SLI performance worse than single 660TI

[troyeb]

Thanks in advance for any help. 3DMark gives much lower score in Fire Strike (3500 SLI vs 5800 Single) and scores matched in Cloud Gate. Games play better as well with Single. Both cards test good individually. I have read my hardware is up top spec but is this the result of a bottleneck?
Hardware:
I7-875k OCd to 4.2Ghz
Antec 620 Liquid Cooler
8 Gigs DDR3 Corsair at 1550Mhz
P7P55D-E Pro Motherboard
PCI slots 8x/8x (not sure how to confirm this, but based on MB spec this should be the case)
Dual GTX 660 TIs
Win7 64 bit
Updated drivers and bios
I upgraded recently to this CPU and these video cards from an I5-661 @ 3.3Ghz with 4 Gigs DDR3 and a 550 TI. At one point I tried SLIing another 550 TI and had the same drop in performance as with the current setup. Any ideas what I can try?

 

[Corey Carroz]

What power supply do you have? Maybe you don't have a powerfully enough supply. As a guess you probably need at least a 750 watt supply. If you have that check and make sure the supply has the proper voltage on the rails to power both cards.

 

[troyeb]

Thanks for your reply Corey, I thought I had included that I have a 750w Corsair PSU. I will google how to check the voltage on the rails, but if you are feeling generous I'd gladly take you advice on how to do this?

 

[Mousemonkey]

What power supply do you have? Maybe you don't have a powerfully enough supply. As a guess you probably need at least a 750 watt supply. If you have that check and make sure the supply has the proper voltage on the rails to power both cards.

Wrong, I'm running my 660Ti's on a 620w PSU and have been for quite some time now.