the 4th generation ('88-'91) honda civic si comes stock with the d16a6 engine. it has 108 hp @ 6,000 rpm and 100 ft. lbs. of torque @ 5,000 rpm. IMO the 4th gen. civic/2nd gen. crx is a great platform to start out with. there are a few low cost engine swaps that offer great power gains. the cheapest, easiest and probably most common engine swap into those cars is the DOHC ZC swap. the DOHC ZC came out of 4th gen JDM civic si's and 2nd gen JDM crx si's. the DOHC ZC has 129 hp @ 6,800 rpm and 106 ft. lbs. of torque @ 5,700 rpm. the DOHC ZC is a very easy swap because it bolts right into your car with the stock mounts, no real wiring (unless you have to convert to MPFI which is not the case with the civic si) and it allows you to use your stock tranny (unless you have a '88, in that case you have to upgrade to a '89-'91 tranny), the si tranny is geared the best for performance. then next two swaps that are very common in the 4th gen civics are the ls (b18a) and b16a SiR swaps. both of those engines require a new b-series tranny and aftermarket engine mounts. the ls (b18a) has 130 hp @ 6,000 rpm and 121 ft. lbs. of torque @ 5,000 rpm. at 9.2:1 compression, the LS is
VERY boost friendly and is the better choice if boost is in your future. the b16a SiR has 160 hp @ 7,600 rpm and 111 ft. lbs. of torque @ 7,000 rpm. the LS is a 1.8L engine and the b16a is a 1.6L engine. because of the extra .2L of displacement the LS has a slight advantage over the b16 in the torque department. the b16 is more of a "performance" engine and has a much higher redline than the LS and has quite a bit more peak hp. with the b16 swap, you will need to run a few wires from under the hood to the ecu in the ecu in the cabin to enable vtec. how complete your swap is will depend on where you get your engine from. most engine resalers have "complete changeover" packages which include just about everything you will need to swap an engine. i suggest you check out
http://www.hmotorsonline.com/ which has complete swaps and very competitive pricing. if you are near a sea port, check out
http://www.flashoptions.com/ they have very good pricing (most of their front clips are about the same price as other companys swaps) the only downside is that they only ship "to a port near you" and from there you have to arrange the transportation. there is one more swap that i might as well mention, it is the ls/vtec. it is pretty much a combo of the b16 and LS. it involves using the LS block with the b16 head and ecu. that way you get the power up top from the vtec of the b16 as well as the LS' displacement. it is not just a stright swap though, and i wouldn't recomend doing it as your first project. if i were you, i'd swap in a b16 and mayber later down the line, get a ls shortblock and do a ls/vtec conversion. the zc is a good swap if that's all your looking for, if you just want to swap an engine and be done with it. but if you want to keep improving, the b16 is the swap for you. just post up any more questions that you may have and i'm sure they'll be answered. good luck.