the S model was a 16v 944 up to 1988, in 1989 the S became the S2 with
the change to the "Turbo" style body and running gear

more...

The 944S

The 944S arrived as a 1987 model year addition with a redesigned 16
valve dual overhead cam engine producing 190 hp.  In addition to the
increased number of valves and cams, the 944S came with an improved
Digital Motor Electronics (DME) system with knock-sensing regulation
capabilities.  Also included on the base 944, the 944S benefited from
the new timing belt tensioner which was implemented for the 1987 model
year.  As well, the 944S received the Bembo brake calipers with optional
ABS.  The initial 944S shared the base models bodywork until 1990.

Porsche Upgrades the 951

Porsche revisited the 951 and in 1988 introduced the 944 Turbo S with
total model year production of 718.  This performance version of the
already impressive 944 Turbo received many significant improvements
including:  bigger Bempo brakes, ABS, firmer springs and torsion bars,
stiffer Koni low-pressure gas adjustable shock absorbers, a thicker
front sway bar, and firmer suspension bushings.  The following year,
Porsche dropped the "S" designation and the standard 944 Turbo was
infact the "S" form factor, building 1385 for 1989.  This was to be the
last year the U.S. market would have access to the 944 Turbo.

944 Series 2

In 1990 Porsche releases the 944 Series 2, designated 944 S2. The S2 was
an extensive revision of the 944S which now included the aerodynamic
bodywork of the 951.  The previous three engines available were replaced
by the 3.0 liter's 16 valve engine and a big turbo 2.5 liter 944 Turbo
(sold in Europe).   As well, a soft-top version of the 944 was
introduced, the 944 S2 Cabriolet.  The new cabriolet was produced
jointly by Porsche and the German division of the American Sunroof
Corporation.

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all-year@rocketmail.com (jereme) wrote in news:e69de3d5.0408261257.e9a9fc0
@posting.google.com:

If I recall, I was advised to consider the S2 in preference to the S years
back. The reasoning being that the 944S engine was nowhere near as robust
as the engine in the S2, the latter has a larger engine with less
compromise on strength, and is less stressed, hence more reliable.

There are a few...

The S2 was available in a cabriolet.
The S2 has a 3.0 liter engine.
The 91 S2 has an "engine management" sensor.
The S2 has a chain/belt timing combo.
The S2 has the turbo body
The S2 has a removable nose for air filter replacement.
The S2 has standard drivers and passenger airbags
The S2 has twin electric seats
The 91 S2 has the 968 spoiler

The S2 was the best of the 944 N/A line.