I just remembered I still have the original low-mileage pump for the 73 455 in my FB. So I checked it out to see how Pontiac meant it to work. Unfortunately, I don't have a digital camera. The impeller is nicely cast with 8 well-shaped, properly designed vanes. It is 4.4" OD. The vanes are 0.70" high above back plate at the inlet and 0.50" high at the OD. The inlet tips of the vanes have exactly the same diameter as the inlet hole in the plate, which is 2.4" so the impeller eye matches the hole perfectly. The vane contour exactly matches the plate, i.e. the gap between the plate and the vanes is 0.06" at the inlet and it stayed 0.06" (I checked with clay) right out to the impeller OD. The impeller has 3 balancing holes in the back plate to balance thrust and reduce load on the bearings.
I then checked a new aftermarket pump I have with a stamped impeller. Impeller eye was bigger at 2.85”. OD was same. Vanes were same height above back plate. Vane contour matched plate well, but gap was a bit high at 0.13” The thing that really struck me was the inefficiency of the large cutouts in the impeller back plate (see photo in this post) which allow water to shortcut back to the inlet behind the back plate. The back plate to pump housing gap was 0.09”. To me, this explains why stamped steel impellers don’t work well.
The cast pump impeller in the second of Old Goat’s photos looks crude by comparison to the Pontiac one and also does not seem to match the plate contour very well. That large cutout in the plate will not help either as it allows water to spill back to suction. The ‘73 455 plate did not have that.
Malcolm
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