The Results

Since I did not have test equipment to test the pumps at actual motor driven RPMs (idle through 6,000 RPM), I had to settle for low RPM tests and extrapolate out to the higher RPMs. The engine pulley and the pump pulley are the same size (they look the same, I didn’t actually measure them) so the pump RPM is the same as the engine RPM.

Since I could not achieve actual performance data, I decided to test only one of the two pump outlets for convenience. I think that if both outlets were testes simultaneously, the flow would not be double, but only about 30% more.

The results indicate relative (not absolute) performance of the pumps. See test data below.


Broken Stamped Impeller

This impeller, even though missing a blade and being mangled, performed surprisingly well (even though it was way out of balance). This means to me that one could have a badly damaged impeller with bent blades, and the pump would still work and one would never even know until a blade came off due to cyclic stressing.


Replacement Stamped Impeller

This produced the highest flows and pressures. However, highest flows and pressures are not necessarily what we want. We want to keep the pressures moderate so that we don’t burst the hoses or have the pump fail. Note that during dead head testing, the pressure was erratic and bouncing between 6 and 10 PSI. (The other pumps did not do this). This is suggesting to me that something like cavitation might be going on during dead heading. I suspect that this is what is causing the stamped pumps to fail.


3.66” Cast Impeller

This pump had the lowest pressure at only slightly reduced flow (even before I beveled the housing inlet). The idle flow was the lowest of the group (as expected), but not as low as I thought it would be. I settled on this pump and also beveled the inlet for better water flow. I did not retest it after beveling the inlet.


4.365” Cast Impeller

The tight tip clearance of this pump turned me off. Any small deviation could potentially cause it to bind up. It had good performance, better than the stamped impeller as far as pressure and flow. It was the best looking of (except that the hub appears to be out of round, but did not cause any noticeable vibration).


Other Information

I decided to install bypass anyway, to a keep the dead head pressures down. Renegade is starting to market a bypass system and I decided to try it. It is very simple. You remove the regular thermostat and install an external Wahler bypass thermostat, connect the discharge to the filler pot (as usual) and run the bypass back to the pump inlet and make the connection with a Tee fitting (pictures later). The Wahler thermostat is for a BMW 2002 or 320i. (W0133-1627641 80 deg C / 176 deg F, W0133-1627306 75 deg C / 167 deg F, W0133-1622197 71 def C / 160 deg F)


Cast Iron Pump Sources

Slits says that he has 30 cores (3.66 diameter cast iron) that can be rebuilt and sold for $55 plus shipping. This is a very good deal. Although I could find many kinds of stamped and cast impeller pumps on the internet, I could find no information about impeller diameters or any other specific dimensions. And, the designs from the various manufacturers are quite different. You would need to use trial and error to find what could fit into your housing. Judging from the pictures that I saw, the marine big block Chrysler pumps seem to have the smaller diameter. The cast iron impeller pumps that I saw ran in the $125 to $200 range. The stamped impeller pumps ran in the $30 to $90 range. My recommendation – go with SLITS.

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