I've mentioned this somewhere before, but I find it amazing so here I go again. It concerns the 1983-84 Washington Capitals:
The Caps in 1984 had a fine season -- 101 points, fifth overall, just three points behind the defending champ Islanders -- but what's amazing is their defensive game. They were 1st in the NHL on defence, with 226 goals allowed. That was nearly 30 goals fewer (!) than 2nd best Buffalo, and 164 goals fewer (!!) than pathetic Pittsburgh (hello, Mario).
But what's most amazing is the Caps in the second half of 1983-84. So, the season started with them going an utterly disastrous 0-7-0. After game six, they pulled off the trade that landed them Larry Murphy. Once Murphy joined, they won 7 out of 8 games (getting stomped by Edmonton once for the only loss) and were back in the hunt.
Even so, up to game 42, the Caps were only 19-20-3.
Amazingly, they then went 29-7-2 the rest of the way, the League's #1 record over that time. (The Caps were actually the NHL's best team over the final 45 games of the regular season.)
Anyway, the Caps' team defence in the second half was out-of-this-world good. Remember, this was a season when Edmonton scored 446 goals and four different clubs allowed around 375 to nearly 400 (Pittsburgh). In the final 40 games, the Caps allowed only 84 goals against (and in one of those games they lost in just-revived overtime, so really it's 83 goals against per all comparables for decades prior). This is a great stat for any team in any (non-DPE) era, but it's staggering for 1983-84. The second-best defensive club over this period (Buffalo) allowed 113 goals against, nearly 30 more in only 40 games (!).
Over this latter 40 games, the Caps' PK was at nearly 90%, which must have been a big part of their success (their "net" PK was almost 92%). On average, they were allowing only 23 shots against, per game (for comparison, Pittsburgh allowed over 37 per game).
Over this super-40-games-of-defence, every Cap skater who played 5 or more games was a "plus" at evens, with Dave Christian topping them all with a +28 in just forty games.
So, looking at other 1980s'-era clubs with strong defensive records, can any team beat the Caps' 84 goals against (or 83, by pre-1983-84 standards) over the first- or second-half of any season?
1980-81 Canadiens
First 40 games = 109 GA
Last 40 games = 119 GA
1981-82 Canadiens
First 40 games = 120 GA
Last 40 games = 103 GA
1982-83 Bruins
First 40 games = 117 GA
Last 40 games = 111 GA
1982-83 Islanders
First 40 games = 124 GA
Last 40 games = 102 GA
1986-87 Flyers
First 40 games = 107 GA
Last 40 games = 138 GA
1988-89 Canadiens
First 40 games = 122 GA
Last 40 games = 96 GA
1988-89 Flames
First 40 games = 114 GA
Last 40 games = 112 GA
Those are just some that I thought to check, but nobody even comes close to the Caps' back-half of 1983-84.
Of course, it's nice if you can throw out Langway, Stevens, and Murphy all night...