OK, in the case of the 51515, Beta 2=19, Beta 20=41.
Beta 2 = 50% of all particles in the relevant SAE multipass test, therefore 50% of 19 micron particles are captured.
Hey thanks! As often as I've looked at the Wix web site data, I've never noticed the beta data before.
Guess I need new glasses.
And they have nominal micron ratings too. Way to go Wix!
I've switched a lot of my air, oil and crankcase filters purchases over to Wix brand, and discoveries like this make me more satisfied that I did. Now, if only other filter manufacturers would be this forthright with product information.