diff --git a/crates/specialized/rtx-cfd/tests/embedded3_flag_reference_2d.rs b/crates/specialized/rtx-cfd/tests/embedded3_flag_reference_2d.rs
index 17ae920..d46193a 100644
--- a/crates/specialized/rtx-cfd/tests/embedded3_flag_reference_2d.rs
+++ b/crates/specialized/rtx-cfd/tests/embedded3_flag_reference_2d.rs
@@ -158,12 +158,14 @@ fn median(v: &[f64]) -> f64 {
 #[tokio::test]
 #[ignore = "S2-3c: the 2D reference for the flag wake's kinematics (minutes on the host)"]
 async fn flag_wake_2d_reference() -> CfdResult<()> {
-    let ny = 62;
+    // S2-9c: `RTX_E3_FLAG_NY` (default 62); the step scales with h so the
+    // reference and the slab march the same dt at every rung.
+    let env_f = |k: &str, d: f64| std::env::var(k).ok().and_then(|v| v.parse().ok()).unwrap_or(d);
+    let ny = env_f("RTX_E3_FLAG_NY", 62.0) as usize;
     let h = H / ny as f64;
     let nx = (L / h).round() as usize;
-    let dt = 8.817e-4;
+    let dt = 8.817e-4 * 62.0 / ny as f64;
     // S2-9: the record's period and the requested number of periods.
-    let env_f = |k: &str, d: f64| std::env::var(k).ok().and_then(|v| v.parse().ok()).unwrap_or(d);
     let period = if recorded().is_some() { env_f("RTX_E3_FLAG_KIN_PERIOD", 0.5225) } else { 1.0 / FREQ };
     let t_end = env_f("RTX_E3_FLAG_PERIODS", 2.0) * period;
     let config = CfdConfig::new()