"KA vs. SR vs. CA Which one is best?" (haha "easy" good luck guys)
Yah...thats an easy one indy....not!!!
Obviously here in the states you will be able to pick up an ka fairly easy, and it shouldnt be very expensive. Now, if you try to NA build an ka you wont see very much in the way of top available hp, but if you do go the boosted route there is an alot of information about what to do and what not to do at ka-t.org. With proper tuning these guys seem to be making ok reliable hp on an engine that was never built for boost. However, stock, the ka is an ok swap into the s12 as it will provide more torque and more power then your 20e boat anchor. Also since this is the only engine that came to the states in its true form, there is an huge amount of parts for it and you wont have to worry about not being able to find oem replacement pieces. The swap for the ka into the s12 is about as difficult as swapping in an sr or ca.
Out of the box the sr puts down more hp stock then the ca, but it should for the price tag you will have to pay for it. There is an wealth of parts online for the sr from stock replacement to aftermarket, but you wont be able to go to your napa in most cases and get some stuff that you may need. The sr has an huge following in the states, and if you search online you will find a wealth of knowledge on how to properly build and tune an sr.
Out of the box it has less hp, and it will be difficult to find one with lower miles. But you will have it out of the car...so no reason not to replace the minor things like the water pump and timing belt and head gasket. There are parts available online...but not as many as the sr, and the ca doesnt have as big a following here in the states as the sr. Stock internals on an 18det can handle up to 300hp, and supposedly the crank can go up to like 450 500hp but you will need forged pistons and the like to make that much hp.The ca18det actually came in the mk2 s12 in japan so all of the mounts are the same...and imho the engine looks like it should be in the s12 chassis...altho i will admit the sr looks at home in it as well. The 18det in the s12 tho is different from the head up from the 180sx 18det. The s12 version has an throttle body that goes over the valve cover and has 8 intake runners instead of 4. Also the engine uses one coil pack for all 4 plugs as opposed to individual coil packs. Most people have told me there is like an 50hp difference and i have also heard that this setup has problems taking higher boost levels. It is very rare tho to find an s12 18det and if you do most likely the harness will be cut so you would have to go and buy an 180sx head, harness, ecu, maf, and coilpacks and ignitor if you go this route...which is the way my build has gone so far.
In the end you have research what each engine is capable of online, figure out what you will be doing with your car and what you want your car to be, and then decide from there. The real point of all this is no one can really tell you which engine is better, because they all do certain things well.