[SOLVED] SLI power throttle on a 750W psu?

Hy guys,

I've recently bought an ASUS 780ti at a sortofabargain price to pair up with my Gigabyte one. So, 780ti SLI, bring in the uberframes and let's get down to some 2k goodness! No, not really...

I had a problematic weekend at the beginning of the month trying to get the second card (an ASUS DirectCU II OC) to actually work. I found out that some people had problems with them and had to underclock them to actually get them stable enough. The factory bios with the factory oc was, well, not good.

The screen went black and the pc rebooted when under load at stock oc speeds and sometimes it dropped out even at underclocked settings. Anyway, after a bit of research and plain old geeking about, I just flashed the bios and VOILA', it worked. No more dropouts and reboots as a single card (and it OC properly, although nothing special). That comforted me in believing that the card hardware was not faulty.

So I put the two cards in, eventually set the same skyn3t bios for the two cards (different brands, different bios, but at least the volts and clocks are the same), and they work fine. But FINE was not what I was expecting...

What's the problem then? Well, I've noticed a drop in FPS in many games after about 25/35 mins of playing. As an example, in Far Cry 4 (max settings at 1080p) I get an average of 95fps and it never drops below 75 in the first 25/35 mins. After that, the FPS count drops, even below 55 and then you really notice it stuttering. 

Both cards work at about the same load, temps are well within the comfort zone (72°C for the top one and 65°C for the bottom one, the Asus). NO OVERCLOCK, just the stock speeds from the modded bios. I tried pushing it just a bit, but it wasn't stable.

My pc case is nothing special, but the temps are good nonetheless, nothing out of the ordinary and the CPU it's definitely not a bottleneck.

My theory is that my PSU is barely just enough to power the system under load, it's a Corsair HX750W, one of the 80plus Silver series. It pushes out 65A on the +12V rail, and MAYBE I'm pushing it outside its normal efficiency range, it overheats and progressively pushes out less current.

What's your opinion? Am I way off?  

Thanks!

 

You may be getting close to maxing out the psu - about 500w for both cards + cpu + everything else you have. Only true way to tell is with a wall meter and take into account the units efficiency. At the end of the day though having a psu that is on a knifes edge for extended periods of time isnt good. Best to have some headroom.

Perhaps it worth putting the setting in the nvidia control panel to 'adaptive' and see how stable the framerate is. Obviously it limits your fps but it wont stress your psu as much and if there noticable dips in fps like you've seen previously you may be able to rule out the psu and put it down to shitty scaling.

Hy deejeta,

I've already set the voltage to adaptive in the ncp. I'll try set it to fixed and see what happens. I'll post the result. I'll also get a wall meter, but I'm leaning toward the new psu solution, I just need to justify it in my head ;)

Thanks for the answer man!

Hi castigo86,

I had problems with Corsair HX850W silver (system crashes after 15min of BF4 and Arma3) changed it with Seasonic X-1050 gold and everything is fine now. You can check my PC specs in my sig, so I think your PSU is too small for that setup.

 

Thanks To9opoB,

I saw your rig and I'm convinced now, I NEED MORE POWER... or at least a bigger PSU...

Thank you!

Right, I got a little longer session this time in Far Cry, but I get random crashes in other games, so I'm going to get a bigger PSU, 1000w or so. 

Thank you guys, you were a big help! :)

Be welcome,
I'm glad that I could help. :)