This topic has been discussed on and off in various other threads, but I have not seen a complete list which includes the 20 MHz and 83 MHz FSB settings. I've tested the PLL clock generator chips on various UMC 486 motherboards and all brands have the same pin-outs and clock options. The most commonly labelled clock generator chip found on Biostar MB8433 motherboards is the UM9515-01 by UMC. It is made by other manufacturers as well, but I forget the brand names.

1MB8433  M919  HOT-433   20   25   33   40   50   60   66   83 MHz
2----------------------------------------------------------------- 
3JP15    JP3A    JP1     0    0    1    0    1    0    1    1 
4JP16    JP3B    JP2     0    0    1    1    0    1    0    1 
5JP17    JP3C    JP3     0    1    1    1    0    0    1    0

The above frequencies were verified using an oscilloscope. 1 = jumpered, 0 = no jumper. For the HOT-433, assume jumper position 1-2 is 1 (ON) and 2-3 is 0 (OFF).

SiS motherboards also seem to support the 60/66 MHz FSB setting, however I do not have a SiS PCI 486 motherboard to verify any other FSB jumper settings. Ideally, it would be nice to find a SiS board which supports a 75 MHz FSB. For 75/83 FSB operation, you'd likely need 10 ns L2 cache.

It is my hope that this post will encourage someone to test out the 83 MHz setting with an appropriate CPU. The CPU must support the 2X multiplier as 2X (not 4X), so the most likely candidate is probably an AMD DX4-120. If this is successful, this would be the first such 486 run at 166 MHz.

The other option is to use a custom PLL chip, or one which is placed just before the CLK signal of the 486 socket. I believe there are such units with over 4000 possible clock settings. They are cheap (under $10) and use the I2C or SPI communication standards, so you could program them with a microcontroller pretty quickly. I am just not sure if you'd need to re-program them each time the power is cut. There are also some discrete value PLL's which I've sourced and have experimented with but have put this custom PLL project on hold for the time being.