diff --git a/firmware/controllers/algo/engine_state.h b/firmware/controllers/algo/engine_state.h
index aa6af6ceac..13082dacdd 100644
--- a/firmware/controllers/algo/engine_state.h
+++ b/firmware/controllers/algo/engine_state.h
@@ -55,6 +55,9 @@ public:
 	angle_t mapAveragingStart[INJECTION_PIN_COUNT];
 	angle_t mapAveragingDuration = 0;
 
+	/**
+	 * timing advance is angle distance before Top Dead Center (TDP), i.e. "10 degree timing advance" means "happens 10 degrees before TDC"
+	 */
 	angle_t timingAdvance = 0;
 	// fuel-related;
 	float fuelCutoffCorrection = 0;