I've got a 1.8TS GT Blackline currently, If you look at my signature, I've had most of the variants of the TS engine and the JTDm. Historically, the engine has it's origins with Fiat and the 1.8 was going to be the top performing motor. It has an almost identical bore and stroke and was of the famous 1750 displacement. Trouble was everyone else was producing 150bhp 2.0 and the engine was seen to be underpowered compared to the competition. Fiat had a load of trouble trying to get the engine to 2litres - it was really rough - and only solved this problem by fitting a balancer shaft. The capacity was increased by lengthening the stroke - not usually a good move in an engine that people expected to be able to rev the nuts off, however the flip side was that it produced a lot more torque. When Fiat took over Alfa they used this engine to replace the famous boxer unit, but they Alfa-ised it with bespoke heads, drawing on Alfas racing history by fitting twin spark plugs and additionally variable valve timing. Later Alfa introduced direct injection with JTS, replacing one of the spark plugs per cylinder with an injector. This was slightly different to other direct injection engines as it relies on a very high compression ratio and exact metering of fuel to ensure smooth running at engine speeds up to 1500rpm. Other makers systems do this at engine speeds up to 3000rpm.
So which is best? 1.8 or 2.0?? Harry Hill would say 'FFIIGHT!' 2.0 will be torquier and (possibly) more powerful (many seemed to ship with 137bhp - Belgium mode!). The 1.8 though is almost as powerful, is simpler (no balancer shaft and accompanying belt) and I suppose has history on it's side.
Finally when I had a 156 2.0 followed by a 1.8, the one thing I noticed was that the 1.8 was a little bit lower geared which mean't for the 0-100kmph run you had to make 2 gear changes - but in the 2.0 you could just reach 100 in second. It was almost as if they geared it to be slower (or more likely to compensate for the lower torque)! I personally didn't think there was that much in it between their performance.
Don't expect the 1.8 to be more economical though!!
146 1.6TS, 156 2.0TS, 156 1.8TS, 147 1.6TS, 147 2.0TS, 159 1.9JTS, 147 1.6TS, 159 1.9JTDm, Mito 1.4, GT, Giulietta 2.0JTDm, Giulietta 1.4tb, Giulietta 1.6JTDm........