Calculating the key OC points on the 7800

[HeavyH20]

After some testing, the best points on the core clock of the nVidia 7800 series cards are whole multiples of the core frequency oscillation of 27 MHz. That calculates to the following root core clocks:

(17 x 27) - 40 = 419
(18 x 27) - 40 = 446
(19 x 27) - 40 = 473
(20 x 27) - 40 = 500
(21 x 27) - 40 = 527
(22 x 27) - 40 = 554
(23 x 27) - 40 = 581

And, here is how a single MHz below the whole number threshold affects 3Dmark05 scores:

445 8052
446 8222

472 8404
473 8508

499 8669
500 8753

Now, what about the other two cores. How are those controlled?

Well, when you adjust the root clock, you actually adjust all three clocks.

The root clock gets bumped by 40 MHz to represent the real geometry or vertex core clock. So, lets pick 450 core, which so many vendors use. 486 is the best vertex clock value (18*27), which translates to 446 root (486-40). So, we would actually have 490 using a 450 root clock, a 4 Mhz pad on the required frequency.

But, the shader and ROP cores start at 415 (really 418.5) in low power, and jump in whole frequency values (ie, 17 x 27 = 459) to a value that is less than 13.5 MHz over or equal to the root frequency. In this case the root frequency is 450 which is 18 MHz over 432 (16 x 27) and 9 MHz shy of 459 (17x 27), so 459 is the ROP/Shader clock.

So, a little confusing, but you do have some control over the other two clocks.

So, here is a list of primary - vertex based (red) and secondary - ROP and shader based (blue) root clock OC targets. The red will yield the larger gain, and you will see a second, albeit smaller gain on the blue clocks where the optimal memory frequency matches the root clock.

419
432 - the nVidia spec root clock
446 - so many vendors use 450
459 - why BFG uses 460
473
486 - why eVGA, XFX, and BFG use 490 on limited cards
500
513
527
540 - How I got 9539 on 3DMark05 with a lowly 3200+ and one 7800 GTX
554
567
581
594
608
621