A spark plug, comprising: an insulator extending along a center axis and presenting an insulator outer surface extending from an insulator upper end to an insulator nose end; said insulator including an insulator body region extending between said insulator upper end and said insulator nose end; said insulator presenting a first radius (R 1) at said insulator body region extending from said center axis to said insulator outer surface; said insulator including an insulator nose region between said insulator body region and said insulator nose end; said insulator presenting a sixth radius (R 6) at said insulator nose region extending from said center axis to said insulator outer surface, said sixth radius (R 6) being less than said first radius (R 1); said insulator including an insulator seat disposed between said insulator body region and said insulator nose region, said insulator seat extending radially toward said center axis at an insulator seat angle ( α i); said insulator including a first transition extending from said insulator body region to said insulator seat, said first transition being convex; said insulator presenting a fifth radius (R 5) at said first transition, said fifth radius (R 5) being a spherical radius at said first transition; said insulator presenting a second transition extending from said insulator seat to said insulator nose region, said second transition being concave, characterized by said insulator presenting a second radius (R 2) extending from said center axis to a point at the intersection of said insulator outer surface of said insulator seat and said insulator outer surface of said insulator nose region adjacent said second transition; said insulator presenting a fourth radius (R 4) at said second transition, said fourth radius (R 4) being a spherical radius at said second transition; said insulator seat angle ( α i) being measured relative to a plane extending perpendicular to the center axis and intersecting the insulator seat, the insulator seat angle ( α i) being from 35° to 50°; and said insulator seat angle ( α i) being greater than or equal to a boundary value provided by the equation: 90°- acos[1- ( R 1 - R 2) ÷ ( R 4 + R 5)]. an insulator extending along a center axis and presenting an insulator outer surface extending from an insulator upper end to an insulator nose end; said insulator including an insulator body region extending between said insulator upper end and said insulator nose end; said insulator presenting a first radius (R 1) at said insulator body region extending from said center axis to said insulator outer surface; said insulator including an insulator nose region between said insulator body region and said insulator nose end; said insulator presenting a sixth radius (R 6) at said insulator nose region extending from said center axis to said insulator outer surface, said sixth radius (R 6) being less than said first radius (R 1); said insulator including an insulator seat disposed between said insulator body region and said insulator nose region, said insulator seat extending radially toward said center axis at an insulator seat angle ( α i); said insulator including a first transition extending from said insulator body region to said insulator seat, said first transition being convex; said insulator presenting a fifth radius (R 5) at said first transition, said fifth radius (R 5) being a spherical radius at said first transition; said insulator presenting a second transition extending from said insulator seat to said insulator nose region, said second transition being concave, characterized by said insulator presenting a second radius (R 2) extending from said center axis to a point at the intersection of said insulator outer surface of said insulator seat and said insulator outer surface of said insulator nose region adjacent said second transition; said insulator presenting a fourth radius (R 4) at said second transition, said fourth radius (R 4) being a spherical radius at said second transition; said insulator seat angle ( α i) being measured relative to a plane extending perpendicular to the center axis and intersecting the insulator seat, the insulator seat angle ( α i) being from 35° to 50°; and said insulator seat angle ( α i) being greater than or equal to a boundary value provided by the equation: 90°- acos[1- ( R 1 - R 2) ÷ ( R 4 + R 5)].