diff --git a/jstests/big_object1.js b/jstests/big_object1.js
index 6bbe11571cfbc..14d216e609b53 100644
--- a/jstests/big_object1.js
+++ b/jstests/big_object1.js
@@ -4,11 +4,11 @@ t.drop();
 
 if ( db.adminCommand( "buildinfo" ).bits == 64 ){
     
-    s = ""
+    var large = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa";
+    var s = large;
     while ( s.length < 850 * 1024 ){
-        s += "x";
+        s += large;
     }
-    
     x = 0;
     while ( true ){
         n = { _id : x , a : [] }
diff --git a/jstests/mr_bigobject.js b/jstests/mr_bigobject.js
index 4466b8dc73b1f..97195e2542e9b 100644
--- a/jstests/mr_bigobject.js
+++ b/jstests/mr_bigobject.js
@@ -2,9 +2,11 @@
 t = db.mr_bigobject
 t.drop()
 
-s = "";
+// v8 requires large start string, otherwise UTF16
+var large = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa";
+var s = large;
 while ( s.length < ( 6 * 1024 * 1024 ) ){
-    s += "asdasdasd";
+    s += large;
 }
 
 for ( i=0; i<5; i++ )