FM Towns revolves around the 386 architecture with the CPU taking 5 volts. Generally you have two different packages or chip sizes and types for CPUs which are the SQFP and PGA. Only generation 1 and 2 tower models have sockets with the remaining ones having their CPU soldered.
- -Models 1 and 2 (very first Towers), S1, S2, 1F, 2F, 1H, 2H
These Towns have a 386DX CPU. You can remove it and place a 486 processor in it without difficulty.
- Models Sf, Sh, Sf2, Sh2, 10F, 20F, 40H, 80H, CX10, CX20, CX40, CX100
These Towns while they have a 386DX CPU it is soldered. If you want to upgrade the processor to a 486 you must perform a total disassembly and desolder the CPU and insert a PGA 138 socket in it’s place. You may also socket the stock crystal near the CPU (should be 32mhz) and insert a 36mhz 5 volt crystal for a marginal speed boost. Clock divider is 2 which sets the computer from 16mhZ to 18mhz. Stability is questioned beyond 18mhz.
The following CPUs should be a drop-in upgrade from 386 to 486
- Cx486DLC, TX486DLC, TX486DLCe, TX486SXL and Cx486DRx2
All have Cache that can be enabled with a script. There is a Cyrix 486 cache enabler script floating around here:
ttp://www.vector.co.jp/soft/towns/hardware/se014605.html
However for the Texas Instruments CPU you will need to find the datasheet and write a cache enabling script specific for the FMT. The DRx2 also has a clock doubler that can be enabled. Generally enabling cache will break most software and clock doubling breaks software as well.
The following have a 386SX processor (100 pin Quad Flat Package) and can be upgraded to a 5 volt 486
- UX10, UX20, UX40, UG10, UG20, UG40, UG80 and Marty
Options for upgrade include the 486SXLC2, Cx486SRX2 and IMB486SLC (warning 3.3v CPU. Clip-on board required with voltage regulator). It is also advised to use a heatsink when you upgrade to a 486. Generally the 486SXLC2 is easier to find. No matter the CPU speed you’re limited to onboard clock at 16mhz except UG model with 20mhz.
TownsWorld 
What are some of the practical benefits of upgrading the CPU? Does the slight speed boost result in noticeably better performance in certain games?
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486 CPU upgrade is 50% times quicker than 386 and has same instruction set as well making it compatible.
Some software runs better. For example SSF2 will have increase performance with 486. With cache enabled performance increases. If you also clock double WITH cache enabled it will break sound and you won’t have sound.
Samurai spirits can have clock doubled and cache enabled for performance boost. However it is still a bit slow and likely just a bad port honestly.
Overall adding 486 increases performance with ZERO negative effects and breaks zero software. When you begin enabling cache or clock doubler if your CPU has it… Games have erratic behavior and usually break. It also opens up possibility of playing the later DOS ports that need that CPU power as some games later on required a 486…not many though.
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Aah, I didn’t realize that you would get a performance increase without overclocking.
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