diff --git a/firmware/config/boards/f407-discovery/config.mk b/firmware/config/boards/f407-discovery/config.mk
deleted file mode 100644
index 11f7c985d8..0000000000
--- a/firmware/config/boards/f407-discovery/config.mk
+++ /dev/null
@@ -1 +0,0 @@
-PROJECT_CPU = ARCH_STM32F4
\ No newline at end of file
diff --git a/firmware/rusefi.mk b/firmware/rusefi.mk
index 9bab68fa9d..399c2ebe0e 100644
--- a/firmware/rusefi.mk
+++ b/firmware/rusefi.mk
@@ -19,7 +19,11 @@ ifeq ($(PROJECT_BOARD),)
   PROJECT_BOARD = f407-discovery
 endif
 
-# minus before 'include' makes this directive optional, we will be good without config.mk file
+ifeq ($(PROJECT_CPU),)
+  # many boards all the way to Proteus use this F4 default
+  PROJECT_CPU = ARCH_STM32F4
+endif
+
 -include $(PROJECT_DIR)/config/boards/$(PROJECT_BOARD)/config.mk
 
 # CPU-dependent defs