Better Kirby Suction
My info corresponds with Rick Asquith, that the 515 had a more powerful motor and better suction. I don't have one handy, however, so I can't look at the serial number plate to check the amperage but it SEEMS to me it's 4 amps.
One of the biggest jumps in performance, however, was when the 516 came out. It had the same motor as the 515, but a big improvement was made in how the rug nozzle and suction coupler attaches to the machine. One the earlier ones, the suction hole was just plain metal, wire-brushed when new to make a smooth surface. However, no matter how smooth it was, there was always some suction loss. With the 516, a rubber gasket was added on the suction hole which ensured an airtight seal and, thus, much better suction.
I =believe= the same motor was used through the first Sanitronic and not "beefed up" until the Dual 50 but I may be wrong there. I will say, they all SOUND the same to me.
Just a final point about the early 500s - especially the 505 - those machines have a very musical, gentle sound. I have always liked the sound of those earlier Kirbys even though they do not perform quite as well. In my 505 in particular, the motor "sings" with two high-pitched tones that give it a very distinctive sound, especially when using the hose and attachments.
