Pastic or Metal water pump impeller?

[Liquidsunshine]

I know it's a knee jerk reaction to think the plastic water pump impeller is cheap, and to get the good one, get the metal one. I did it myself, and the parts counter person says that 90% of sales are of the metal one.

As soon as I compared the new metal one with the OEM composite impeller, I was sorry that I insisted on metal. The OEM water pump is not plastic, it's a resin composite. The reason that they use a resin composite rather than "Chevy" sytle metal is that the impeller is very intricate and has very thin sections. This can't be done in metal. This isn't a large, old-fashioned water pump. This is a high performance BMW water pump. The impeller is quite small and has very delicate details.

I put the metal one in my engine, because I didn't have any other easy choice, and I'm sure it will be just fine, I won't be racing or whatever, but I regret it. The metal impeller looked kinda like a blob when compared with the composite impeller. I'm sure it will work just fine, but it won't be as efficient or move as much water as the composite version.

Use the OEM syle composite impeller. It's better.

I've attached photos for your own judgement.

[MirrorrorriM]

I really don't think it matters either way. Both are going to accomplish the same task, by the looks of it.

[atlantisvip]

Metal won't come apart like your composite will. High performance BMW water pumps .. :lmao

[Moosecakes]

The benefit of the metal impeller is that it will not grenade inside your motor and obliterate things. That was the first thing I did when I got my old e46 was pull the cooling system out, only to find cracks all over the composite impeller and a piece ready to come off. Metal impeller went in the next day. I'm not gonna risk that happening.

Speaking of.. need to order a Stewart performance pump for my car soon

[winkosmosis]

They look like the same design except the plastic one is thinner

[islandtime]

In a race engine you actually try to slow the water pump down. They try to move too much water and cavitate at higher RPM's. Remember the thermostat allows far less water through it than the water pump moves, and a constant flow is all you need. When the water moves too fast it doesn't remove as much heat so even the metal impeller will move far more water than needed to keep the motor cool.

[Liquidsunshine]

Yes, I have heard the argument of the composite impeller coming apart. I think that could be prevented by just replacing it every 185,000 miles whether it needs it or not. That's how many miles mine had with no signs of cracks. Also, I can agree that it might be a bad idea to install an off-brand or cheap aftermarket composite water pump.

After spending some time with BMW technology, I think I'll stop questioning them. I bought the $300 piston rings from BMW after I bought the $150 aftermarket rings. The BMW rings were REALLY different. Surprisingly supple. Surprisingly tighter machining tolerances. Surprisingly easier to slide the pistons with them on. I don't know how they do it. There are many odd things that BMW does differently than other manufacturers. Odd details like two passages through the muffler for different engine loads, the VANOS system, etc. I am convinced that they don't do it because it's the cheapest way. There may be readers here who know more than I, but I am going to stop saying I know more than the engineers at BMW. ... and parts stores are going to continue to sell metal impeller water pumps 90% of the time...

[r33p04s]

metal or plastic who cares...if it's a quality replacement it most likely will last longer than you need it to unless you plan on going another 150k miles...don't buy cheap oem manufacturer or better

[atlantisvip]

No doubt OEM is best but don't forget the "tech" in these cars is about 20 years old. Not exactly cutting edge. So believe it that some replacement parts are better and made a certain way for a reason. Just curious, how old are you?

[PpRecuero]

The two cents I can drop here are these:
Composite resin WILL (and so help me, it WILL) become exhausted and break into smithereens, causing some major problems inside the engine. read: "been there, seen that".
The resin is not as resistant to thermal change as metal is, specifically because the metal one is aluminum with can't remember what else, making it expand and contract very comfortably within its housing.

So, tl;dr, get the metal one. It's the same urban legend as the VW/Audi metal vs. composite plastic oil pump. When it breaks, it's so macho it will fudge things up around.