Can I replace my 1990 prelude motor "B20B" with a B21A1 1991 will it fit? do I need to change anything?

You do not have a B20"B" you have a B20A, the B20b is out of a Honda CR-V and not like our engines even though the engine code would have you believe they are. They do not fit in the Preludes without custom motor mounts and other work. To try and answer your question, it would be very helpful to know what submodel Prelude you have: S model which has the carberated B20A3, the 2.0Si with the B20A5, or the Si with the B21A1. If you have the 2.0Si or Si, then the engine swap to go B21A1 will be very straightforward. There are some differences between the B20A5 and B21A1 that you will want to know about, but to overcome most of the differences you would want to choose to keep the original engine's intake and exhaust manifolds and all associated parts connected to them, just swap the engine (engine block and cylinder head) itself but using the current engine's Intake manifold assembly and exhaust manifold. For much more precise and thorough answer, however, it will demand that you go on the PreludePower forums. That is the largest 3rd gen Prelude community and their knowledge-base has only been growing for the last 11+ years. All your answers and Prelude gurus are on there, but it will do you good to go through all the "stickied" threads on top before requesting help via new thread. There is a prominent member on the PP forum under the name MustardCat who has made a very helpful website of his own (Prelude3g dot com) that covers many topics. Good luck!

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