Just informing the root cause is in the CF to IDE adapter. Got the CF card working now in VLB controller with bracket type adapter.
Always try easy and probable solutions first. Lessons learned.

This is the one I has issues with.

What rev AP4 do you have printed on the motherboard.
Using latest bios (Award 11/25/97) and a 1GB CF card connected as master to the onboard IDE works for me. This is using a cheap chinese CF > IDE adaptor with floppy power connector. It is slow though, limited to PIO-3, I'm not sure if the onboard IDE support PIO mode 4.
In the end I skipped CF cards and a VLB option and used a sata controller( SAT2-MV8 PCI-X) in one of the PCI slots with a cheap 128GB SSD, installed DOS7.1, fat32 to use full space. This was the only controller I had available and even though it is 64bit PCI-X it works on the 32 bit PCI slots on the AP4. Speedsys shows 61MB/S read benchmark in DOS, it uses UDMA-66 with the controller bios. I noticed in some of the more demanding games a clear speedup and smoother without pauses in flight unlimited, Nascar racaing. Obviously dma frees up the cpu overhead compared to PIO. Much cheaper than SCSI alternatives I was looking in to.
IF your looking for alternatives, Promise TX4 SATA PCI controllers are cheap on that auction site.

Board version is 1.6 if I remember correctly.

I used Vision QDIMark to benchmark drive speed.

This is without QD6580 driver: ~2.2 MB/s

QD6580 DOS driver installed: ~6.8 MB/s

Not that I care, just wanted to see the effect. Speed tripled. I think 6+ MB/s is just enough for me and my simple games...

Pretty sure the onboard controller is local bus, probably PCI.

AvalonH wrote on 2020-10-12, 17:46:

Speedsys shows 61MB/S read benchmark in DOS

Are you sure? On a 486 machine, we could not cross the 25mb/s threshold.
Don't you have a screenshot of speedsys?

i use the QD6580W atm on a Asus VL/I-486SV2GX4 with 4021 Bios. On primary IDE is a CF-IDE adapter with a transcent 4GB .
So my Bios see the CF with 2GB in bios LBA. I created than the first parti for dos with max size and installed dos 6.22 .
System boots fine.
But when i try to install the DOS Driver (V3.7) system freeze when the QD6580.sys is loaded. I got a screen that the driver ist init successfull but thats it.
I disabled also on my bios IDE Block Mode Transfer, but no effect.
I tryed running the installer with all the Speed settings and second options.. nothing.
When i uninstall the driver system boot fine..
Can this be a problem with my drive size?