The P1800 started life with Volvo's workhorse engine, the B18 (a 1.8L OHV Pushrod engine, with just over 100HP; Equipped with a pair of SU Carburettors ), gradually being upgraded to the 2 Litre B20, and eventually replacing the Carbs with Bosch D-Jetronic Fuel injection (in the 1800E). The Final P1800's had approximately 130-135HP.
The P1800's Body design was concieved under contract by Frua, an Italian design company. This gave the P1800 it's distinct lines and shape. The P1800's appearance changed little over it's production run, with the most important changes being a slightly different grille, changes in the side trim, and an interior refresh. These changes took effect in 1967 and were present on cars from that point onward. Changes strictly aesthetic, as the base sheetmetal did not change at all throughout the run (with the exception being the ES)
The 1800's shape may have suggested a sports car, but while it's performance was decidedly not lackluster (The 1961 model could do 0-60 in about 12-14 Seconds, and had a top-speed of about 105Mph), it was simply outclassed by the cars with which it competed (Jaguar's E-class, BMW, etc), though one of it's selling points was that it cost much, much less than these cars ($3995 for a P1800, vs $10,000 for the BMW, at the time). All in all, it was more of a Grand-tourer than a sports car. Volvo capitalized upon these facts, even running ads that called it "a souped-down" or "Poor-man's" Ferrari.
It was also significantly more reliable than most competing models, and Volvo capitalized upon this fact as well. A true testament to this model's reliability and longevity is the fact that the car with the Highest accrued mileage ever is a P1800 (Who's engine has only been rebuild once in it's 2.5 million mile run.)