diff --git a/src/collision/Collision.js b/src/collision/Collision.js
index 8d21b32..1f24db9 100644
--- a/src/collision/Collision.js
+++ b/src/collision/Collision.js
@@ -152,8 +152,10 @@ var Pair = require('./Pair');
             supports[supportCount++] = supportsB[0];
         }
 
-        // update supports array size
+        // update supports array size if changed
+        if (supports.length !== supportCount) {
             supports.length = supportCount;
+        }
 
         return collision;
     };
diff --git a/src/collision/Detector.js b/src/collision/Detector.js
index a4626e2..c6059b0 100644
--- a/src/collision/Detector.js
+++ b/src/collision/Detector.js
@@ -22,6 +22,7 @@ var Collision = require('./Collision');
     Detector.create = function(options) {
         var defaults = {
             bodies: [],
+            collisions: [],
             pairs: null
         };
 
@@ -45,6 +46,7 @@ var Collision = require('./Collision');
      */
     Detector.clear = function(detector) {
         detector.bodies = [];
+        detector.collisions = [];
     };
 
     /**
@@ -57,12 +59,13 @@ var Collision = require('./Collision');
      * @return {collision[]} collisions
      */
     Detector.collisions = function(detector) {
-        var collisions = [],
-            pairs = detector.pairs,
+        var pairs = detector.pairs,
             bodies = detector.bodies,
             bodiesLength = bodies.length,
             canCollide = Detector.canCollide,
             collides = Collision.collides,
+            collisions = detector.collisions,
+            collisionIndex = 0,
             i,
             j;
 
@@ -104,7 +107,7 @@ var Collision = require('./Collision');
                     var collision = collides(bodyA, bodyB, pairs);
 
                     if (collision) {
-                        collisions.push(collision);
+                        collisions[collisionIndex++] = collision;
                     }
                 } else {
                     var partsAStart = partsALength > 1 ? 1 : 0,
@@ -126,7 +129,7 @@ var Collision = require('./Collision');
                             var collision = collides(partA, partB, pairs);
 
                             if (collision) {
-                                collisions.push(collision);
+                                collisions[collisionIndex++] = collision;
                             }
                         }
                     }
@@ -134,6 +137,10 @@ var Collision = require('./Collision');
             }
         }
 
+        if (collisions.length !== collisionIndex) {
+            collisions.length = collisionIndex;
+        }
+
         return collisions;
     };
 
@@ -180,6 +187,13 @@ var Collision = require('./Collision');
      * @default []
      */
 
+    /**
+     * The array of `Matter.Collision` found in the last call to `Detector.collisions` on this detector.
+     * @property collisions
+     * @type collision[]
+     * @default []
+     */
+
     /**
      * Optional. A `Matter.Pairs` object from which previous collision objects may be reused. Intended for internal `Matter.Engine` usage.
      * @property pairs
diff --git a/src/collision/Pair.js b/src/collision/Pair.js
index 8f7ed81..b4ae5bb 100644
--- a/src/collision/Pair.js
+++ b/src/collision/Pair.js
@@ -74,19 +74,27 @@ var Contact = require('./Contact');
         pair.slop = parentA.slop > parentB.slop ? parentA.slop : parentB.slop;
 
         collision.pair = pair;
-        activeContacts.length = 0;
+
+        var activeContactIndex = 0,
+            supportsLength = supports.length;
         
-        for (var i = 0; i < supports.length; i++) {
+        for (var i = 0; i < supportsLength; i++) {
             var support = supports[i],
                 contactId = support.body === parentA ? support.index : parentAVerticesLength + support.index,
                 contact = contacts[contactId];
 
             if (contact) {
-                activeContacts.push(contact);
+                activeContacts[activeContactIndex++] = contact;
             } else {
-                activeContacts.push(contacts[contactId] = Contact.create(support));
+                contact = Contact.create(support);
+                activeContacts[activeContactIndex++] = contact;
+                contacts[contactId] = contact;
             }
         }
+
+        if (activeContacts.length !== activeContactIndex) {
+            activeContacts.length = activeContactIndex;
+        }
     };
     
     /**
diff --git a/src/collision/Pairs.js b/src/collision/Pairs.js
index 2c62cd3..bb223b5 100644
--- a/src/collision/Pairs.js
+++ b/src/collision/Pairs.js
@@ -39,21 +39,19 @@ var Common = require('../core/Common');
     Pairs.update = function(pairs, collisions, timestamp) {
         var pairsList = pairs.list,
             pairsListLength = pairsList.length,
+            pairsListIndex = pairsListLength,
             pairsTable = pairs.table,
             collisionsLength = collisions.length,
             collisionStart = pairs.collisionStart,
             collisionEnd = pairs.collisionEnd,
             collisionActive = pairs.collisionActive,
+            collisionStartIndex = 0,
+            collisionEndIndex = 0,
+            collisionActiveIndex = 0,
             collision,
-            pairIndex,
             pair,
             i;
 
-        // clear collision state arrays, but maintain old reference
-        collisionStart.length = 0;
-        collisionEnd.length = 0;
-        collisionActive.length = 0;
-
         for (i = 0; i < pairsListLength; i++) {
             pairsList[i].confirmedActive = false;
         }
@@ -66,10 +64,10 @@ var Common = require('../core/Common');
                 // pair already exists (but may or may not be active)
                 if (pair.isActive) {
                     // pair exists and is active
-                    collisionActive.push(pair);
+                    collisionActive[collisionActiveIndex++] = pair;
                 } else {
                     // pair exists but was inactive, so a collision has just started again
-                    collisionStart.push(pair);
+                    collisionStart[collisionStartIndex++] = pair;
                 }
 
                 // update the pair
@@ -80,14 +78,14 @@ var Common = require('../core/Common');
                 pair = Pair.create(collision, timestamp);
                 pairsTable[pair.id] = pair;
 
-                // push the new pair
-                collisionStart.push(pair);
-                pairsList.push(pair);
+                // add the new pair
+                collisionStart[collisionStartIndex++] = pair;
+                pairsList[pairsListIndex++] = pair;
             }
         }
 
         // find pairs that are no longer active
-        var removePairIndex = [];
+        pairsListIndex = 0;
         pairsListLength = pairsList.length;
 
         for (i = 0; i < pairsListLength; i++) {
@@ -95,20 +93,32 @@ var Common = require('../core/Common');
             
             if (!pair.confirmedActive) {
                 Pair.setActive(pair, false, timestamp);
-                collisionEnd.push(pair);
+                collisionEnd[collisionEndIndex++] = pair;
 
+                // remove inactive pairs
                 if (!pair.collision.bodyA.isSleeping && !pair.collision.bodyB.isSleeping) {
-                    removePairIndex.push(i);
+                    delete pairsTable[pair.id];
                 }
+            } else {
+                pairsList[pairsListIndex++] = pair;
             }
         }
 
-        // remove inactive pairs
-        for (i = 0; i < removePairIndex.length; i++) {
-            pairIndex = removePairIndex[i] - i;
-            pair = pairsList[pairIndex];
-            pairsList.splice(pairIndex, 1);
-            delete pairsTable[pair.id];
+        // update array lengths if changed
+        if (pairsList.length !== pairsListIndex) {
+            pairsList.length = pairsListIndex;
+        }
+
+        if (collisionStart.length !== collisionStartIndex) {
+            collisionStart.length = collisionStartIndex;
+        }
+
+        if (collisionEnd.length !== collisionEndIndex) {
+            collisionEnd.length = collisionEndIndex;
+        }
+
+        if (collisionActive.length !== collisionActiveIndex) {
+            collisionActive.length = collisionActiveIndex;
         }
     };
 
diff --git a/src/core/Engine.js b/src/core/Engine.js
index 91b55ac..cdf03c4 100644
--- a/src/core/Engine.js
+++ b/src/core/Engine.js
@@ -60,6 +60,7 @@ var Body = require('../body/Body');
         engine.world = options.world || Composite.create({ label: 'World' });
         engine.pairs = options.pairs || Pairs.create();
         engine.detector = options.detector || Detector.create();
+        engine.detector.pairs = engine.pairs;
 
         // for temporary back compatibility only
         engine.grid = { buckets: [] };
@@ -138,7 +139,6 @@ var Body = require('../body/Body');
         Constraint.postSolveAll(allBodies);
 
         // find all collisions
-        detector.pairs = engine.pairs;
         var collisions = Detector.collisions(detector);
 
         // update collision pairs