Spark plug gapping

Hi, I have a 1976 Vette, 4 speed. Looking to change the spark plugs, but my Vette does'nt have the sticker on it indicating the gap, nor the timing.

Thanks stngray1

Timing varies year to year.  Probably around 8 degrees, but I don't have the specs handy.  You may want to look back at some past threads to set timing.  Original spec was best for emissions, not necessarly performance or economy.

The plug gap on all of the early HEI systems is .045

 76 L48  6degreesBTDC ,L-82 12 degrees BTDC ,   both engines take R44TX plugs gapped at .060 according to my Chiltons manual

Indeed, many HEI systems did use a .060 gap, and some even used .080 gap (several  Olds 307 motors).

GM learned after the fact that the gap at 80 was way too big, and was causing problems with arcing through out the ignition system.  Most of the time it would burn through the rotor.  To correct this problem, they changed the spec to .060.  This helped quite a bit.

Shortly after they also found ignition life could further be improve with less gap.  So some of the .060 systems was now recommended at .045.   Granted to do this you needed to use a different plug.  You did not want to just close up the gap on the existion plug.  This would bend the ground electrode, and it was no longer flat to the center of the plug.  This made performance suffer insted of improving the system.  There were several TSBs from GM about this.  Of course the original specs did not show it.

The advantage of the larger gap is that it creates a hotter spark.  This is not the same thing as heat range.  The disadvantage besides stressing the ignition system, it also created a shorter duration spark. Sometimes the hotter spark is preffered, usually with a rich condition.  The leaner (not lean) mix likes a longer duration, which you get with a smaller gap, and a bit cooler spark.

The flip side to that is at high cylinder pressures and rpm, the longer duration from the smaller gap is preffered.  MSD creates a longer duration spark by pulsing the spark, thus streaching it out for a longer time.

'morning -

I'm in the process of replacing my plugs with ACs - R45TS and with a .045 gap. Have not rechecked the timing yet, but plan to set at 8 BTDC and go from there. If this fails - will post an update of what worked for me.

Not sure how long existing (Bosch) plugs where in there, but they looked to have a gap of .045 and were burning fine. I don't know what number they were, but will check if you want.

Good Luck

(ps: I also have a 76 L48 4 Speed)

[QUOTE=Ron 78]Hey PUMPS,,could you down size your picture a little?it fills my whole screen and I can't read what you say [/QUOTE]

Done!