B21A1 to B20A5 Engine Swap - Honda Prelude Forum

SFRigger · 06-06-2003, 06:20 PM

This may be a somewhat stupid question considering the wealth of knowledge available on this board, but here goes:

Are the intake & exhaust manifolds interchangeable on the B21A1 and the B20A5? In fact, are there ANY compatibility issues I need to know about? I am putting a B20A5 into a 91 Si 4WS with a B21A1 that the timing belt broke on.

Any advice would be helpful.

sped · 06-07-2003, 12:21 PM

i have never heard of any compatibility issues. should be a direct swap.

marcucci · 06-09-2003, 06:24 AM

The problem is that the B21A1 uses a single O2 sensor since it was a 90-91 motor. The B20A5 is the same for the same years, but 88-89 it had dual O2's.

If you retain the 91 ECU and just swap the engine and keep your header, you will be fine. Not sure what power will look like on the B20A5 w/the B21 ECU.

Another consideration is that you won't have the dual runner manifold, or at least the B21 ECU doesn't have the signal to open them. You can run the B21 manifold, though, you will just be down a hair on low-end.

SFRigger · 06-10-2003, 10:19 PM

Thanks for the info guys. I'll let you know how it goes when I'm finished.

Si4WS · 06-15-2003, 10:58 AM

A little OT but,
Is the B20A5 better to tune then the B21?

marcucci · 06-16-2003, 11:26 AM

Very OT. Start another thread if you want to dive into that one, or check out the 3g forum (search).

Si4WS · 06-17-2003, 01:51 PM

Quote:

Originally posted by marcucci
Very OT. Start another thread if you want to dive into that one, or check out the 3g forum (search).

I knew it was, I just want the general consenscious(sp?). I don't really know. I had a 3g and the B20 seemed way weak compared to the B21.

SFRigger · 06-21-2003, 10:03 PM

Alright guys. Update on the swap.

First things first, when you order an engine for your vehicle, and your vehicle is a manual, be sure to request an engine from a manual car. I ordered my engine and when it arrived i discovered that the coolant pipe that runs along the back of the engine was about 3 inches longer than on my old engine because the new engine came from an automatic car, so I hacksawed it off to the proper length and dressed up the end.

Next, as I was putting the front engine mount back onto the bracket on the block, I discovered that the bracket on the new engine was narrower than the bracket on the old engine. This discovery came after I had already put the front of the new engine back together after replacing all the old seals and the timing belt, adjusting the valves, and seating the timing belt covers, and the valve cover.

I have been delayed finishing the transplant due to lack of time, so I went over tonight to finish putting various other brackets and mounts on, and found a crack in my exhaust header. No problem...Use the header from the new engine, right? Nope, no place to attach the EGR pipe on the header from the new engine. I'm either going to see if the crack in the old header can be welded, see if a machinist friend of mine can tap the new header to match the old, or I'm going to buy an old B21 header from a salvage yard.

I'll let you guys know when I'm up and running again. Thanks for your advice.

rconway75 · 01-22-2010, 04:50 PM

I have a 91 honda prelude 1991 B21A1 was the original motor I spun the main bearings and tore up the bore line so the block was un rebuildable so I took a B20A5 (any year 88 - 90) and slapped the B21A1 head on top every thing that went to the head went back on my flywheel went on no problem seal on the block and were different sizes and the harmonic balancer was a different modle based on the year of the block but I am now running nitrous on the car and it runs awsome low end compression so if you ever want to throw a turbo kit your already with low end compression. ok hope this helps

tom27897 · 01-22-2010, 06:41 PM

probably wouldve helped seven years ago.....