Hey, I designed this trumpets quite sometime ago, to run on my car and ended up being a test before deciding on buying metal ones.

I ran them on Dellorto's DHLA 40 (Alfa 75 1.6) but it should also be compatible with vertical ones and Weber DCOE 40. I designed a template to test fit previous to print the full thing.

Template was printed on regular PLA jsut for testing and the velocity stacks I had printed in raw ABS as it was what I had laying around at the time. It hold on better then expect for 1 or 2 months but started getting stained from the gasoline.

If you have doubts or need any changes let me know.
(The ones in the photos were printed just to show it works without supports and that print profiles work without a lot of trouble. I would recommend not to use layers higher then 0.2mm, though, and either reinforing the infill on the screws part or doint a high infill percentage. 

Also consider that for engine use, you need to consider a material that is resistant to chemicals, temperature, and vibration)