diff --git a/src/hydro/GizmoMFM/hydro_iact.h b/src/hydro/GizmoMFM/hydro_iact.h
index 5bed20d7f894a76d5fe3642c7438dc03195e43d6..2fc80ccb56fd86c5ac4445d9fb9dc94b6e6a6ca6 100644
--- a/src/hydro/GizmoMFM/hydro_iact.h
+++ b/src/hydro/GizmoMFM/hydro_iact.h
@@ -267,8 +267,7 @@ __attribute__((always_inline)) INLINE static void runner_iact_fluxes_common(
   const float dvdotdx = min(dvdr, 0.0f);
 
   /* Get the signal velocity */
-  /* the magical factor 3 also appears in Gadget2 */
-  vmax -= 3.0f * dvdotdx * r_inv;
+  vmax -= 2.f * const_viscosity_beta_over_alpha * dvdotdx * r_inv;
 
   /* Store the signal velocity */
   pi->timestepvars.vmax = max(pi->timestepvars.vmax, vmax);
diff --git a/src/hydro/GizmoMFV/hydro_iact.h b/src/hydro/GizmoMFV/hydro_iact.h
index c766ce3cc9048f8da8b3438c3c27e6998dd5df7e..cf5be8d4ec2cf7f6efa741710abf441d0b1adcf7 100644
--- a/src/hydro/GizmoMFV/hydro_iact.h
+++ b/src/hydro/GizmoMFV/hydro_iact.h
@@ -271,8 +271,7 @@ __attribute__((always_inline)) INLINE static void runner_iact_fluxes_common(
   dvdotdx = min(dvdotdx, 0.f);
 
   /* Get the signal velocity */
-  /* the magical factor 3 also appears in Gadget2 */
-  vmax -= 3.f * dvdotdx * r_inv;
+  vmax -= 2.f * const_viscosity_beta_over_alpha * dvdotdx * r_inv;
 
   /* Store the signal velocity */
   pi->timestepvars.vmax = max(pi->timestepvars.vmax, vmax);