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I have a very strange (to me) thing going on with my 7970's. Let me start off with some of my hardware\OS\driver stats:

OS:            Win7 x64
GPU's:   Sapphire 7970 OC - (11197-03-40G) (http://www.amazon.com/gp/product/B009B6Y01Y)
RAM:           8GB
AMD Driver: 13.1
SDK:          2.7

I have gone through all the threads that discuss getting the 7970 running optimized for Scrypt mining:
1) https://bitcointalk.org/index.php?topic=117221.msg1256502#msg1256502
2) https://bitcointalk.org/index.php?topic=173938.0
3) https://bitcointalk.org/index.php?topic=28402.msg1635964#msg1635964
4) and many others, but those are the main ones.

So just using stock voltages (don't want to mess with tweaking those), I was able to determine by highest stable memory settings for me are 1625. I ended up using these settings to allow cgminer to find me some good numbers:

cgminer.exe --scrypt -o stratum+tcp://pool.com:3333 -u Jazkal -p X --intensity 13 -g 2 --thread-concurrency 8192 --lookup-gap 2 --gpu-powertune 20 --auto-fan --auto-gpu

With this, I was able to boost my numbers from low 525 or so to 630-650. Looking at what cgminer defaulted to, it choose 950/1625. Sometimes if would choose 1650 which would crash the system. So I tweaked around and found that 1650 was the outside edge of stability for this card. So I added a hard code for 1625 which seemed to run just fine and was stable.

Now since I run my rigs out in an outdoor shed in the middle of the Texas summer heat (100F+), I let cgminer change up my settings downward when the heat climbs. So I was adding in this setting:

--gpu-memclock 1000-1625

This is what I find strange...

And when I re-launched cgminer, it defaulted to 950/1000 but was still running at 630-650 stable.

Why would it run at the same hashing power at 950/1000 and 950/1625? This doesn't make any sense to me. Can someone help me understand why this is? I thought Scrypt mining was all about super high memory settings.

edit: forgot to add my driver and sdk info

I remember the scrypt readme saying that higher memclock does not imply higher hashrate. The relation between memclock and gpuclock are more important.