An extremely rare Blower Bentley will cross the auction block at a Bonhams sale scheduled for Aug. 16 in Carmel, Calif. Experts agree on its rarity, but differ on how pristine an example this machine truly is.

In the 94-year history of Bentley Motors, perhaps none of its creations is held in more esteem than the Blower Bentley.

The more powerful supercharged version of the 4.5-liter, 4-cylinder engine was the idea of Tim Birkin, who along with Dudley Benjafield and Woolf Barnato, comprised the original “Bentley Boys” team of racers. The Blower was fast, but it consumed profligate amounts of fuel and was not particularly reliable. Ettore Bugatti derided the Blower Bentley as “the world’s fastest truck.”...