s13, s14, s15 whats it all mean

Without going into a novella describing all the Silvias and their engines, do a search for "Nissan Silvia". In a nutshell, the Silvia in Japan is the 240SX in North America. The S1? describes the both the body style (which came in a coupe and fastback model) and the engines. The engines of the S1? are SR20DET. All the 240SXs had KA24 engines.

In a nutshell, it goes like this:

In the US the S13 chassis was sold from 89-94, and both coupes and fastbacks were called 240SX's. They came with the KA24E in 89 & 90, and KA24DEin 91-94. There was also a convertible available in the later S13's (the conv was the only S13 available in 94).

In Japan, the S13 coupe was called the Silvia and built from 89-94. The S13 fastback was called the 180SX. The JDM S13's used the CA18DE, CA18DET, SR20DE and SR20DET.

The S14 was sold in the US (240SX) and Japan (Silvia) from 95-98. US cars had the KA24DE, Japan the SR20DET.

The S15 was sold in Japan from 99 to (I believe) 2002. Engine was the SR20DET with a 6 speed.

FWIW, the 180SX was popular enough in Japan to be built for a couple of years after the S14 was introduced, and the S14 & S15 were sold in Australia as the 200SX.

The various front end conversions are pretty popular and can be done with varying degrees of difficulty. An S13 Silvia front end is pretty much a bolt-on conversion on a fastback. This 'Sil80' conversion alledgedly came about because it was less expensive to put Silvia sheetmetal on your 180SX if you wrecked it. S14 & 15 to S13 and S15 to S14 front conversions can be done with varying levels of fabrication and custom parts are available, but pricey. There's also a kit floating around that puts a 280ZX style front on the S13.

Hopefully this will lower your confusion level some!