Designated series BN4 with 11,294 built, the "6" in the model's name represents the 6-cylinder engine that replaced the 4-cylinder of the original Big Healey. However, despite the additional cylinders, this engine actually provided less performance. The 100-6 also acquired two small "occasional" seats behind the front buckets, making it a four-seater and thereby giving potential buyers some plausible justification for buying it even if they had small children. However, the additional seating robbed the car of some of its sporting character.
Make no mistake, the 100-6 is a very different car from the 100. Between the reduced performance and the "family" seating, the 100-6 was initially regarded as somewhat of a disappointment. Both of these problems were later addressed (see below), but the 100-6 never fully recovered its reputation and is still today regarded as the least powerful Big Healey.
In 1957, an important engine improvement was introduced. Initially presented to the public and the motoring press as the "Mille Miglia" model—or simply "MM"—after the famous road race in Italy, it provided an important power boost via an improved cylinder head/manifold and fitting of larger carburetors. Once introduced, all 100-6 cars had this improved engine, and a kit was offered to upgrade the earlier production cars that had been sold with the original engine design. The improved engine was introduced at chassis number 48863. Note, however, that the name "Mille Miglia" was used only briefly when the model was introduced, and it never even appeared in factory sales literature. The improved engine is much better known as the "six-port" engine, while the original engine is known as the "integral head" design.
To further address market demands, a strict two-seater was also later introduced, this being the BN6 series car, of which 4,150 were built beginning in 1958. Note that all BN6 Series cars had the improved "six-port" engine. Today many people prefer the lines of this two-seater, and this is reflected in the prices. If you do opt for the four-seat BN4, go for a later one with the improved "six-port" engine.