Low volume stuff, single digits of parts, unless I find that niche, and i might outsource then. I'll keep the send-cut-send type stuff in mind. There are variations to certain parts that I'd like to iterate on, for example, intake runner lengths, diameters and shapes, various adapters/converters, panels/covers, etc. Plus, it's fun to do things yourself and learn along the way. It's not just about the end result.

Using a service would be better for a final part in a better material for the sake of longevity, not just 3d printed either, but cnc'd aluminum for example. Plastics can only go so far, but for short durations in some stressful situations, like intake manifolds, they can give you information. Having a printer in house will let me prototype to verify functionality of some engine parts, and being able to print in resistant materials will let me make final parts of less stressed parts in areas exposed to fuels, oils, etc.