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Discussion Starter · #1 ·
I have a 1991 SI prelude with the 2.1L engine and I was wondering if the 2.0L engine would be able to just fit the engine bay without any alterations or anything. I want to say its should fit since the engine it in the other 3rd gens but I just want to be sure
 

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I have a 1991 SI prelude with the 2.1L engine and I was wondering if the 2.0L engine would be able to just fit the engine bay without any alterations or anything. I want to say its should fit since the engine it in the other 3rd gens but I just want to be sure
All Bastard B motors will fit in any 3rd gen chassis. B20a a5/6/7/9 a4/a3, B21a a1.
No other motors drop in without my swap kits or some other modifications.

You have to transfer your stock engine mounts and side post mount over to the new block and transmission.
 

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Discussion Starter · #3 ·
All Bastard B motors will fit in any 3rd gen chassis. B20a a5/6/7/9 a4/a3, B21a a1.
No other motors drop in without my swap kits or some other modifications.

You have to transfer your stock engine mounts and side post mount over to the new block and transmission.
I'm confused about what you mean in that last part
 

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I'm confused about what you mean in that last part
Not every Bastard B motor bolts correctly too the chassis with some variants.

The B20a5 vs the B21a1 are different driver side mounts and post mount.

B21a1:

Motor vehicle Automotive tire Hood Vehicle Wheel


This part is different than a B20a5 post mount.
Also the driver side motor bucket is different than the B20a5.

B20a5:

Motor vehicle Automotive tire Automotive design Automotive lighting Automotive fuel system
 

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There's more involved than just motor mounts. Vacuum lines, wiring harnesses, and sensors are different, as well as the ECU. The b20a5 is OBD0, and the b21a1 is OBD1 (mostly). The easiest way to do this swap is to keep the intake and exhaust manifolds from the b21a1, and install those on the b20a5. That way you won't need to swap wiring harnesses, ECU's, or modify the vacuum system. I did this swap (b21a1 to b20a) about 12 years ago, so there are possibly things I'm forgetting. But it's an easy swap if you do what I suggested.
 

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Discussion Starter · #6 ·
Okay, so if I bought a 1989 prelude that has a B20a5, what all would I have to do for the engine swap? Id have the whole car but its an auto so I don't know if that adds anything to the process. And would it be best to keep the intake and exhaust manifolds, or since I have everything I just change out other things?
 

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You'll need the flywheel from the b21 as well as the intake and exhaust manifolds. And the motor mounts Sean mentioned. If your AC works you just want to unbolt the compressor from the engine and leave it in the engine bay. Power steering pump and alternator should be compatible with either, but since you just bought the '89 you probably don't know their condition. So you might want to reuse the ones from your '91.

I would suggest you get familiar with the FAQ sticky before you tackle this. Tons of great info there. And Prelude3G.com as well. Good luck and have fun!
 

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Seriously this was already been discussed a ton on this forum, and I even made a slmple writing up it.
Just pull a search:

 

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It's true that this has been discussed a lot in the past. The problem is in sorting through the results, trying to find something relevant. Regarding your thread, there are a couple of grey areas in it. You mention engine wiring harness and ECU. If he keeps his b21a1 intake and exhaust manifolds (and the sensors contained within) he will reuse the engine harness and ECU from his car, not the donor car. He will essentially be converting the b20a5 into an obd1 engine, so there's no need to remove those from either car. Good call with the EGR tube and water pipe though. I had forgotten about the issues with those.
 
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