[Alien]

Originally Posted by Web-Junkie

Thinking about making a CPU duct so I can suck in cooler air from outside the case and blow it onto the CPU fan, not sure what to make it out of though! CPU is an AMD 2.8GHz Dual Core with stock cooler. Or should I be sucking hot air FROM the CPU and not BLOWING cooler air onto it?

It generally tends to work better feeding cool air onto a cooler, rather than trying to suck hot air from it.

Originally Posted by Web-Junkie

Ideally I want to use an 80mm fan at the back of the case and make a right angle duct down to the 60mm fan on the CPU. I could do one from the side as I have 2 fans there too pulling air out so I could turn one around and it would be more or less straight but fancy the right angle approach as it would give more space for air flow.

Thought about square rubber tube (if that exists) or round rubber tube with thin walls so it can be stretched over a square 80mm fan and square 60mm fan, not sure if that can be 'bought' offshelf?

Well, you've got multiple options for ducting, but I think your rubber idea is stretching things a bit.

To start with, you've got 2 types here:
Linkage
If you're only dealing with 80mm fixings, then the square ones look like a good choice, though you might have to do a bit of improvising somewhere along the line, as odds are that the straight & corner ducts won't add up to the exact length you need.

There's a few options here. The 80mm flexible tube kit looks like it'll cover most of your requirements, plus the various fan adapters would mean you could use it with different sized fans if you need.

Originally Posted by Web-Junkie

The reasoning behind this is my mobo/gfx card use passive cooling, no fans, and so radiates heat into the case. I have two 80mm fans in the side pulling air out and one 80mm fan at the back exhausting too and space for 1 more fan at the rear which is the one I want to use for the duct fan.

I also have a 120mm fan inside the case hung under the HDD bay blowing directly across the gfx card towards the rear of the case and another 120mm fan hung under the PSU with an inch gap blowing stright down onto the gfx card too, so all hot air is being blown towards the 80mm fans and exhausted.

Some pics might help.

Originally Posted by Web-Junkie

But I think some of the hot air is getting sucked in by the CPU fan and making it spin faster and gets noisier, so that's the reason to make a CPU duct rather than fork out more money for different CPU cooler!

I doubt the temperature would increase the speed of the fan, considering that the resistance of items that conduct electricity increases as they get hotter. Admittedly your average fan in your average PC isn't likely to encounter temperatures that would affect it by a noticeable amount.

Originally Posted by Web-Junkie

Maybe flawed logic but I like the idea of a duct as it won't be recycling hot air from inside the case!

That does make sense, and as I said, getting cool air into where it's needed is your best bet. Then, depending on the layout of your PC, you might be able to get some more improvement by doing something about how the warm air exits the system.