with a simple delay-line modification you will likely see about 33Mhz (23 MIPS) - (not all A3640's will do this without further modification) - I have modded a 3.1 and 3.2 revision for 33MHz and they worked fine - if you are wanting more with a little more effort you will need to change some of the CPLD Code and a few traces - this will see about 40MHz (30 MIPS ) (with a later revision MC68040). If memory serves SpeedGeek managed to spank 50 MHz on his 040 - quite impressive indeed!
I would humbly say to invest in an 060 CPU and the A3640 to A3660 adaptor - boom 060@50Mhz = 65 MIPS
I would highly recommend you get someone to replace a few of the GAL chips with SpeedGeek's State Machine mod to increase the memory access performance back to the level of the 030 CPU card.
Also as Zetr0 mentions get an 060 CPU and adaptor board.
I could only get my A3640 to run with a 60mhz crystal (30mhz 040 / 60mhz 060) but I think that is down to my PSU, which I am replacing at the min.
At 60mhz with an 060 CPU it's just shy of 80MIPS.
I had mine running at maximum 34.5MHz with a 33MHz 68040, 69MHz Oscillator and the delay line mod. This also needed 60ns FastRam on the mainboard since the whole Amiga Mainboard will also run at 34.5MHz (aside from the AA Chipset) which not every A4000 mainboard can do. If this is not possible on your A4000, it could be due to the Ram, the CPU, the logic on the A3640, the mainboard logic and IC's, the Zorro Cards, a 3rd party daughtercard or the PSU....
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