How can I decode Lycoming engine number suffices? What does D3G mean in o-320-D3G or D2A in o-320-D2A? Is there any chart or tabel for decode suffixes?
I got the following information for this website:
Also if you scroll down here you can see the model numbers that you talk about in your question.
The last character seems to be an indication of the variant of the engine as shown in the document. The D indicates Dual Magnetos and the 2 or 3 seems to indicate some type of counterweight application.
This PDF on Lycoming's web site covers it. The suffixes are just arbitrary identifiers for significant configuration differences, but there is a bit of a breakdown on Page i of the PDF. Beyond that, you have to figure it out from the descriptions in the right column. If you study them the logic should start to emerge.