diff --git a/crates/specialized/rtx-fsi/tests/fsi2_harness/overset_march.rs b/crates/specialized/rtx-fsi/tests/fsi2_harness/overset_march.rs
index d7b4bbf..5c5f6a8 100644
--- a/crates/specialized/rtx-fsi/tests/fsi2_harness/overset_march.rs
+++ b/crates/specialized/rtx-fsi/tests/fsi2_harness/overset_march.rs
@@ -91,10 +91,25 @@ pub struct OversetMarchResult {
 }
 
 pub fn run_march_overset(case: BenchmarkCase, config: &OversetMarchConfig) -> OversetMarchResult {
-    let cfg = config.clone();
+    let mut cfg = config.clone();
     let mut fluid = OversetFluid::build_case(case, cfg.ny, cfg.flag_nx, cfg.sweeps, cfg.max_rounds)
         .expect("overset fluid");
     let dt_fluid = fluid.dt_fluid;
+    // R4-d (2026-09-21): the acceleration-scaled floor. The loop converges
+    // the displacement; the fluid feels its second difference over dt², so
+    // a fixed floor's admissible acceleration noise quadruples per rung
+    // (R4: the FSI3 force spikes). `RTX_FSI2O_TOL_FLOOR_DT2=<floor at the
+    // ny 62 fluid step 3.44e-4>` sets floor = value · (dt_fluid / 3.44e-4)².
+    if let Some(f0) = std::env::var("RTX_FSI2O_TOL_FLOOR_DT2")
+        .ok()
+        .and_then(|v| v.parse::<f64>().ok())
+    {
+        cfg.tol_floor = f0 * (dt_fluid / 3.44e-4).powi(2);
+        println!(
+            "  floor scaled with dt² (R4-d): {:.3e} at dt {:.3e} (from {f0:.1e} at 3.44e-4)",
+            cfg.tol_floor, dt_fluid
+        );
+    }
     let dt = dt_fluid * cfg.subcycle as f64;
     let zero_d = vec![0.0; 2 * fluid.interface.wetted.len()];
     // Every setting the acceptance rule reads, printed once: P5-3 lost a