DSM transmissions come in two types with different output shafts. 1990 and some 1991 cars have 22-spline output shafts. Later 1991 through 1994 cars have 23-spline output shafts. Please note that 1991 cars that are 'later' (i.e. have later production or serial numbers) may still have the 22-spline shaft. The best way to check is to look, or to check part numbers with a dealership.
The number of splines on the transmission output shaft must match the number of splines on the transfer case input shaft, or else the transmission will not fit. Some people have found it possible to swap shafts between the transmissions in question to match the parts up correctly. Alternatively, the transfer case can also be changed to match up with the transmission, with an associated greater cost.
There have been several syncro changes over the years to try and combat problems with 1G cars not shifting well. It is generally accepted that later transmissions have superior syncro designs, so if all other things are equal a later version may be superior to an earlier version. Also, 2G transmissions can be put onto 1Gs with some simple modifications, and can provide superior performance. However, most people do not have free choice of transmissions due to economic reasons.
It must also be noted that the manual transmissions from turbo FWD and non-turbo FWD cars are different. Non-turbo FWD 1Gs have a F5M22 transmission (manual) or F4A22 transmission (automatic). Non-turbo 2Gs have a F5MC1 transmission (manual) or F4AC1 transmission (automatic). Turbo FWD 1Gs and 2Gs have a F5M33 transmission (manual) or F4A33 transmission (automatic). These transmissions are not interchangeable, and they are geared differently.
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