Don't confuse taking off at an intersection with taking off at the end.
The three minutes applies to an intersection takeoff only. The rule
for taking off behind a heavy is two minutes after he starts his takeoff
roll, it has nothing to do with where or when he gets airborne. There
is no delay for you taking off behind a DC9. A controller may also use
radar separation instead of time.

Peter R. wrote:

A couple of days ago I flew into Logan Airport (Boston, MA, USA) in a C172
for an Angel Flight. Taxing to the departing runway, we were behind a B767
and a DC9 (in that order), with several other large airliners behind us.

I noticed that there seemed to be no wake turbulence delay for the DC9
behind the B767, as he was cleared for TO less than a minute after the B767
departed.

Tower then positioned me on the runway, and again, less than a minute later
(after awaiting a crossing runway landing), gave me a 90 degree right turn
after takeoff heading, cautioned wake turbulence, then cleared me to go.

My question has to do with the ATC's wake turbulence procedures. At the
class C airport where I am based, I constantly hear about the 3 minute rule
from ATC. In other words, if I am departing from an intersection mid-
field, tower will say that they are required to make me wait three minutes
for wake turbulence avoidance (unless I wave it, which I normally do not).

In the case of Boston's tower, did her "wake turbulence caution" and/or
right turn heading allow her to clear me sooner than the three minutes?

BTW, the DC9 ahead of me took at least three quarters of the runway to lift
off, then turned left. When I departed, I dropped a notch of flaps to
lift off very quickly, climbed a few hundred at Vx as per the obstacle DP,
then turned the 90 degrees right as per the instruction to be well away
from the previous two aircrafts' wake turbulence.