Elbow Impacts On Pump Performance

Posted: January 5, 2013 in How To & Misc, Uncategorized
Tags: , , , , ,

Ever since doing pump top testing for the DDC series pumps, there was always some thought that gains from the DDC tops was larger in part (vs D5 tops) due to the sharp and small elbow at the pump inlet in the factory top that is removed.  In addition, past testing of aftermarket tops with alternate inlets also showed some rather large losses when using those alternate elbow inlets.   Which brings me to this fairly simple test of a single DDC pump + top using Bitspower 90 degree elbows and straight barbs as the variable.

While I didn’t request a sponsor specifically for this test, I did use some parts sponsored from a long time ago. The top was provide by XSPC many years ago.  This is the king of DDC pump tops that I tested back here in 2008. I have tested other newer tops and have yet to find one that outperforms it on a like DDC motor.  It has a fairly typical thicker top with a reduced inlet opening so I figured it represents the DDC tops out there fairly well.


Bitspower also sent me the elbows many moons ago when these handy little swivel elbows first came to market.  They have the same quality barb and have a larger 10mm or so.  The internal transition is not quite a smooth radius, but it’s a fairly large diameter which makes a big difference.  These elbows are exceptionally nice and easy to use with the swivel feature.


So, the mission of this test, do modern 90 degree elbows like the larger 10mm ID bistpower swivel 90s create a significant loss on pumps when installed directly on the pump top itself?

We shall see…

Test Setup

The obligatory test setup picture, below is during the straight in and straight out configuration:


I am using:

  • Mastech HY3005D DC power supply to regulate voltage
  • Cen-tech P98674 Digital multimeter to read molex plug voltage
  • Crystalfontz CFA-633 + WinTest B1.9 to monitor and smooth RPM
  • Dwyer 477-5 Digital Manometer to read Pressure Differential across the pump
  • King 7520 (Valved) to measure flow rate and adjust restriction

The large reservoir bleeds out the loop almost instantly, I simply swapped out the barbs for elbows in the other conditions.

Detailed Test Results

Straight In and Straight Out


Elbow In and Straight Out


Straight In & Elbow Out




That’s not quite what I expected.  I had expected the inlet side to be the bigger loss than the outlet AND I expected the losses to be much larger than that.  While you can see upwards of a 30-40% pressure loss at 1.5GPM using aftermarket pump top alternate inlets with their tiny little drilled passageways and close proximity to the impeller, the larger ID bitspower 90 degree elbows and more distant proximity is not affecting pump performance very much at all on this particular pump top.  I think the reduced diameter inlet built into the top probably straightens out the flow pretty well and we are mostly just measuring restriction added.  Some of the DDC tops such as the MCP35X likely have more impact, but there isn’t much showing when the inlet has a step down diameter after the elbow.

In an earlier elbow testing experiment here on blocks, I found this same elbow to have roughly .15PSI loss at 1.5GPM and in this test I’m getting around .1-.2PSI loss depeding on the location, so pretty close.


That data is fairly good for this discussion as well, so for those that like speaking in “Degrees”, adding an elbow to your pump inlet or outlet is about equivalent to a 0.05C temperature loss.  Probably not something to worry about much.

So, that’s that.  While I used to be a skeptic about installation of elbows on pumps, I’m not so worried about it now. At least with your typical DDC top with reduced inlet opening, the larger ID Bitspower elbows do not seem to cause much more than a little restriction which is not going to add up to more than a tenth of a degree and really not worth worrying about.


  1. Systemlord says:

    Great review Martin, do you think the D5’s would mimic your test here of fair worse?

    • Martinm210 says:

      Probably very similar. I have heard before that you should try to get 4x the ID in length between elbow and pump inlet as a rule of thumb. With a 10mm or so ID, the elbow itself has quite a bit of that just in the swivel plus whatever the pup top thickness is. I don’t current have any recent D5 tops other than bay reservoirs to test that, but pretty comfortable this is similar for common D5 tops as well.

  2. Acurax says:

    Nice testing. I am uncertain what you mean by this line: ” Some of the DDC tops such as the MCP35X likely have more impact…” Not sure what to take away from that.

    • Martinm210 says:

      The XSPC top I tested with has a reducing inlet nozzle that takes the usual 10mm or so ID and gets it down to a size smaller than the impeller. It has a thicker top as well so there is a little more space between impeller and elbow. Between the extra space and reduced nozzle, this combo probably has less of an elbow impact than a 35x top which has no reduction in ID nor the extra space. That is just a guess though. The rule of thumb for industrial pumps is to retain 4x the pipe diameter in space between elbow and pump, so I am going on that rule and their slight differences.

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  4. GIBSON says:

    Hi Martin,
    I have been enjoying your articles/tests for the last few days. the testing you have been doing gives me a lot of input on what to expect as pressure drop vs flow with several components.

    Anyhow, I’d like to point you at one significant difference between what you have tested here and with the factory DDC top. As you have fitted here a 90° elbow on top of the installed top you are adding a certain length between rotor and elbow which allows the flow to realign itself. On the factory top the distance between the 90° bend the fluid has to make is so small that I suspect the fluid is hitting the rotor under an angle which impacts performance. (As you most probably know the incoming and outgoing angle of the fluid is of utmost importance for a turbopumps performance. Hence why you would also see large differences in performance on significantly differrent rpm’s than the nominal rpm – the relative in and outgoing angles between flow direction of the fluid and the rotors blades is not aligned at that moment.)


    • Martinm210 says:

      Good clarification. I know in industrial applications I have heard 4x ID for inlet to elbow spacing as a rule of thumb which supports that. Also most of these tops have smaller ID at the actual impeller opening so I am sure that has some benefit to straightening flow as well.

      • GIBSON says:

        Regarding straightening of the flow, sometimes fins are added in the housing on the inlet side (these remain stationary) so they prevent the fluid from having any pre-rotation. Maybe something that could be tried 🙂 (e.g. inserting a razorblade). No idea if it would have any effect. Might be an idea to do some finite element calculations to see how the flow behaves in a DDC pump.
        If I’m not mistaken the impeller opening will be more related to the other dimensions of the rotor as it needs to support the mass rate continuity (basically needing to have a continuous total cross section from inlet to outlet – though sometimes some diffusion might be built into the rotor by having a little more cross section at outlet than inlet)