BMW E36 – Stewart Warner Water Pump vs OEM Style Welded Steel Impeller

One of the biggest known issues with BMW E36 chassis cars is the fragility of certain critical cooling system components. The main OEM components that have known weaknesses (even in non-racing applications) are the plastic thermostat housing, radiator with plastic end tanks, plastic clutch driven cooling fan and water pump with plastic impeller. Notice a trend here? Plastic is not the most suitable material to use for these components as the constant cycling from ambient temperature to hot temperature over the years can lead to material failure. BMW engineering apparently didn’t intend for these to last the life of the car but instead to be periodically replaced. I’m going to focus on a couple ways that you can address the OEM plastic impeller water pump replacement.

Plastic Impeller Failure

There are a few popular replacements for the OEM BMW plastic impeller water pump:

  1. Updated BMW OEM “composite” impeller water pump. It looks the same as the plastic impeller but supposedly is fiber reinforced.
  2. OEM style water pump with welded stainless steel impeller
  3. Stewart Warner water pump with stainless steel impeller

I’m not sold on the “composite” impeller as it seems like it could still fatigue due to the heat cycling. The main benefits are that it has lower interia due to the impeller being light weight, the OEM style bearing supports this lower weight well, and it’s relatively low cost.

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Let’s focus first on the OEM style water pump with the welded stainless steel impeller. You can see in the picture above that the vanes of the impeller are welded onto the rotor of the water pump. This style impeller should hold up well over time and completely eliminate the chances of disintegration. The negative would be that the added weight of the steel impeller will potentially tax the OEM style bearing over time. Obviously, it’s all about the quality of the bearing used in the water pump then. I haven’t performed all the research on the various brands of metal impeller water pumps available, but I’m sure there’s a range for bearing quality. If you go this route then I suggest you look into this further.

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Here’s another view of the welded stainless steel impeller. This water pump was pulled from my S52 with an unknown amount of miles but I’d guess at least 20,000+ miles. It still looks new to me. Bearing seemed fine.

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That brings us to the Stewart Warner water pump. They describe themselves on their website as the following:

“Stewart Components is an industry leader in high performance automotive cooling system research and development. For the past 25+ years, Stewart water pumps have been the #1 choice for professional engine builders, race teams, and performance enthusiasts, from street rodding and OEM replacement applications, to the most extreme forms of motorsports in the world. The development and production of high performance water pumps has resulted in innovations like special high strength castings, larger flow passages, more efficient impellers, and stronger bearings.”

The Stewart Warner water pump is generally viewed as the best water pump available for the BMW E36. Stewart Warner says that it uses a “heavy duty” bearing, stainless steel impeller and the flow of the pump is 20% greater than the OEM style pump. All of this and they claim that it’s twice as efficient as the OEM water pump (much less parasitic drag). You can see in the flow chart provided by Stewart Warner in the picture above. The water pump also carries a lifetime warranty.

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Here’s a view of the Stewart Warner stainless steel impeller. It looks to be cast and then goes through finishing machining.

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And a view of the underside of the impeller.

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Here’s a picture showing the OEM style water pump with welded stainless steel impeller versus the Stewart Warner water pump. In this picture you can clearly see the stainless steel shroud on the Stewart Warner water pump. This looks to help decrease the rotational inertia of the impeller (by reducing the size needed) while also increasing the efficiency of the pump.

img_2682 Here’s a view of the side of the water pump where the pulley will mount. You can see that the main housing uses a different casting from the OEM style (to incorporate the heavy duty bearing) but all the mounting holes are identical.

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You can see that the coolant flow hole in the Stewart Warner water pump shroud lines up with the main coolant passage that flows into the cylinder head.

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This picture shows the Stewart Warner water pump fully installed (with the pulley on the front as well). It’s a very tight fit and I had to slowly tighten the nuts on the water pump studs in order to “pull” the pump into place. Don’t beat on the shaft to get it seated!

You can see that I chose to use the Stewart Warner pump for my S52 engine. I think it should hold up well to the racing and will serve me for a long time!

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Stewart Warner BMW E36 Water Pump

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