I once tried 6 x 7990s, but it wouldn't work with more than 8 GPUs in Windows, so i gave up trying 
I'm surprised you can still mine with your 7990s.. all of mine died after 1 year of mining. 7990s are terrible for mining unless you have them water-cooled.
I'm surprised you can still mine with your 7990s.. all of mine died after 1 year of mining. 7990s are terrible for mining unless you have them water-cooled.
I have 2 7990 which ran between 80°C - 90°C with stock fans and now run at 56°C - 75°C with 2 x 140mm yate loon fans zip tied to the card mining Equihash.
The stock fans are useless for mining - I hit over 100°C when the card's airflow was restricted i.e. in a case.
Only way is a higher static pressure and higher airflow solution