GPU using only 8 PCIe lanes

[jbushey94]

H Folks.  I have a brand new build - ROG Z790 Hero, Intel 13th gen i7, a Gigabyte Nvidia GeForce rtx 4070 ti, and two NVMe's.  One NVMe is installed in the top n.2 slot and the second m.2 is installed in the second slot.  The GPU is installed in the top PCIe slot.   

What I've noticed is that the GPU negotiates only 8 lanes and will only scale up to 8 lanes under load.  I've also noticed that in BIOS, under Advanced / System Agent Configuration / PCI Express Configuration, the card is reported as being plugged in to PCIEX16(G5)_2 (at x8) despite definitely being plugged to the top slot PCIEX16(G5)_1.  

The manual for the motherboard seems to indicate that the top m.2 slot, and the top two PCIe slots share 20 lanes from the CPU.  

My question is - is my system behaving as expected?  (ie, reporting in the second slot at 8 lanes despite definitely being plugged in to the top slot).  And if this is expected, why?

Thanks for any help.

[JohnAb]

No idea why the GPU is reported as being in the wrong slot, but leave M.2_1 unpopulated and you should get x16 😃

[Shinchan0125]

something definitely is not right.  When I had mine Hero, i  had EVGA 3090ti on slot 1 X16 4.0.    With M2.1 and M2.2 populated running 4x4.

Maybe check bios to see if you can force it to 16 4.0

[jbushey94]

I haven't yet found any switches in the BIOS that would allow me to control this, beyond being able to force a particular Gen. on the PCIe's and m.2 slots.  Forcing things to Gen4 doesn't seem to change what I'm observing.  

[Shinchan0125]

what type of M2 is in M2.1 slot?  If you are only getting X8, most likely your M2.1 is GEN5.

[Griff7520]

I believe if you use a gen5 m.2 drive in the top slot then the gpu will run at x8 lanes. I am sure I have read it some where