Big or Small water pump impeller and housing?

[Stewart Mayer]

Hi,

On one of our 7.5hp Evinrudes from the 50s, I think it is a 7520, the water pump I found on it has a smaller impeller than should be there.  Everything fits fine, but can you recommend which one to use?  I’ve got an extra normal-size impeller housing, so either solution would work, but I’m not sure of the pros/cons of the smaller over original larger impeller.  Someone made that change in the motor’s previous life, and I’m not sure if there was a good reason for it or not. Thanks!

 

 

[RICHARD A. WHITE]

The smaller one is the upgraded pump and housing….
Either will work…

http://www.richardsoutboardtools.com
classicomctools@gmail.com

1 user thanked author for this post.

Stewart Mayer

[Don]

Well either is fine, but you must ensure you are using all the correct parts, nothing in common between the two pumps.  Look up the individual part numbers for the plastic housing/smaller impeller under a 1979 6hp.   Needless to say, the old aluminum housings got worn easily, and are expensive to replace now.

1 user thanked author for this post.

Stewart Mayer

[JACQUES]

it was my understanding that the  newer ones were more efficient..     true  false   ?   who knows …. was it just economics (profit%) or a mechanical advantage ?   who knows… ?    but if does the job  go for it   and tell us how it worked out.

 

here is a perspective from an AI source  🙂

🚤 1. Impeller Size & Efficiency — What It Really Means

Impeller diameter, pitch, and housing geometry affect how much water the pump moves through the engine’s cooling system at a given RPM.

• A larger impeller generally moves more water at lower engine speeds → more cooling flow at idle/low speed.
• A smaller impeller moves less water per revolution → potentially less cooling flow at low RPM and slightly less engine drag.

“Efficiency” depends on what you’re trying to optimize:<br data-start=”719″ data-end=”722″ />• Fuel efficiency / less drag → smaller impeller might help marginally.<br data-start=”799″ data-end=”802″ />• Better cooling at low speeds / heavy use in hot water → larger impeller is usually better.

So, smaller doesn’t inherently mean more efficient in terms of cooling — it just changes the flow characteristics.

<hr data-start=”1022″ data-end=”1025″ />

🛠 2. Why Someone Might Use a Smaller Impeller / Different Housing

Here are the common reasons you see this happen:

✅ a) Different Operating Needs

If the original setup was designed for:

• heavy load towing,
• cold water,
• slow cruising

…then a large impeller was chosen to ensure lots of cooling water.<br data-start=”1349″ data-end=”1352″ />Someone who uses the engine differently (e.g., fast cruising, in cooler water) might elect a smaller impeller.

<hr data-start=”1464″ data-end=”1467″ />

✅ b) Improved High-RPM Performance / Reduced Parasitic Drag

A smaller impeller usually:

• produces less resistance in the water pump
• reduces load on the engine’s accessory drive
• can slightly improve top-end RPM and fuel economy

This feels like “more efficient” because the engine is easier to turn at speed, not because it’s cooling better.

<hr data-start=”1826″ data-end=”1829″ />

✅ c) Housing Compatibility

Sometimes the replacement impeller is installed with a different housing because:

• the original housing was worn/damaged
• the replacement housing has different flow geometry
• the new housing fits a different aftermarket impeller

Small changes in housing shape can change the pressure and volume characteristics, which may improve or worsen cooling performance.

<hr data-start=”2239″ data-end=”2242″ />

✅ d) Parts Availability

Old Evinrude engines often have scarce OEM parts — people substitute housings with more easily found modern parts that require a different impeller size.

<hr data-start=”2431″ data-end=”2434″ />

⚠️ 3. Is This Safe for Your Engine?

Cooling is critical — if the impeller/housing combo doesn’t move enough water at the RPMs you operate in, the engine can overheat quickly.

Signs you don’t have enough cooling flow:

• thermostat opening late
• engine runs hot at idle/low rpm
• white steam or overheating alarms

Signs you might have too much resistance (from too large an impeller):

• harder to get full RPM
• high fuel burn for RPM achieved

<hr data-start=”2902″ data-end=”2905″ />

📌 Summary

Aspect	Larger Impeller	Smaller Impeller
Water flow @ low RPM	High	Lower
Engine drag	More	Less
Cooling safety margin	Higher	Lower
High-speed drag	More	Less

👉 Smaller impeller ≠ inherently more efficient — it changes the balance between pump load and water flow.

 

Joining AOMCI has priviledges 🙂

1 user thanked author for this post.

Stewart Mayer

[mike n]

i believe the newer style impeller is taller,so it might not be that much smaller. i installed one on a 5.5 and didnt put too many miles on it but didn’t see any problems

mn

1 user thanked author for this post.

Stewart Mayer

[Don]

mike n wrote:

i believe the newer style impeller is taller,so it might not be that much smaller. i installed one on a 5.5 and didnt put too many miles on it but didn’t see any problems

I would agree.  The taller height of the new style impeller compensates for the lack of diameter.   Keep in mind, the impeller only tells half the story here.  The wear of the older aluminum housings is often over looked/missed when replacing impellers.  The SS liners on the newer style pumps seem to wear less than the aluminum housings.  I would not be concerned about possible lower water pump output when replacing the old style set up with the new style.

I used to use a relatively simple test when servicing older motors with the aluminum pump housings while running in a test tank (not a barrel).   Just let the engine run in neutral or reverse at high idle for several minutes.  A worn pump would usually start reducing pump output slowly after a few minutes, but output would return to normal once back in forward gear taking advantage of water being passed by and directed into the pump inlets.

The one short coming of the plastic housings shows up in salt water though.  Salt seems to make its way between the plastic and the SS sleeves for the housing screws, splitting the plastic open around the sleeves.  You can’t use a torch to loosen stuck screws without igniting the nasty plastic as well.  But, there is no excuse for not cleaning and lubing impeller hsg retaining screws properly during service either.

So, in the end, I would not have a problem using either style set up on an older engine.  Just stick with OEM parts/kits,  I wouldn’t trust most amazon/chinese aftermarket “stuff”.

[Author]

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