diff --git a/src/main/scala/chiseltest/simulator/VerilatorSimulator.scala b/src/main/scala/chiseltest/simulator/VerilatorSimulator.scala
index 3a200305..90fb5fe7 100644
--- a/src/main/scala/chiseltest/simulator/VerilatorSimulator.scala
+++ b/src/main/scala/chiseltest/simulator/VerilatorSimulator.scala
@@ -229,12 +229,11 @@ private object VerilatorSimulator extends Simulator {
   }
 
   private def DefaultCFlags(topName: String) = List(
-    "-O1",
+    "-Os",
     "-DVL_USER_STOP",
     "-DVL_USER_FATAL",
     "-DVL_USER_FINISH", // this is required because we ant to overwrite the vl_finish function!
     s"-DTOP_TYPE=V$topName",
-    s"-include V$topName.h"
   )
 
   private def DefaultFlags(topName: String, verilatedDir: os.Path, cFlags: Seq[String], ldFlags: Seq[String]) = List(