when you use a good pump, is better to use in parallel, so every vga get's the same cooling, in serie you will get more resistance in loop of all the vga blocks behind eachother, and temp will be different between then, even when it's small difference.
When you use a triple sli cooling bidge, it's in parallel.
A cpu block need some more flow then a vga block.
When you put vga's in series, the flow can be to slow for cpu block and in that way temp will be not so good on cpu.
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