1956 - 1959
Series BN4 and BN6
The 100-6 - spoken as "Hundred-Six" - was a very different car from the model that it replaced. Designated Series BN4 with 11,294 built, the "6" in the model's name represents the 6-cylinder engine that replaced the 4-cylinder engine 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 changed the character of the car and robbed it 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.
Be that as it may, in 1957 there was an important engine improvement 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 - it was not an option - and a kit was even 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.
Upside: The beautiful clean lines of Gerry Coker's original design were definitely altered, but it still "works" resulting in a tremendously handsome car; this model is generally available for less money than any other big Healey - especially the four-seat Series BN4 - often representing a very good value.
Downside: Weather protection is still lacking; creature comforts are still few; early models with the lower-power engine have the least performance of any big Healey.
Bottom line: A reasonable choice for a first big Healey, delivering all of the qualities of the marque while generally requiring the least money to become a big Healey owner, but if performance is important to you avoid the early examples with the low-power engine.