Water pumps for the LA and Magnum series engines

[Kern Dog]

I stumbled upon a discovery the other day regarding the water pumps used on the 1970 and later 318-340-360 engines.
I was assembling the timing cover and water pump on a 1990 360 I'm rebuilding and when the shaft was spun, I heard some scratchy noises which encouraged me to replace the pump. Mine looked like this:



Note the large holes around the snout.



Note the size of the impeller and 6 blade count.

[Kern Dog]

If the rotation of the shaft made no noise, I would have gone ahead and used it. By outward appearances, it seems like a quality pump. It had a Chrysler Pentastar cast into it so it is a Mopar part. This 360 I'm rebuilding blew two head gaskets and I don't know what caused that to happen.
My friend is here rebuilding a 2001 MP 360/380 crate engine. He is reusing his water pump:



At a glance, it looks the same but it has none of the big holes in the casting around the snout. The impeller is bigger too.



Mine on the left, his on the right:



Normally I don't measure my stuff against another man but hey.....
His is 8 blade and 4 3/8", mine is 6 blade and 3 1/2". I'd read about how there was a standard duty water pump and a heavier duty one that was used. I found the article last night in an issue of Mopar Action magazine:





Rick Ehrenberg from Mopar Action has strong opinions on the matter...

[Kern Dog]

I bought another pump yesterday but they had to bring it up from their warehouse so I got it today.



No holes around the snout, the fan and pulley flange is bigger but the impeller is still small:



The fact that this engine blew the head gaskets on both sides and 2 cylinders each had me wondering what led to the failure. Could it be the pump itself? I'm not sure. It does turn but makes noise. It isn't sloppy or loose but it doesn't bind up either, it just makes a scraping noise. Some say that the fact that the LA series engines only have 4 bolts around each bore means that gasket failures are more likely than in a big block.
I wondered about the water pumps used on the revised LA engines when they evolved into the "Magnum" 5.2 and 5.9 designation. Here is an image of a Magnum water pump courtesy of Rock Auto:



The bolt pattern looks the same but the hub is huge! These were driven by a serpentine belt and are reverse rotation. Given that these were never used in cars but instead trucks, vans and SUVs, I'd expect it to be more robust and move more water.

[70 sublime]

I always thought the water pumps were different with the A/C motors vs non A/C stuff because they used a different size pulley on the pump to make the fan blades spin faster or not to draw more air through the rad and that is why the pump had a bigger or smaller blade inside to keep the water moving the same amount ??

[John_Kunkel]

   Some aftermarket pump makers use the smaller impeller which doesn't move enough coolant when using non-AC pulleys.

[Kern Dog]

There is debate about coolant flow and how it matters.
Some say it needs to be slower to allow the coolant to stay in the radiator long enough to shed the heat but while the coolant does linger longer in the radiator, there is coolant staying in the engine longer setting hotter.
The water pump I bought does have a slightly bigger impeller than the old pump....3 3/4" compared to 3 1/2". The blades are slightly larger too.
O Rielly had no listing difference for A/C or Non A/C.
I have pumps that look like this new one in 2 other cars that I have here. They don't overheat but I'm not out stuck in traffic with them as a torture test.

[John_Kunkel]

The purpose of using the 6-blade impeller on A/C cars that drive the pump faster (40% faster than non-A/C) is cavitation...it's thought that the 8-blade impeller spinning that fast leads to the formation of small vapor-filled bubbles that form shock waves that disturb the coolant flow. 

As an inventory-saving practice, many pump makers offer only the 6-blade of varying blade design. Again, pulley choice determines coolant flow.

[Kern Dog]

That makes sense.