B20a or b21a?

[Ludechix]

So I have an 89 with the b20a engine in it currently. It needs the top half redone. Local junkyard just got in a 90 with the b21a in it, I am unsure of condition really but it does have a guarantee to fire. So my question is this will the top half of the b21a fit on my b20a block? Should I swap whole engine or should I just have my b20a rebuilt...

[spiffyguido]

First of all, I assume that when you say 'B20A' and 'B21A' you're probably talking about a B20A5 and B21A1 respectively. If you actually have a B20A you're a very lucky person and should rebuild it as is.

Assuming you do have a B20A5 and a B21A1, the easiest solution would be to just pop the head from the B21A1 onto the B20A5 and get on with life. This wouldn't take too much effort, and yes, the head should fit without issue. However, for a little more power, if you're able to get the harness and ECU for the B21A1 as well, just put the B21A1 in your car. It will mate up to your tranny with ease, and you'll get 10 more HP right off the bat. Since the B20A5 is OBD0 and the B21A1 is OBD1, you do have to use the ECU, distributor, etc from the B21A1, but in the end, you'll have a little more grunt. The B21A1 is a great engine.

[Ludechix]

No I actually have the b20a engine, ran when I got it but had some electrical issues. It was not well taken care of and ran really rough. I think it may have jumped timing but have not yet torn it apart. I took the wire harness off a 90 with a b21a (no other stamps or numbers) I also took the entire top half (including exhaust and intake manifolds) i was thinking if I hav e to rebuild I would like to use this top half for the increased hp... Will this top half work with my b20a? Or am I just crazy...

[spiffyguido]

I'm not sure that the top half of B21A would work on the B20A. The B20A is actually a different engine than the series that makes up the B20A5/B20A3/B20A7/B20A8/B20A9/B21A1.

Whether or not it fits is beside the point. The B20A makes 160hp, which is 15 more than the B21A makes, so that's the one I imagine you would want to keep and restore. Understand that both of these are very rare engines, the B21A in particular. You're VERY lucky to have them. I think I speak for everyone when I say that we would love to see pictures.

If you're really in a pickle and need a new head for the B20A, then we can investigate further about the B21A fitment, but I'm not certain it will work. The B21A, as I understand it, is not a version of the B20A the same way that the B21A1 is a version of B20A5. The B21A uses the PK3 ECU, same as the B21A1, so I think they might be more closely related.

Also, there were several different engines labeled as 'B20A'. Before you go further, you should get the serial number of the engine (same place as the engine stamp) and confirm which one you have. Full information is here: http://en.wikipedia.org/wiki/Honda_B20A_engine

[Ludechix]

Thanks so much for ur info. I will double check the serial number and post pictures soon!
One more question tho, my car is an '89 4ws si, it was owned by some "crack heads" that messed with the wiring, I even had some house wire in there! Crazy! So my question is that the harness I took from the junk yard came for
An 89 si (not 4ws) but a few of the sensors seem to be different... Before I go to the effort should I just order the proper harness???

[spiffyguido]

Which sensors are different? You don't have to worry about the 4WS / no 4WS deviance between the harnesses. The 4ws system on 3rd gen preludes is fully mechanical doesn't include any sensors.