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  • RB26 N1 Water Pump Rumours, Theories, and Lies - Demystified

    Posted by andre perras on


    It's commonly thought by many that more expensive parts are superior to less expensive parts.  We see this all the time in the high performance industry.  Lots of people think that if they spend more money on an oil pump, or a water pump that they'll get better results.  This originates in our industry in businesses pushing their sales to increase their bottom lines and profits.  "You need this" is something we've all heard.  But has anyone ever taken the time to justify that claim?

    This mentality has lead to a gross misunderstanding of what's actually better for you, the end-user of said parts.  Often times, you'll hear consumers argue what the best part for X application is, bragging about how much they spent, preaching "Do it once, do it right, bro" or about how they "Didn't want to take any chances" 

    Today, we set the records straight in regards to the N1 Water Pump - which is extremely misunderstood as being a superior water pump.  In reality, it's not - it's just made for a very specific application, that 95% of the people buying them aren't taking advantage of.  It's actually a downgrade if you aren't using it as it was intended. 


    Let's go over the technical and visual differences in the two pumps;


    OEM Water Pump (RB20, 25, 26)

    Part Number: 21010-21U26

    Price: ~ $100 USD / $135 CAD



    N1 Water Pump (RB20, 25, 26)

    Part Number: 21010-24U27

    Price: ~ $235 USD / $315 CAD


    Comparing them, you can see there are not many significant differences.  When looking at the outside/front of the water pumps, there are no visible differences.


    The Slotted Hole

    Some people believe that the slotted M6 hole was evidence of an N1 water pump.  While all N1 Water pumps certainly do have the slotted hole, many OEM water pumps also have the slotted hole.  In fact, it's what we see most commonly now. 

    Originally, Nissan manufactured quite a few variations of the RB water pump.  This was to accommodate for different castings in the blocks - as there were many variations of the RB over the span of a decade in time.  If you were to try bolting a water pump without the slotted hole to an N1 block, you wouldn't be able to get that bolt threaded into the hole - however a slotted hole water pump will fit on any RB.

    We have encountered instances where the edge of the slotted hole was actually not supported by a machined surface behind the water pump, which resulted in coolant leaking through the slotted hole.  There really isn't much you can do in this case, other than get a water pump without a slotted hole, or build a small sealing plate to cover the slot.  There doesn't appear to be any rhyme or reason to whether or not the slotted hole water pump will seal or not, but we can say that you have about a 98% chance of never having any issues.


    The Impeller and Backing Plate

    You surely noticed the N1 water pump has 6 blades on the impeller, and that the OEM water pump has 8.  Have you ever wondered why?  Why on earth would the almighty N1 water pump have LESS blades than the plain old OEM water pump?

    But - Did you notice the plate behind the N1 Pump's impeller?  That's an anti-cavitation plate.  (We'll get into cavitation in depth below) 

    We did - and we have researched the issue fundamentally, technically, and through real world experience & testing.

    The answer lies in hydrodynamics;




    • 1. the branch of science concerned with forces acting on or exerted by fluids (especially liquids).


    Why did Nissan reduce the amount of blades on the N1 pump?

    In order to understand why Nissan chose to reduce the amount of blades in the believed to be superior pump, we need to understand it's intended application, and the issues with the OEM pump design.

    The N1 pump was designed to be used in the N1 R32 Skyline GT-R, which competed in various forms of competitive racing.

    Group N and A racing is nothing like your every-day drive to work.  The engines in these cars would typically operate in the 4000-8000 RPM range, and never really drop below unless mechanical faults, pit stops, or special circumstances occurred.

    Do you ALWAYS drive your car in the 4000-8000 RPM rev range?  Probably not, unless you're competing in racing. 

    That is the only circumstance under which you will benefit from the N1 Water pump.  If you're a daily driver and weekend warrior, you may benefit from the N1 water pump, but most likely your daily driving will suffer.  Many will argue they've got one and never had a problem, which is totally possible.  But have they noticed any benefits?  Unlikely.

    If you've ever watched a movie that featured submarines, you surely have seen the bubbles that follow the torpedoes, as well as the propeller behind the submarine.  Ever wonder why or even how bubbles somehow manifested themselves under water?  It's not like there's air down there, right?

    The answer is cavitation.

    1. the formation of an empty space within a solid object or body.
      • the formation of bubbles in a liquid, typically by the movement of a propeller through it.

    How does cavitation occur?  We'll use the submarine's propeller as an example as many of you can relate and understand it fundamentally.  When a propeller moves through a fluid (water, coolant, etc...) it displaces the fluid (pumping).  This displacement of water causes very rapid changes in pressure - at the inlet, the pressure drops as it is pulled into the propeller, and at the outlet - the pressure rises.  This difference in pressure results in the phenomenon we know as cavitation. Technically speaking, the fluids actually evaporating due to the drop in pressures.  A reduction in pressure will actually reduce the temperature at which the coolant will boil, resulting in evaporation.  This is one of the reasons why cooling systems are pressurized.  Pressurized coolant will have a much higher boiling point than if it were at atmospheric pressure. 

    Evaporation is simply a phase change, much like how ice melts, and water becomes a gas when boiled.  You can actually evaporate water at room temperature under a strong enough vacuum.  Pressure changes the temperature at which fluids will evaporate as well as condense.

    The evaporation of the water (or coolant, in our case) through the process of cavitation results in "bubbles" of evaporated fluid.  These bubbles then suddenly implode (or collapse) on themselves, and erode the surfaces on which they are near.  The damage appears similar to that of media blasting.  Below is an example of cavitation damage on a propeller.  You can see how this is not something you want occurring inside your engine.


    You can witness cavitation in many every-day things. Any pump with an inlet restriction will likely cause cavitation, or an impeller spinning to quickly for the fluid's dynamics and application for which it was designed.


    So, how does the N1 pump prevent cavitation? 

    By reducing the amount of blades on the pump as well as integrating a backing-plate behind the impeller, Nissan was able to reduce the pressure differentials across the inlet side and the outlet sides of the impeller.  Taking two of the blades away actually reduces the flow of the water pump - assuming all other variables are constant (Speed, temperature, fluid type, etc...).



    So, simply said, the N1 water pump actually flows LESS under the exact same circumstances as an OEM water pump.  It won't cavitate and destroy your block and water pump, though.

    So - knowing all of this now, does it really make sense to spend 3X the money on a cooling system downgrade, to prevent a phenomenon that you'll likely never have a problem with?


    Feel free to like, comment, and share this article on social media platforms. 

    Let's clean up the misinformation on the internet.  Power through education, knowledge, and social responsibility.



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