diff --git a/AMSS_NCKU_source/Z4c_class.C b/AMSS_NCKU_source/Z4c_class.C
index 6f4cd27..3a53dc0 100644
--- a/AMSS_NCKU_source/Z4c_class.C
+++ b/AMSS_NCKU_source/Z4c_class.C
@@ -485,25 +485,7 @@ void Z4c_class::Step(int lev, int YN)
   }
 #endif
 
-  Parallel::Sync(GH->PatL[lev], SynchList_pre, Symmetry);
-
-#ifdef WithShell
-  if (lev == 0)
-  {
-    clock_t prev_clock, curr_clock;
-    if (myrank == 0)
-      curr_clock = clock();
-    SH->Synch(SynchList_pre, Symmetry);
-    if (myrank == 0)
-    {
-      prev_clock = curr_clock;
-      curr_clock = clock();
-      cout << " Shell stuff synchronization used " 
-           << (double)(curr_clock - prev_clock) / ((double)CLOCKS_PER_SEC) 
-           << " seconds! " << endl;
-    }
-  }
-#endif
+  // CA-RK4: skip post-prediction sync (redundant; ghost cells computable locally)
 
   // for black hole position
   if (BH_num > 0 && lev == GH->levels - 1)
@@ -868,25 +850,28 @@ void Z4c_class::Step(int lev, int YN)
     }
 #endif
 
-    Parallel::Sync(GH->PatL[lev], SynchList_cor, Symmetry);
+    // CA-RK4: only sync after last corrector (iter_count == 3); stages 1 & 2 are redundant
+    if (iter_count == 3) {
+      Parallel::Sync(GH->PatL[lev], SynchList_cor, Symmetry);
 
 #ifdef WithShell
-    if (lev == 0)
-    {
-      clock_t prev_clock, curr_clock;
-      if (myrank == 0)
-        curr_clock = clock();
-      SH->Synch(SynchList_cor, Symmetry);
-      if (myrank == 0)
+      if (lev == 0)
       {
-        prev_clock = curr_clock;
-        curr_clock = clock();
-        cout << " Shell stuff synchronization used " 
-             << (double)(curr_clock - prev_clock) / ((double)CLOCKS_PER_SEC) 
-             << " seconds! " << endl;
+        clock_t prev_clock, curr_clock;
+        if (myrank == 0)
+          curr_clock = clock();
+        SH->Synch(SynchList_cor, Symmetry);
+        if (myrank == 0)
+        {
+          prev_clock = curr_clock;
+          curr_clock = clock();
+          cout << " Shell stuff synchronization used "
+               << (double)(curr_clock - prev_clock) / ((double)CLOCKS_PER_SEC)
+               << " seconds! " << endl;
+        }
       }
-    }
 #endif
+    } // end CA-RK4 guard
     // for black hole position
     if (BH_num > 0 && lev == GH->levels - 1)
     {
@@ -1558,7 +1543,7 @@ void Z4c_class::Step(int lev, int YN)
     }
   }
 
-  Parallel::Sync(GH->PatL[lev], SynchList_pre, Symmetry);
+  // CA-RK4: skip post-prediction MPI ghost sync (redundant; ghost cells computable locally)
 
   if (lev == 0)
   {
@@ -2120,7 +2105,9 @@ void Z4c_class::Step(int lev, int YN)
       }
     }
 
-    Parallel::Sync(GH->PatL[lev], SynchList_cor, Symmetry);
+    // CA-RK4: only MPI sync after last corrector (iter_count == 3); stages 1 & 2 are redundant
+    if (iter_count == 3)
+      Parallel::Sync(GH->PatL[lev], SynchList_cor, Symmetry);
 
     if (lev == 0)
     {
diff --git a/AMSS_NCKU_source/bssnEM_class.C b/AMSS_NCKU_source/bssnEM_class.C
index e06b701..4867600 100644
--- a/AMSS_NCKU_source/bssnEM_class.C
+++ b/AMSS_NCKU_source/bssnEM_class.C
@@ -1221,25 +1221,7 @@ void bssnEM_class::Step(int lev, int YN)
   }
 #endif
 
-  Parallel::Sync(GH->PatL[lev], SynchList_pre, Symmetry);
-
-#ifdef WithShell
-  if (lev == 0)
-  {
-    clock_t prev_clock, curr_clock;
-    if (myrank == 0)
-      curr_clock = clock();
-    SH->Synch(SynchList_pre, Symmetry);
-    if (myrank == 0)
-    {
-      prev_clock = curr_clock;
-      curr_clock = clock();
-      cout << " Shell stuff synchronization used " 
-           << (double)(curr_clock - prev_clock) / ((double)CLOCKS_PER_SEC) 
-           << " seconds! " << endl;
-    }
-  }
-#endif
+  // CA-RK4: skip post-prediction sync (redundant; ghost cells computable locally)
 
   // for black hole position
   if (BH_num > 0 && lev == GH->levels - 1)
@@ -1683,25 +1665,28 @@ void bssnEM_class::Step(int lev, int YN)
     }
 #endif
 
-    Parallel::Sync(GH->PatL[lev], SynchList_cor, Symmetry);
+    // CA-RK4: only sync after last corrector (iter_count == 3); stages 1 & 2 are redundant
+    if (iter_count == 3) {
+      Parallel::Sync(GH->PatL[lev], SynchList_cor, Symmetry);
 
 #ifdef WithShell
-    if (lev == 0)
-    {
-      clock_t prev_clock, curr_clock;
-      if (myrank == 0)
-        curr_clock = clock();
-      SH->Synch(SynchList_cor, Symmetry);
-      if (myrank == 0)
+      if (lev == 0)
       {
-        prev_clock = curr_clock;
-        curr_clock = clock();
-        cout << " Shell stuff synchronization used " 
-             << (double)(curr_clock - prev_clock) / ((double)CLOCKS_PER_SEC) 
-             << " seconds! " << endl;
+        clock_t prev_clock, curr_clock;
+        if (myrank == 0)
+          curr_clock = clock();
+        SH->Synch(SynchList_cor, Symmetry);
+        if (myrank == 0)
+        {
+          prev_clock = curr_clock;
+          curr_clock = clock();
+          cout << " Shell stuff synchronization used "
+               << (double)(curr_clock - prev_clock) / ((double)CLOCKS_PER_SEC)
+               << " seconds! " << endl;
+        }
       }
-    }
 #endif
+    } // end CA-RK4 guard
     // for black hole position
     if (BH_num > 0 && lev == GH->levels - 1)
     {
diff --git a/AMSS_NCKU_source/bssn_class.C b/AMSS_NCKU_source/bssn_class.C
index eb84f8e..09881c6 100644
--- a/AMSS_NCKU_source/bssn_class.C
+++ b/AMSS_NCKU_source/bssn_class.C
@@ -3349,27 +3349,7 @@ void bssn_class::Step(int lev, int YN)
   }
 #endif
 
-  Parallel::AsyncSyncState async_pre;
-  Parallel::Sync_start(GH->PatL[lev], SynchList_pre, Symmetry, sync_cache_pre[lev], async_pre);
-
-#ifdef WithShell
-  if (lev == 0)
-  {
-    clock_t prev_clock, curr_clock;
-    if (myrank == 0)
-      curr_clock = clock();
-    SH->Synch(SynchList_pre, Symmetry);
-    if (myrank == 0)
-    {
-      prev_clock = curr_clock;
-      curr_clock = clock();
-      cout << " Shell stuff synchronization used "
-           << (double)(curr_clock - prev_clock) / ((double)CLOCKS_PER_SEC)
-           << " seconds! " << endl;
-    }
-  }
-#endif
-  Parallel::Sync_finish(sync_cache_pre[lev], async_pre, SynchList_pre, Symmetry);
+  // CA-RK4: skip post-prediction sync (redundant; ghost cells computable locally)
 
 #ifdef WithShell
   // Complete non-blocking error reduction and check
@@ -3709,27 +3689,30 @@ void bssn_class::Step(int lev, int YN)
     }
 #endif
 
-    Parallel::AsyncSyncState async_cor;
-    Parallel::Sync_start(GH->PatL[lev], SynchList_cor, Symmetry, sync_cache_cor[lev], async_cor);
+    // CA-RK4: only sync after last corrector (iter_count == 3); stages 1 & 2 are redundant
+    if (iter_count == 3) {
+      Parallel::AsyncSyncState async_cor;
+      Parallel::Sync_start(GH->PatL[lev], SynchList_cor, Symmetry, sync_cache_cor[lev], async_cor);
 
 #ifdef WithShell
-    if (lev == 0)
-    {
-      clock_t prev_clock, curr_clock;
-      if (myrank == 0)
-        curr_clock = clock();
-      SH->Synch(SynchList_cor, Symmetry);
-      if (myrank == 0)
+      if (lev == 0)
       {
-        prev_clock = curr_clock;
-        curr_clock = clock();
-        cout << " Shell stuff synchronization used "
-             << (double)(curr_clock - prev_clock) / ((double)CLOCKS_PER_SEC)
-             << " seconds! " << endl;
+        clock_t prev_clock, curr_clock;
+        if (myrank == 0)
+          curr_clock = clock();
+        SH->Synch(SynchList_cor, Symmetry);
+        if (myrank == 0)
+        {
+          prev_clock = curr_clock;
+          curr_clock = clock();
+          cout << " Shell stuff synchronization used "
+               << (double)(curr_clock - prev_clock) / ((double)CLOCKS_PER_SEC)
+               << " seconds! " << endl;
+        }
       }
-    }
 #endif
-    Parallel::Sync_finish(sync_cache_cor[lev], async_cor, SynchList_cor, Symmetry);
+      Parallel::Sync_finish(sync_cache_cor[lev], async_cor, SynchList_cor, Symmetry);
+    } // end CA-RK4 guard
 
 #ifdef WithShell
     // Complete non-blocking error reduction and check