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Originally posted by 8-Ball
If you reread my post, though it is possible I didn't make the point clear enough, there is no waterblock in the situation I have described.
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Oh no, I got that clearly. That's why I said what I did. In the WC system you don't get multiple chances to cool the water. You only get one. That's an important difference.
Please correct my impression if it is wrong, but I get the impression that you think the radiator doesn't need to cool as much because the water temperature is lower. This, I'm afraid, is completely wrong. Yes, the water is cooler, but the rad has to cool more water for a given time period than with a slower flow rate. So the radiator really does have more energy to transfer. It must, as that is the only way to reduce the temperature of the CPU.
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Ask yourself, this, how is it that increasing the efficiency of both the waterblock and the radiator would result in an increase in the cpu temp, as this seems to be what you're implying.8-ball
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I'm glad you asked!
In a typical system, we have water, at a given temperature, going into the waterblock. The water is heated to a higher temperature and then goes to the radiator. There, it is cooled back to the starting temperature and the cycle begins again.
When we increase flow, the starting water temperature is assumed to stay the same. This means that the increase to waterblock performance is based solely on increased flow. The performance of the radiator, however, will be affected by two factors. The fundamentals of heat transfer tell us that the water exiting the waterblock is now at a lower temperature. Since we are now starting with a lower temperature for the radiator, the performance is affected by increased flow, *and* decreased temperature. Since a temperature decrease will reduce performance, the benefit from increased flow will be countered. Yes, the radiator can now move heat faster, but the waterblock's performance increase is still bigger. The radiator will get more heat than its performance increase can account for. The bottom line is that the radiator is going to have to work harder.
Furthermore, the decrease in radiator temperature means that the radiator/fan relationship is not quite as good as it used to be. This fact exacts another toll on the radiator. The result from these additional influences is that the radiator is walloped with lots of extra work.
So, what happens to CPU temperature? If you have a good radiator that can rise to the challenge, and cool water down to the point it did before, then you will have a lower CPU temperature.
However, if the radiator can't rise to the challenge and can't remove the heat, then it will leave the heat in the water. This will undo the flow benefit, and temperatures will rise to the point where there is once again balance between the heat taken from the CPU and the heat expelled by the radiator. This balance will occur at a higher CPU temperature.
So we see that it all depends on the radiator. Anything you can do to help the radiator do its job better, will help you realize the benefit of increased flow. Pushing more air through, or changing the rad outright, may be required. If you had a good radiator to begin with, and you weren't using all of the radiator's cooling capacity, then increasing flow will drop your CPU temperature by putting that extra capacity to work.