Doing my fall maintence this day and I pulled the spark plugs to check how
things were burning in my inline 4 jetta powered van.  Easy as pie, btw, on
that motor.
  As is usually  the case they look to be in perfect tune with the mixture
but again "as usual" the gap has grown a lot over about 20k miles of mainly
highway driving.  I put them in gapped at 30 and they were eroded or
whatever out to nearly 40.  What causes that?  and how critical is the gap?
The motor was running fine despite the way over spec gap in every plug.
Re-gapped and re-inserted, I could detect no difference in how the motor
sounds or runs...Would I have more power show on a dyno with the proper
gap?  Better starting?  Cooler running? Less chance of pre-ignition?
  Just curious..
  Don Hanson