diff --git a/src/collision/Manager.js b/src/collision/Manager.js
index 8ee4e91..e996ba9 100644
--- a/src/collision/Manager.js
+++ b/src/collision/Manager.js
@@ -16,7 +16,7 @@ var Manager = {};
      * @param {object} pairs
      * @param {collision[]} collisions
      */
-    Manager.updatePairs = function(pairs, collisions) {
+    Manager.updatePairs = function(pairs, collisions, timestamp) {
         var pairsList = pairs.list,
             pairsTable = pairs.table,
             collisionStart = pairs.collisionStart,
@@ -53,10 +53,10 @@ var Manager = {};
                     }
 
                     // update the pair
-                    Pair.update(pair, collision);
+                    Pair.update(pair, collision, timestamp);
                 } else {
                     // pair did not exist, create a new pair
-                    pair = Pair.create(collision);
+                    pair = Pair.create(collision, timestamp);
                     pairsTable[pairId] = pair;
 
                     // push the new pair
@@ -70,7 +70,7 @@ var Manager = {};
         for (i = 0; i < pairsList.length; i++) {
             pair = pairsList[i];
             if (pair.isActive && activePairIds.indexOf(pair.id) === -1) {
-                Pair.setActive(pair, false);
+                Pair.setActive(pair, false, timestamp);
                 collisionEnd.push(pair);
             }
         }
@@ -81,10 +81,9 @@ var Manager = {};
      * @method removeOldPairs
      * @param {object} pairs
      */
-    Manager.removeOldPairs = function(pairs) {
+    Manager.removeOldPairs = function(pairs, timestamp) {
         var pairsList = pairs.list,
             pairsTable = pairs.table,
-            timeNow = Common.now(),
             indexesToRemove = [],
             pair,
             collision,
@@ -97,12 +96,12 @@ var Manager = {};
             
             // never remove sleeping pairs
             if (collision.bodyA.isSleeping || collision.bodyB.isSleeping) {
-                pair.timeUpdated = timeNow;
+                pair.timeUpdated = timestamp;
                 continue;
             }
 
             // if pair is inactive for too long, mark it to be removed
-            if (timeNow - pair.timeUpdated > _pairMaxIdleLife) {
+            if (timestamp - pair.timeUpdated > _pairMaxIdleLife) {
                 indexesToRemove.push(i);
             }
         }
diff --git a/src/collision/Pair.js b/src/collision/Pair.js
index a8ad105..0e089be 100644
--- a/src/collision/Pair.js
+++ b/src/collision/Pair.js
@@ -14,10 +14,9 @@ var Pair = {};
      * @param {collision} collision
      * @return {pair} A new pair
      */
-    Pair.create = function(collision) {
+    Pair.create = function(collision, timestamp) {
         var bodyA = collision.bodyA,
-            bodyB = collision.bodyB,
-            timestamp = Common.now();
+            bodyB = collision.bodyB;
 
         var pair = {
             id: Pair.id(bodyA, bodyB),
@@ -35,7 +34,7 @@ var Pair = {};
             slop: Math.max(bodyA.slop, bodyB.slop)
         };
 
-        Pair.update(pair, collision);
+        Pair.update(pair, collision, timestamp);
 
         return pair;
     };
@@ -46,12 +45,13 @@ var Pair = {};
      * @param {pair} pair
      * @param {collision} collision
      */
-    Pair.update = function(pair, collision) {
+    Pair.update = function(pair, collision, timestamp) {
         var contacts = pair.contacts,
             supports = collision.supports,
-            activeContacts = [];
+            activeContacts = pair.activeContacts;
         
         pair.collision = collision;
+        activeContacts.length = 0;
         
         if (collision.collided) {
             for (var i = 0; i < supports.length; i++) {
@@ -65,11 +65,11 @@ var Pair = {};
                 }
             }
 
-            pair.activeContacts = activeContacts;
             pair.separation = collision.depth;
-            Pair.setActive(pair, true);
+            Pair.setActive(pair, true, timestamp);
         } else {
-            Pair.setActive(pair, false);
+            if (pair.isActive === true)
+                Pair.setActive(pair, false, timestamp);
         }
     };
     
@@ -79,13 +79,13 @@ var Pair = {};
      * @param {pair} pair
      * @param {bool} isActive
      */
-    Pair.setActive = function(pair, isActive) {
+    Pair.setActive = function(pair, isActive, timestamp) {
         if (isActive) {
             pair.isActive = true;
-            pair.timeUpdated = Common.now();
+            pair.timeUpdated = timestamp;
         } else {
             pair.isActive = false;
-            pair.activeContacts = [];
+            pair.activeContacts.length = 0;
         }
     };
 
diff --git a/src/core/Engine.js b/src/core/Engine.js
index a30b62d..7a0190e 100644
--- a/src/core/Engine.js
+++ b/src/core/Engine.js
@@ -106,6 +106,9 @@ var Engine = {};
             if (!engine.enabled)
                 return;
 
+            // timestamp is undefined on the first update
+            timestamp = timestamp || 0;
+
             // create an event object
             var event = {
                 timestamp: timestamp
@@ -264,9 +267,10 @@ var Engine = {};
         var collisions = broadphase.detector(broadphasePairs, engine.metrics);
 
         // update pairs
-        var pairs = engine.pairs;
-        Manager.updatePairs(pairs, collisions);
-        Manager.removeOldPairs(pairs);
+        var pairs = engine.pairs,
+            timestamp = engine.timing.timestamp;
+        Manager.updatePairs(pairs, collisions, timestamp);
+        Manager.removeOldPairs(pairs, timestamp);
 
         // wake up bodies involved in collisions
         if (engine.enableSleeping)