Introduction I just wanted to investigate the phenomena of chasing ever larger pumps, and bring in some figures to explore just when "too much pump is bad". First let's start with our challengers, all configured for in-line operation. Eheim 1046, adds around 1.5W of heat to the loop Eheim 1048, adds 3W of heat Eheim 1250, adds 9W of heat MCP600, adds 8W of heat MCP650, adds 15W of heat Iwaki MD-15R @ 60Hz, adds 22W of heat Iwaki MD-20RZ @ 60Hz, adds 31W of heat Iwaki MD-30RZ @ 50Hz, adds 50W of heat Let's assume that we're watercooling an exceptionally hot CPU, that's putting 100W of heat into the water cooling loop under load. Let's assume that we're using a Cascade waterblock, and for our CPU die it has a C/W curve relation that's roughly shaped like this: Our theoretical loop consists of the Cascade block, 2 meters of 1/2" ID tubing, 1/2" barbs, and a Thermochill 120.2 radiator. Against each of the above pumps, we arrive at the following flow charts: For our radiator performance curve, we'll use the following graph, and use the pink line on the graph as befits two moderately powerful (35dBA) fans, being good performance while being fairly loud, but not insanely so. Okay, so that's all the information we basically need to make our predictions. Results For each of the pumps we see that we'll get the following flow rates, and also the corresponding radiator C/W, and waterblock C/W at that flow rate: Eheim 1046, 3.2 LPM, 0.045, 0.187 Eheim 1048, 4.2 LPM, 0.044, 0.178 Eheim 1250, 5.2 LPM, 0.043, 0.172 MCP600, 5.7 LPM, 0.042, 0.169 MCP650, 5.8 LPM, 0.042, 0.168 MD-15R, 6.7 LPM, 0.041, 0.165 MD-20RZ, 8.7 LPM, 0.040, 0.159 MD-30RZ, 10.0 LPM, 0.039, 0.157 Okay, so the CPU heats up by the CPU wattage dissipated by the waterblock's C/W at that flow rate. The water temperature rises above ambient by the radiator's C/W at whatever flow rate multiplied by the CPU wattage plus the pump heat wattage being added to the water. This now gives us a predicted correlation for the relationship between the final CPU temperature as affected by the flow rate, but more importantly also after factoring in the pump heat. Eheim 1046, WB delta = 100W * 0.187C/W = 18.7C, Water delta = (100 + 1.5)W * 0.045C/W = 4.6C, Total CPU temperature = 23.3C above ambient Eheim 1048, WB delta = 17.8C, Water delta = 103 * 0.044 = 4.5C, Total CPU Temp = 22.3C above ambient Eheim 1250, WB delta = 17.2C, Water delta = 109 * 0.043 = 4.7C, Total CPU Temp = 21.9C above ambient MCP600, WB delta = 16.9C, Water delta = 108 * 0.042 = 4.5C, Total CPU Temp = 21.4C above ambient MCP650, WB delta = 16.8C, Water delta = 115 * 0.042 = 4.8C, Total CPU Temp = 21.6C above ambient MD-15R, WB delta = 16.5C, Water delta = 122 * 0.041 = 5.0C, Total CPU Temp = 21.5C above ambient MD-20RZ, WB delta = 15.9C, Water delta = 131 * 0.040 = 5.2C, Total CPU Temp = 21.1C above ambient MD-30RZ, WB delta = 15.7C, Water delta = 150 * 0.039 = 5.9C, Total CPU Temp = 21.6C above ambient Discussion Overall I think the patterns established speak for themselves. All of the high-flow focused pumps lose out significantly due to the wasted strength of their motors in comparison to the flow rates that can be realistically pushed. The high-head, low-moderate flow pumps (MCP600, MD-20RZ) perform the best as the ratio of motor heat to the final corresponding flow rate means that they are operating more efficiently for our proposed water-cooling scenario. If we doubled the radiator effectiveness, which could only be achieved through exceedingly noisy fans, or using two such radiators (and a corresponding amount of fan noise), we get to the situation where the MCP600, 650, and the MD-15R all pretty much still fall on each other, the MD-20RZ pulls a slightly more significant lead, but the MD-30RZ still fails to catch the MD-20RZ while consuming a boat load of power in the process. Of interest is the relative closeness of the Eheim 1048 and the Eheim 1250. If instead we chose a single 120mm radiator the difference between the two drops to 0.2C, or next to nothing, and so the Eheim 1048 would have to get the choice every time out of the Eheims, but the MCP600 still would be the first preference if its small amount of extra noise is not an issue. All of the other pumps become rather unattractive in a single 120mm radiator scenario where even the MD-20RZ should be avoided. The Eheim 1046 is totally behind the knee of the curve in all respects. So to answer: How much pump is enough, and how much pump is too much? I can make the following general recommendations: Single 120mm radiator: First choice: Swiftech MCP600 or AquaXtreme 50Z. Silence Choice: Eheim 1048 Dual 120mm radiator: First choice: Iwaki MD-20RZ, Second choice MCP600. Silence Choice: Eheim 1048 or 1250 Larger radiators: Iwaki MD-20RZ all the way, with the MCP600 as a good second option. I think it could be safe to say that if you didn't know what pump you need, and don't have the dollars to spring for the Iwaki MD-20RZ, then the Swiftech MCP600 (aka AquaXtreme 50Z) would have to be the single safest bet. For the performance freaks, the Iwaki MD-20RZ can't be overlooked. Avoid anything stronger than the 20-RZ though - it is a total waste - your CPU will be hotter - your water will be hotter - and you'll be sucking down more electricity while being worse off - there's no good news here at all. The Eheim 1048 puts in an admirable show, with the Eheim 1250 perhaps giving the best performance for low noise, and the Eheim 1046 should simply be avoided unless you have a specific need for it (low space and low noise). The Iwaki MD-15R holds a bit of a no-mans-land position, being large, powerful and hot, but not really offering anything much to show for its go. Both the MCP650 and MCP600 are better, cooler, and less power hungry choices. The MCP650 fits the bill adequately as a very close runner up to the MCP600 in all scenarios, and is a worthy successor to the MCP600 once expected pump life-span is taken into the equation. Summary & Conclusion Recommended pumps: Performance: Iwaki MD-20RZ Jack-of-all-trades: Swiftech MCP600/AquaXtreme 50Z Silence focused: Eheim 1048 Broad guidelines on pump selection: DO NOT select a pump that draws more than 50W of power. Performance will not improve further and the pump will be drawing more power than is needed, effectively being a waste of electricity which just shows up on your power bill. Pumps drawing much more power than 50W will actually make your CPU hotter. In general avoid pumps that put more than 16W of heat into the water per 120x120mm of radiator area (or equivalent). Match your pump to your available radiator capacity so that pump heat is not a major player in your water's temperature. In general, try to keep pump heat to radiator capacity down below 12W of pump heat per 120x120mm of radiator area. Try to choose a separated armature pump (spinning magnet around the impeller) as opposed to a canned motor (eg. Laing style), or electro-magnet motor (eg. Eheim style) as the separated armature design minimises motor heat transferral into the water Avoid pumps with less than 1.0mH2O of pumping pressure at 3.5LPM flow rate (or 3' of pressure @ 1gpm), unless you have a very specific need for some particular pump (space requirements). Such pumps are too weak and performance will suffer noticably. Try to avoid "high flow" pumps (pumps with >20LPM peak flow rates). Such pumps tend to have the wrong impeller design and flow characteristics for water-cooling use. Choose pumps with at least 6LPM of peak flow rate Choose pumps with at least 1.5mH2O (5') of peak pressure When trying to decide between two pumps where one pump has more peak flow than another pump, then so long as the rated peak flow rates are at or above 10LPM, always choose the pump that has the higher peak pressure rating, over the pump that has the higher peak flow rating. If the peak pressures are about equal, but both offer peak flow rates above 10LPM, choose the pump with the lower peak flow rate as it will add less heat to your system.