diff --git a/lib/server.js b/lib/server.js
index 644d6b9ef..0d09afb90 100644
--- a/lib/server.js
+++ b/lib/server.js
@@ -229,7 +229,7 @@ class Server extends EventEmitter {
    * @api private
    */
   async handshake(transportName, req) {
-    const protocol = req._query.EIO === "3" ? 3 : 4; // 4th revision by default
+    const protocol = req._query.EIO === "4" ? 4 : 3; // 3rd revision by default (this makes engine.io truly backward compatible)
     if (protocol === 3 && !this.opts.allowEIO3) {
       debug("unsupported protocol version");
       sendErrorMessage(
diff --git a/lib/transport.js b/lib/transport.js
index 7fb2603a5..461bda2fc 100644
--- a/lib/transport.js
+++ b/lib/transport.js
@@ -22,8 +22,8 @@ class Transport extends EventEmitter {
     super();
     this.readyState = "open";
     this.discarded = false;
-    this.protocol = req._query.EIO === "3" ? 3 : 4; // 4th revision by default
-    this.parser = this.protocol === 3 ? parser_v3 : parser_v4;
+    this.protocol = req._query.EIO === "4" ? 4 : 3; // 3rd revision by default (this makes engine.io truly backward compatible)
+    this.parser = this.protocol === 4 ? parser_v4 : parser_v3;
   }
 
   /**