So I recently OC'ed my GPU and was running some tests.
With my side fan disconnected I am actually finding a 2C temperature drop during stress testing.
Does that sound normal?
My side fan pulls air in, and maybe it is messing up with the fans on my GPU.
current fans include one at front pulling air to cool HDs, and one at back pulling air- both always on
If both fans pull air inside the case then you have a problem of positive air flow.
Basically what this means is that there's more air getting inside the case than getting out.
That's bad because this means that air is getting trapped inside the case which in turn means it's getting warmer.
The reason that when you turn the side fan on things get worse is because when it's off air is being released through the fan hole whereas when it's on, it's just trapping more air in.
Usually what you want is to aim for a neutral to slightly negative air flow.
That means that the fans are pulling air slightly faster than taking it in so there's always fresh air inside the case.
Definitely put the back fan to push air outside the case and if possible, add a second fan on the top of the case, also to pull air out.
Then turn the side fan back on and you should see some big improvements!
You have the right idea...your side fans are disrupting the air the gpu fan is attempting to pull in to cool with...once you aren't disturbing the air directly over the gpu then it performs more efficiently. Although if the air the gpu is forcing through it's heatsink is cooler...that's a good thing...just make all the ambient air inside the case cooler by moving large amounts of cool "outside" air through the machine...that way the air the gpu cooler is using can move more heat!!!
I'm not so sure about negative pressure inside one's case....that would hold air in...not force heat out...exhaust...I would rather force "Cold" air IN all case fans as Intake...with high RPM fans and have ALL TOP OR BACK FANS EXHAUST out at the top or top back of the machine...Heat naturally goes in which direction??? UP isn't it...ie ALL exhaust should be at the top of the machine...everything else should be INTAKE!!!
I have some experience with this...the last 4 machines and 6 years...all my rigs...have used this configuration...antec 900...antec 1200 x2...Rosewill Blackhawk Ultra Supertower...current machine...temps are 30-32C cpu...40C gpu...same in all four machines...with differing hardware...2 quad amd...i7 960 and 2700K...6870 and 6970 gpus...
My experience is not unusual....just common sense and actual results.