I bought an 8th gen Civic SI with no motor
Everything is there including the transmission, just no motor

Is K20A3 a direct swap into this car? Can I just drop in a K20A3 and an ECU from a base model RSX and go?

Before anyone says it's illegal to put an older model year engine into a never car, the Civic is a 2006 - and the K20A3 was still in production in 2006 so it is not illegal.

I just can't justify buying a K20Z3 which is going to cost more than the whole car

 

You can do it but you are going to need the K20z3 throttle body (drive by wire), and most likely the K20z3 motor harness and a hondata flash pro and a tune. That alone will add up a bit in terms of cost.
Get a k24a2 from the tsx, that will make it more worthwhile swap and cheaper that a K20Z3.
The car still has the ECU? You cant use the A3 ECU

 

I don't think you will want the A3... not much power either!

 

k24a2.... that's a very good suggestion actually. Thank you.

So I cannot use the A3 ECU.. that is good to know.

Can I use the K24A2 ECU without having to go the Hondata route?

 

Nope, you have to use the original ECU, engine harness and some sensors that came with Civic Si in conjunction with the Flashpro. You will need a tune sine the stock Z3 map will run the engine lean.

Here a link to some the things you will need for the k24 swap. You can just drop the k24a2 straight with just the oil pan from the z3 (there a engine mount on it), but you will be limited to the stock rev limiter of the K24.

K24/K20 Frankenstien swap - What needs to be done! - 8th Generation Honda Civic Forum

 

If you don't want to use Flashpro why not just get a K20Z3 and call it a day?

I don't know why the A3 was even brought in to this really cause it's a real crappy motor when compared to the Z3.

 

True...K20A base model is not going to work. I understand K20Z3 is an expensive engine. I remember my buddy paying about $2500 for a rebuilt engine.

 

You can find k20z3 for 1000-1500$ if you look around honestly... no shortage of that motor!