Section 1051.230 - How do I select engine families?(a) For purposes of certification, divide your product line into families of vehicles as described in this section. Except as specified in paragraph (f) of this section, you must have separate engine families for meeting exhaust and evaporative emissions. Your engine family is limited to a single model year.(b) For exhaust emissions, group vehicles in the same engine family if they are the same in all the following aspects:(1) The combustion cycle.(2) The cooling system (liquid-cooled vs. air-cooled).(3) Configuration of the fuel system (for example, port fuel injection vs. carburetion).(4) Method of air aspiration.(5) The number, location, volume, and composition of catalytic converters.(7) The number, arrangement (such as in-line or vee configuration), and approximate bore diameter of cylinders.(8) Numerical level of the emission standards that apply to the vehicle. For example, an engine family may not include vehicles certified to different family emission limits, though you may change family emission limits without recertifying as specified in § 1051.225 .(c) For evaporative emissions, group vehicles in the same engine family if fuel tanks are similar and fuel lines are similar considering all the following aspects: (1) Type of material (including additives such as pigments, plasticizers, and UV inhibitors).(2) Emission-control strategy.(3) Production methods. This does not apply to differences in production methods that would not affect emission characteristics.(d) You may subdivide a group of vehicles that is identical under paragraph (b) or (c) of this section into different engine families if you show the expected emission characteristics are different during the useful life.(e) You may group vehicles that are not identical with respect to the things listed in paragraph (b) or (c) of this section in the same engine family, as follows: (1) In unusual circumstances, you may group such vehicles in the same engine family if you show that their emission characteristics during the useful life will be similar.(2) If you are a small-volume manufacturer, you may group engines from any vehicles subject to the same emission standards into a single engine family. This does not change any of the requirements of this part for showing that an engine family meets emission standards.(f) You may divide your product line into engine families based on a combined consideration of exhaust and evaporative emission-control systems, consistent with the requirements of this section. This would allow you to use a single engine-family designation for each engine family instead of having separate engine-family designations for exhaust and evaporative emission-control systems for each model.(g) Select test engines from the engine family as described in 40 CFR 1065.401 . Select test components related to evaporative emission-control systems that are most likely to exceed the applicable emission standards. For example, select a fuel tank with the smallest average wall thickness (or barrier thickness, as appropriate) of those tanks you include in the same family. 70 FR 40495 , July 13, 2005, as amended at 73 FR 59249 , Oct. 8, 2008; 75 FR 23024 , Apr. 30, 2010