Thank you all for the kind words. The burgers tasted great.
Here is the recipe for the special sauce. (Thanks to Cooks Illistrated)
1/2 Cup mayonnaise
1/4 Cup ketchup
2 Teaspoons sweet pickle relish
2 Teaspoons sugar
2 Teaspoons white vinegar
1 Teaspoons ground black pepper
Makes about 1 cup.