diff --git a/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/cutwall.rs b/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/cutwall.rs
index 4713db4..33674e9 100644
--- a/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/cutwall.rs
+++ b/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/cutwall.rs
@@ -25,6 +25,9 @@ use super::wall::{FaceKind, Mask};
 pub(super) const INERTIA_FLOOR: f64 = 0.1;
 /// The wall-distance floor of a face, in units of the smallest spacing.
 pub(super) const DISTANCE_FLOOR: f64 = 0.05;
+/// Virtual merging: a cell whose fluid fraction (at either end of the
+/// step) stays below this shares its pressure unknown with a neighbour.
+pub(super) const MERGE_FRACTION: f64 = 0.1;
 
 /// Lattice addressing of faces and cells with the periodic wrap in z as
 /// data: a face of component `c` at `p = [i, j, k]` (its own coordinate is
@@ -175,7 +178,7 @@ impl Mask {
                 }
             }
         }
-        Ok(Self {
+        let mut mask = Self {
             grid: g,
             periodic_z: periodic,
             cell_fluid,
@@ -190,7 +193,53 @@ impl Mask {
             cut: Some(cut),
             step_apertures: None,
             step_open: None,
-        })
+            merge_master: Vec::new(),
+        };
+        mask.compute_merging(None);
+        Ok(mask)
+    }
+
+    /// The virtual merging map: a small cell (fraction < `MERGE_FRACTION`
+    /// at both ends of the step) takes as master its active face
+    /// neighbour of largest fraction that is not small itself; a small
+    /// cell without such a neighbour keeps its own row.
+    pub(super) fn compute_merging(&mut self, old: Option<&Mask>) {
+        let Some(cut) = self.cut.as_ref() else {
+            return;
+        };
+        let g = self.grid;
+        let n = g.cells();
+        let lat = self.lattice();
+        let frac = |idx: usize| {
+            let v = cut.vol[idx];
+            old.and_then(|o| o.cut.as_ref())
+                .map_or(v, |oc| v.max(oc.vol[idx]))
+        };
+        let small: Vec<bool> = (0..n)
+            .map(|idx| self.cell_active(idx) && frac(idx) < MERGE_FRACTION)
+            .collect();
+        let mut master = vec![usize::MAX; n];
+        for idx in (0..n).filter(|&i| small[i]) {
+            let (k, j, i) = g.kji(idx);
+            let p = [i as i64, j as i64, k as i64];
+            let mut best: Option<(f64, usize)> = None;
+            for d in 0..3 {
+                for side in [-1i64, 1] {
+                    let mut q = p;
+                    q[d] += side;
+                    if let Some(nb) = lat.cell(q) {
+                        let v = cut.vol[nb];
+                        if self.cell_active(nb) && !small[nb] && best.is_none_or(|b| v > b.0) {
+                            best = Some((v, nb));
+                        }
+                    }
+                }
+            }
+            if let Some((_, m)) = best {
+                master[idx] = m;
+            }
+        }
+        self.merge_master = master;
     }
 
     /// Set the step-averaged apertures and the space-time classification
@@ -214,6 +263,7 @@ impl Mask {
             .collect();
         self.step_open = Some((open(&au), open(&av), open(&aw), active));
         self.step_apertures = Some((au, av, aw));
+        self.compute_merging(Some(old));
     }
 
     pub(super) fn lattice(&self) -> Lattice {
diff --git a/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/poisson/hierarchy.rs b/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/poisson/hierarchy.rs
index d31bcbd..814085b 100644
--- a/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/poisson/hierarchy.rs
+++ b/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/poisson/hierarchy.rs
@@ -27,6 +27,8 @@ pub(crate) struct Level<T: MgScalar> {
     pub(crate) top: Vec<usize>,
     pub(crate) bot: Vec<usize>,
     pub(crate) coarse_of: Vec<usize>,
+    /// Off-stencil links per cell (finest level only; empty elsewhere).
+    pub(crate) links: Vec<Vec<(usize, T)>>,
 }
 
 struct Work<T: MgScalar> {
@@ -97,6 +99,15 @@ impl<T: MgScalar> Level<T> {
             .filter(|&idx| parity(idx) == 1)
             .collect();
         let cast = |v: &[f64]| v.iter().map(|&x| T::from_f64(x)).collect::<Vec<T>>();
+        let links: Vec<Vec<(usize, T)>> = if problem.links.is_empty() {
+            Vec::new()
+        } else {
+            problem
+                .link_lists()
+                .into_iter()
+                .map(|l| l.into_iter().map(|(o, c)| (o, T::from_f64(c))).collect())
+                .collect()
+        };
         Self {
             ae: cast(&problem.ae),
             aw: cast(&problem.aw),
@@ -113,6 +124,7 @@ impl<T: MgScalar> Level<T> {
             top,
             bot,
             coarse_of: Vec::new(),
+            links,
         }
     }
 
@@ -145,6 +157,11 @@ impl<T: MgScalar> Level<T> {
         if ab != T::ZERO {
             s += ab * x[self.bot[idx]];
         }
+        if !self.links.is_empty() {
+            for &(other, c) in &self.links[idx] {
+                s += c * x[other];
+            }
+        }
         s
     }
 
@@ -356,6 +373,7 @@ impl Components {
         let mut members = Vec::new();
         let mut singular = Vec::new();
         let mut stack = Vec::new();
+        let links = problem.link_lists();
         for &seed in cells {
             if id[seed] != usize::MAX {
                 continue;
@@ -395,6 +413,9 @@ impl Components {
                 if let Some(b) = problem.bottom(idx, k) {
                     visit(b, problem.ab[idx]);
                 }
+                for &(other, c) in &links[idx] {
+                    visit(other, c);
+                }
             }
             members.push(list);
             singular.push(!has_dirichlet);
diff --git a/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/poisson/pcg.rs b/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/poisson/pcg.rs
index 204a499..825f302 100644
--- a/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/poisson/pcg.rs
+++ b/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/poisson/pcg.rs
@@ -17,6 +17,7 @@ pub(crate) struct OperatorKey {
     periodic_z: bool,
     active: Vec<bool>,
     coefficients: Vec<u64>,
+    links: Vec<(usize, usize, u64)>,
     smoother_sweeps: usize,
     coarsest_cells: usize,
     smoother: MgSmoother,
@@ -44,6 +45,11 @@ impl OperatorKey {
             periodic_z: problem.periodic_z,
             active: problem.active.clone(),
             coefficients: Self::bits(problem).collect(),
+            links: problem
+                .links
+                .iter()
+                .map(|&(a, b, c)| (a, b, c.to_bits()))
+                .collect(),
             smoother_sweeps: params.smoother_sweeps,
             coarsest_cells: params.coarsest_cells,
             smoother: params.smoother,
@@ -59,6 +65,12 @@ impl OperatorKey {
             && self.coarsest_cells == params.coarsest_cells
             && self.smoother == params.smoother
             && self.active == problem.active
+            && self.links.len() == problem.links.len()
+            && self
+                .links
+                .iter()
+                .zip(&problem.links)
+                .all(|(&(a, b, c), &(pa, pb, pc))| a == pa && b == pb && c == pc.to_bits())
             && self.coefficients.iter().copied().eq(Self::bits(problem))
     }
 }
diff --git a/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/poisson/problem.rs b/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/poisson/problem.rs
index 749301c..e342096 100644
--- a/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/poisson/problem.rs
+++ b/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/poisson/problem.rs
@@ -21,6 +21,12 @@ pub struct Problem {
     pub ab: Vec<f64>,
     pub extra_diag: Vec<f64>,
     pub rhs: Vec<f64>,
+    /// Off-stencil symmetric links `(a, b, coefficient)` between two active
+    /// cells (a virtually merged small cell's master to the small cell's
+    /// other neighbours); each adds `coefficient` to both diagonals and
+    /// `−coefficient` off-diagonal both ways. Carried by the finest level
+    /// only (the coarse levels stay seven-point: preconditioner quality).
+    pub links: Vec<(usize, usize, f64)>,
 }
 
 impl Problem {
@@ -42,9 +48,31 @@ impl Problem {
             ab: vec![0.0; n],
             extra_diag: vec![0.0; n],
             rhs: vec![0.0; n],
+            links: Vec::new(),
         }
     }
 
+    /// The link coefficients per cell (`(other, coefficient)` lists).
+    #[must_use]
+    pub fn link_lists(&self) -> Vec<Vec<(usize, f64)>> {
+        let mut out = vec![Vec::new(); self.nx * self.ny * self.nz];
+        for &(a, b, c) in &self.links {
+            out[a].push((b, c));
+            out[b].push((a, c));
+        }
+        out
+    }
+
+    /// The sum of the link coefficients on a cell.
+    #[must_use]
+    pub fn link_diagonal(&self, idx: usize) -> f64 {
+        self.links
+            .iter()
+            .filter(|&&(a, b, _)| a == idx || b == idx)
+            .map(|&(_, _, c)| c)
+            .sum()
+    }
+
     #[inline]
     #[must_use]
     pub fn index(&self, k: usize, j: usize, i: usize) -> usize {
@@ -86,13 +114,18 @@ impl Problem {
     #[inline]
     #[must_use]
     pub fn diagonal(&self, idx: usize) -> f64 {
-        self.ae[idx]
+        let stencil = self.ae[idx]
             + self.aw[idx]
             + self.an[idx]
             + self.as_[idx]
             + self.at[idx]
             + self.ab[idx]
-            + self.extra_diag[idx]
+            + self.extra_diag[idx];
+        if self.links.is_empty() {
+            stencil
+        } else {
+            stencil + self.link_diagonal(idx)
+        }
     }
 
     /// Pure Neumann: no active cell has a Dirichlet contribution.
@@ -109,6 +142,7 @@ impl Problem {
     #[must_use]
     pub fn residual_l1(&self, p: &[f64]) -> f64 {
         let (nx, ny, nz) = (self.nx, self.ny, self.nz);
+        let links = self.link_lists();
         let mut sum = 0.0;
         for k in 0..nz {
             for j in 0..ny {
@@ -144,6 +178,9 @@ impl Problem {
                             nb += self.ab[idx] * p[b];
                         }
                     }
+                    for &(other, c) in &links[idx] {
+                        nb += c * p[other];
+                    }
                     sum += (self.rhs[idx] - (ap * p[idx] - nb)).abs();
                 }
             }
@@ -170,6 +207,11 @@ impl Problem {
                 return Err(format!("{name}: length {len}, expected nx*ny*nz = {n}"));
             }
         }
+        for &(a, b, c) in &self.links {
+            if a >= n || b >= n || a == b || !self.active[a] || !self.active[b] || c <= 0.0 || c.is_nan() {
+                return Err(format!("invalid link ({a}, {b}, {c})"));
+            }
+        }
         let (nx, ny, nz) = (self.nx, self.ny, self.nz);
         let symmetric = |a: f64, b: f64| (a - b).abs() <= 1e-12 * a.abs().max(b.abs());
         for k in 0..nz {
diff --git a/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/step/projection.rs b/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/step/projection.rs
index 9ab868d..71d653c 100644
--- a/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/step/projection.rs
+++ b/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/step/projection.rs
@@ -80,13 +80,98 @@ impl Solver {
                 }
             }
         }
+        self.merge_small_cells(&mut problem);
         problem
     }
 
+    /// Virtual merging (item 10b): every small cell's row is folded into
+    /// its master's — the small cell becomes inactive, its faces to other
+    /// cells become links from the master (to the neighbour, or to that
+    /// neighbour's master), its Dirichlet contribution moves to the
+    /// master; the face between the two is internal to the merged cell.
+    fn merge_small_cells(&self, problem: &mut Problem) {
+        let Some(mask) = self.mask.as_ref() else {
+            return;
+        };
+        if mask.merged_cells() == 0 {
+            return;
+        }
+        let g = mask.grid();
+        let (nx, ny, nz) = (g.nx, g.ny, g.nz);
+        let target = |idx: usize| mask.master(idx).unwrap_or(idx);
+        for k in 0..nz {
+            for j in 0..ny {
+                for i in 0..nx {
+                    let s = g.cell(k, j, i);
+                    let Some(m) = mask.master(s) else { continue };
+                    if !problem.active[s] {
+                        continue;
+                    }
+                    problem.active[s] = false;
+                    // (neighbour, coefficient on s, the mirror coefficient on the neighbour)
+                    let mut faces: Vec<(usize, f64)> = Vec::with_capacity(6);
+                    if i + 1 < nx {
+                        faces.push((s + 1, problem.ae[s]));
+                        problem.aw[s + 1] = 0.0;
+                    }
+                    if i > 0 {
+                        faces.push((s - 1, problem.aw[s]));
+                        problem.ae[s - 1] = 0.0;
+                    }
+                    if j + 1 < ny {
+                        faces.push((s + nx, problem.an[s]));
+                        problem.as_[s + nx] = 0.0;
+                    }
+                    if j > 0 {
+                        faces.push((s - nx, problem.as_[s]));
+                        problem.an[s - nx] = 0.0;
+                    }
+                    if let Some(t) = problem.top(s, k) {
+                        faces.push((t, problem.at[s]));
+                        problem.ab[t] = 0.0;
+                    }
+                    if let Some(b) = problem.bottom(s, k) {
+                        faces.push((b, problem.ab[s]));
+                        problem.at[b] = 0.0;
+                    }
+                    problem.ae[s] = 0.0;
+                    problem.aw[s] = 0.0;
+                    problem.an[s] = 0.0;
+                    problem.as_[s] = 0.0;
+                    problem.at[s] = 0.0;
+                    problem.ab[s] = 0.0;
+                    for (n, c) in faces {
+                        let t = target(n);
+                        if c > 0.0 && t != m {
+                            problem.links.push((m.min(t), m.max(t), c));
+                        }
+                    }
+                    problem.extra_diag[m] += problem.extra_diag[s];
+                    problem.extra_diag[s] = 0.0;
+                }
+            }
+        }
+    }
+
+    /// Fold the small cells' right-hand sides into their masters'.
+    fn merge_rhs(&self, problem: &mut Problem) {
+        let Some(mask) = self.mask.as_ref() else {
+            return;
+        };
+        for s in 0..problem.rhs.len() {
+            if let Some(m) = mask.master(s) {
+                let v = problem.rhs[s];
+                problem.rhs[m] += v;
+                problem.rhs[s] = 0.0;
+            }
+        }
+    }
+
     /// The operator with `field.sp` as the right-hand side.
     pub(crate) fn poisson_problem(&self, field: &Field, dt: f64) -> Problem {
         let mut problem = self.poisson_operator(field.grid, dt);
         problem.rhs.copy_from_slice(&field.sp);
+        self.merge_rhs(&mut problem);
         problem
     }
 
@@ -94,9 +179,10 @@ impl Solver {
     /// the 2D `(1, 1)` at `k = 0`), or `None` with an outlet.
     pub(crate) fn anchor_cell(&self, g: Grid) -> Option<usize> {
         (!self.params.boundaries.any_outlet()).then(|| {
-            self.mask
-                .as_ref()
-                .map_or(g.cell(0, 1, 1), super::super::wall::Mask::anchor)
+            self.mask.as_ref().map_or(g.cell(0, 1, 1), |m| {
+                let a = m.anchor();
+                m.master(a).unwrap_or(a)
+            })
         })
     }
 
@@ -274,8 +360,20 @@ impl Solver {
                 }
             }
         }
-        let inner_stop = self.inner_stop(g, source_scale);
         let problem = self.poisson_problem(field, dt);
+        // The source scale reads the merged right-hand side (a merged small
+        // cell's own divergence is not zero, its pair's is); identical to
+        // the per-cell sum when nothing is merged.
+        if self.mask.as_ref().is_some_and(|m| m.merged_cells() > 0) {
+            source_scale = problem
+                .rhs
+                .iter()
+                .zip(&problem.active)
+                .filter(|&(_, &a)| a)
+                .map(|(r, _)| r.abs())
+                .sum();
+        }
+        let inner_stop = self.inner_stop(g, source_scale);
         let mut p_prime = vec![0.0; g.cells()];
         if warm_start {
             for k in 0..nz {
@@ -310,6 +408,13 @@ impl Solver {
             c0 + 1,
             k0 + solution.iterations as u64,
         );
+        if let Some(mask) = self.mask.as_ref() {
+            for s in 0..p_prime.len() {
+                if let Some(m) = mask.master(s) {
+                    p_prime[s] = p_prime[m];
+                }
+            }
+        }
         field.p_prime.copy_from_slice(&p_prime);
         solution
     }
@@ -401,7 +506,7 @@ impl Solver {
                 }
             }
         }
-        let mut mass_imbalance = 0.0;
+        let mut cell_flux = vec![0.0; g.cells()];
         for k in 0..nz {
             for j in 0..ny {
                 for i in 0..nx {
@@ -422,10 +527,17 @@ impl Solver {
                     if cut {
                         divergence_flux += rho * self.wall_flux(idx);
                     }
-                    mass_imbalance += divergence_flux.abs();
+                    // A merged small cell's flux counts with its master's.
+                    let owner = self
+                        .mask
+                        .as_ref()
+                        .and_then(|m| m.master(idx))
+                        .unwrap_or(idx);
+                    cell_flux[owner] += divergence_flux;
                 }
             }
         }
+        let mass_imbalance: f64 = cell_flux.iter().map(|f| f.abs()).sum();
         let reference_flux = self.reference_flux(g);
         if reference_flux > 0.0 {
             mass_imbalance / reference_flux
diff --git a/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/wall.rs b/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/wall.rs
index 723f1d4..3d5eff6 100644
--- a/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/wall.rs
+++ b/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/wall.rs
@@ -88,6 +88,10 @@ pub struct Mask {
     /// step (a dying cell empties through the apertures it had); `None` =
     /// the instantaneous kinds.
     pub(super) step_open: Option<(Vec<bool>, Vec<bool>, Vec<bool>, Vec<bool>)>,
+    /// Virtual merging (item 10b): the master cell of every small cell
+    /// (`usize::MAX` = its own row) — a small cell shares its pressure
+    /// unknown with its largest active face neighbour in the projection.
+    pub(super) merge_master: Vec<usize>,
 }
 
 /// The z lattice position of a query: the lower plane index, the upper
@@ -497,9 +501,27 @@ impl Mask {
             cut: None,
             step_apertures: None,
             step_open: None,
+            merge_master: Vec::new(),
         })
     }
 
+    /// The master of a virtually merged small cell.
+    #[inline]
+    #[must_use]
+    pub fn master(&self, idx: usize) -> Option<usize> {
+        match self.merge_master.get(idx) {
+            Some(&m) if m != usize::MAX => Some(m),
+            _ => None,
+        }
+    }
+    #[must_use]
+    pub fn merged_cells(&self) -> usize {
+        self.merge_master
+            .iter()
+            .filter(|&&m| m != usize::MAX)
+            .count()
+    }
+
     // The projection's unknowns (the instantaneous kinds at rest).
     #[inline]
     #[must_use]
diff --git a/crates/specialized/rtx-cfd/tests/embedded3_sphere/mod.rs b/crates/specialized/rtx-cfd/tests/embedded3_sphere/mod.rs
index 72177e4..433d593 100644
--- a/crates/specialized/rtx-cfd/tests/embedded3_sphere/mod.rs
+++ b/crates/specialized/rtx-cfd/tests/embedded3_sphere/mod.rs
@@ -197,11 +197,14 @@ pub fn measure(n: usize, scheme: WallScheme, c: (f64, f64, f64)) -> Measurement
     let body = solver.body().expect("body");
     let t = solver.time();
     // The apertured divergence per unit volume, the porous surface's flux
-    // through the wall included (the plain divergence on the binary wall).
+    // through the wall included (the plain divergence on the binary wall);
+    // a virtually merged small cell's flux counts with its master's (only
+    // the pair's continuity holds).
     let mut max_div = 0.0_f64;
     let mut at_vol = 1.0;
     let mut sum_flux = 0.0;
     let (wall_fluxes, _) = mask.wall_flux_table(body, t);
+    let mut cell_flux = vec![0.0; g.cells()];
     for k in 0..n {
         for j in 0..n {
             for i in 0..n {
@@ -220,15 +223,18 @@ pub fn measure(n: usize, scheme: WallScheme, c: (f64, f64, f64)) -> Measurement
                             * h
                             * h
                         + wall_fluxes[idx];
-                    sum_flux += flux.abs();
-                    if (flux / (h * h * h)).abs() > max_div {
-                        max_div = (flux / (h * h * h)).abs();
-                        at_vol = mask.vol(idx);
-                    }
+                    cell_flux[mask.master(idx).unwrap_or(idx)] += flux;
                 }
             }
         }
     }
+    for (idx, &flux) in cell_flux.iter().enumerate() {
+        sum_flux += flux.abs();
+        if (flux / (h * h * h)).abs() > max_div {
+            max_div = (flux / (h * h * h)).abs();
+            at_vol = mask.vol(idx);
+        }
+    }
     println!(
         "    [{scheme:?} n {n}] max div {max_div:.2e} in a cell of fluid fraction {at_vol:.3e}; Σ|flux| {sum_flux:.2e}; last step residual {:.2e}",
         last.final_residual