The following information relates to the nature of this thread in that it is about water pump performance. But it also relates to some other recent threads about the cooling system:
https://xwebforums.com/forum/index.php?threads/thermostat-housing-and-coolant-passages.37706/
https://xwebforums.com/forum/index.php?threads/engine-bay-cooling-options.37681/
After some "blueprinting" of my coolant passages, T-stat housing, head/block, water pump, etc, I took a closer look at the water pump's impeller fitment in the housing (body). I'm referring to the gap between the distal end of the impeller and the tapered surface at the pump body's inlet. There is a specification for this gap in the factory manual and it has been discussed as an important aspect of pump performance. But frankly I had not addressed it until now.
On the AC style pump that I am working on I noticed the gap seemed very large. Like 7 or 8 mm too large! This is a new pump from a well recognized maker and the pump housing (body) is also relatively new (less than a couple hundred miles), and using a stock gasket. After checking everything over I put the pump in the hydraulic press to move the impeller further out (closer to the body). It needed to be moved a lot more than I ever would have imagined. But as it got closer to the specification range I noticed something else. The gap is not consistent around the circumference of the impeller. It could be within spec on one side but over double or even triple that on the other side. In addition I noticed some irregularities in the cast iron housing (AC style pump). Mainly large casting pores and excess flash, but also a little erosion from rust. However the largest deficiency was in the actual shape of the casting; the angled surface that the impeller rides next to wasn't level or consistently even around the full circular surface. Yet another issue I also discovered is with the impeller itself. The 'blades' on it are inconsistent in shape/size. This allows some parts of the impeller to be closer to the housing than other parts. So the gap changes as the pump is rotated.
I used some marking blue (same as used for port matching the head) and adjusted the impeller on the pump to where it just rubbed the housing. Then rotated the impeller to find the high spots. Sanded (ground) them down with a 36 grit Roloc 2" disc and die grinder. Repeated the process several more times until the impeller no longer rubbed, then set the impeller a little closer and did it all again a couple times. Eventually I was able to get the full circumference of the impeller within the specification range for the gap. Although it still isn't perfect; it is toward the high end of the range on one side and the low end on the other.
I also did a little touch up work on the impeller blades to make them more consistent. But I did not want to remove any more material here than absolutely necessary. So most of the work was done to the housing.
I had the thought that maybe this pump was a "bad" one. So I checked two others. They were basically the same. I also checked another fairly new pump housing and found the same irregularities in it. So I've come to the conclusion this is typical. However it may not be obvious in most cases. If you only looked at the gap where it is easily visible (through the window on the housing where it passes to the block), then you would never know that other areas of the impeller are still way out of spec. Therefore your pump may appear to be in spec when it really isn't. I still have a non-AC style pump to do the same checking on, and will be curious to see if any of this is different with the aluminum cast housing and small impeller on that style.
Someone else (sorry I do not recall who it was) suggested that the main reason why some X's have lots of cooling issues while others don't is due to the impeller gap being in or out of spec. After seeing just how bad things can be I tend to agree this may be a major issue for some.
This also may help to justify the use of a electric water pump as suggested in one of the referenced threads. If the stock pump housings are this poorly cast, and the standard replacement pumps have impellers this far out, then going to a completely different type of pump may be a good answer. Especially when you consider that my 'customized' water pump fitting/grinding will have to be completely redone if a new pump is installed in the same housing.