. See 40 CFR 1039.801 for the definition of “noncommercial fuels”.