Why does Crossfire not scale as well as SLI?

[sasquatch451]

I've seen many many performance benchmarks and it seems to be very consistent in situations where multiple cards make a different (i.e. we are talking 1920x1200 or greater with some AA etc...). Adding an extra Nvidia card usually almost doubles performance (80%-100% performance increase). Adding an extra ATI card almost never doubles performance (40%-60% performance increase).

I'm not interested in an argument over my estimates in numeric values - the trend is fairly clear regardless of the actual numbers. I'm interested in *why* this happens. I've also looked at older cards where (presumably) drivers are more mature and should have eliminated this as a variable.

Thanks.

 

[uncfan_2563]

80 - 100% on nvidia? umm no. especially the higher end of that estimate

 

[Mousemonkey]

80 - 100% on nvidia? umm no. especially the higher end of that estimate

It did for me in COD4, 85-95fps with one card and 200+ with two, but that is only one game that sticks out because I had to enable SLi from the game menu as well as the Nv control panel.

 

[Kari]

well some games scale over 100% on ati cards.... so?

edit
http://www.xbitlabs.com/articles/video/display/gtx285sli-hd4890cf.html
meh

 

[pinaplex]

scaling with SLI and CF is extremely dependant on the individual game. unless the game is specifically designed to take advantage of one or both of the technologies, you won't get that "doubling" of performance just by addiing a particular card.