diff --git a/docs/scheduled-mac.md b/docs/scheduled-mac.md
index 22eb345..d3ec34a 100644
--- a/docs/scheduled-mac.md
+++ b/docs/scheduled-mac.md
@@ -186,6 +186,11 @@ witness capture of a 9-copy truth. Kestrel has no CCX `tx.report` path, so
 the witness copy-count is the retry ground truth there — the fw-level
 delivery outcome stays invisible until a receipts-tier consumer counts it.
 
+The ACK window itself is a knob — `DEVOURER_ACK_TIMEOUT_US`, the
+hardware-ARQ *range* lever (round-trip propagation eats ~6.7 µs/km; the
+per-chip defaults and the bench proof live at the field doc in
+`src/DeviceConfig.h`).
+
 Choosing the limit (`tests/arq_retry_sweep.sh`, collision regime: a ~1 k fps
 retrying unicast flood into an 8812EU duplex ground station airing
 PixelPilot-shaped feedback bursts, near-field): retries are backoff-spaced,
diff --git a/examples/common/env_config.cpp b/examples/common/env_config.cpp
index 7d98775..606d266 100644
--- a/examples/common/env_config.cpp
+++ b/examples/common/env_config.cpp
@@ -121,6 +121,10 @@ devourer::DeviceConfig devourer_config_from_env() {
       std::fflush(stderr);
     }
   }
+  if (env_long("DEVOURER_ACK_TIMEOUT_US", &v) && v >= 1)
+    /* 1..255; out-of-range low keeps the library default (a 0 collapsing
+     * to 1 us would write off every frame). */
+    cfg.tx.ack_timeout_us = static_cast<int>(v > 255 ? 255 : v);
   if (env_long("DEVOURER_TX_RETRY_LIMIT", &v))
     cfg.tx.retry_limit = static_cast<int>(v < 0 ? 0 : (v > 63 ? 63 : v));
   if (const char *e = env_str("DEVOURER_TX_RETRY_FALLBACK")) {
diff --git a/src/DeviceConfig.h b/src/DeviceConfig.h
index d90b4f0..de8d8e3 100644
--- a/src/DeviceConfig.h
+++ b/src/DeviceConfig.h
@@ -213,15 +213,38 @@ struct DeviceConfig {
      * Runtime equivalent: StartCwTone/StopCwTone on the concrete device. */
     bool cw_tone = false;
     uint8_t cw_tone_gain = 0;
-    /* env: DEVOURER_TX_RETRY_LIMIT — per-frame hardware retry limit (0..63).
-     * Maps to the TX descriptor DATA_RETRY_LIMIT / RTS_DATA_RTY_LMT field
-     * (Dword4 bits 18-23). 0 = no retries (WFB default: FEC provides
-     * reliability, not MAC retries). On a busy half-duplex link retries flood
-     * the air and blind the receiver. Hardware-ARQ (SetAckResponder + unicast
-     * TA, docs/scheduled-mac.md) needs a nonzero value. Inert on Kestrel
-     * (firmware-level retry) and on the 8814A die (vendor DATA_RETRY_LIMIT=0
-     * carve-out kept). */
+    /* env: DEVOURER_TX_RETRY_LIMIT — per-frame hardware retry limit (0..63;
+     * Kestrel ceiling 62 — its attempts-counting WD field folds +1). Maps to
+     * the TX descriptor DATA_RETRY_LIMIT / RTS_DATA_RTY_LMT field on the
+     * 11ac generations and wd_info DATA_TXCNT_LMT on Kestrel. 0 = no retries
+     * (WFB default: FEC provides reliability, not MAC retries). On a busy
+     * half-duplex link retries flood the air and blind the receiver.
+     * Hardware-ARQ (SetAckResponder + unicast TA, docs/scheduled-mac.md)
+     * needs a nonzero value. Inert on the 8814A die only (vendor
+     * DATA_RETRY_LIMIT=0 carve-out kept pending its bench). */
     int retry_limit = 0;
+    /* env: DEVOURER_ACK_TIMEOUT_US — hardware ACK response window in µs
+     * (1..255, clamped), the hardware-ARQ RANGE lever: the MAC writes a
+     * frame off (and retries) when no ACK is counted within this window,
+     * and round-trip propagation eats ~6.7 µs per km. ONE default, 128 µs,
+     * programmed identically on every generation at bring-up — the same
+     * knob value means the same range budget (~15 km round trip) no matter
+     * which die is plugged. 128 is the vendor's interop-blessed J1/J2
+     * value and covers the slowest narrowband ACK in the tree (the 5 MHz
+     * per-bandwidth vendor value is 117 µs), so it also replaces the
+     * per-chip / per-bandwidth vendor defaults (which ranged 33..128 µs
+     * and made hardware-ARQ range silently die-dependent). The register:
+     * REG_ACKTO 0x640 on the 11ac generations, R_AX_RSP_CHK_SIG 0xCC00
+     * byte0 on Kestrel; the CTS window (REG_CTS2TO 0x641) is separate and
+     * untouched. Sizing: ~6.7 µs x round-trip km + ~50 µs ACK flight and
+     * detection margin; a longer window is NOT free — every retry of a
+     * LOST frame waits the full window, measured (dead RA, retry 8, max
+     * duty): 2719 write-offs/8 s at 33 µs vs 2015 at 128 vs 1507 at 255.
+     * Bench proof the register gates the ARQ verdict: at 8 µs (below the
+     * ACK's flight time) retries pin at the limit with 0% ok against a
+     * live responder; at 128/255 the responder cell runs 100% ok,
+     * retries ~0. */
+    int ack_timeout_us = 128;
     /* env: DEVOURER_TX_RETRY_FALLBACK — "off" | unset. Unset = the firmware
      * fallback ladder with its own floor (the current behaviour, descriptors
      * byte-identical). "off" disables per-retry rate fallback (DISDATAFB /
diff --git a/src/jaguar1/RtlJaguarDevice.cpp b/src/jaguar1/RtlJaguarDevice.cpp
index 27013bf..70829fa 100644
--- a/src/jaguar1/RtlJaguarDevice.cpp
+++ b/src/jaguar1/RtlJaguarDevice.cpp
@@ -80,6 +80,12 @@ void RtlJaguarDevice::InitWrite(SelectedChannel channel) {
    * DEVOURER_DIS_CCA. Always applied — the enable path is what programs
    * the BB EDCCA thresholds off their parked never-trigger table value. */
   SetCcaMode(_cfg.tuning.disable_cca);
+  /* ACK window (DEVOURER_ACK_TIMEOUT_US): one library default on every
+   * generation — see the DeviceConfig field doc. */
+  _device.rtw_write8(0x0640, static_cast<uint8_t>(
+      _cfg.tx.ack_timeout_us > 255   ? 255
+      : _cfg.tx.ack_timeout_us < 1 ? 1
+                                     : _cfg.tx.ack_timeout_us));
 
   /* DEVOURER_XTAL_CAP — crystal-cap trim (issue #217, narrowband CFO lever). */
   if (_cfg.tuning.xtal_cap)
@@ -1333,6 +1339,12 @@ void RtlJaguarDevice::Init(Action_ParsedRadioPacket packetProcessor,
    * DEVOURER_DIS_CCA. Always applied — the enable path is what programs
    * the BB EDCCA thresholds off their parked never-trigger table value. */
   SetCcaMode(_cfg.tuning.disable_cca);
+  /* ACK window (DEVOURER_ACK_TIMEOUT_US): one library default on every
+   * generation — see the DeviceConfig field doc. */
+  _device.rtw_write8(0x0640, static_cast<uint8_t>(
+      _cfg.tx.ack_timeout_us > 255   ? 255
+      : _cfg.tx.ack_timeout_us < 1 ? 1
+                                     : _cfg.tx.ack_timeout_us));
 
   /* DEVOURER_XTAL_CAP — crystal-cap trim (issue #217, narrowband CFO lever). */
   if (_cfg.tuning.xtal_cap)
diff --git a/src/jaguar2/RtlJaguar2Device.cpp b/src/jaguar2/RtlJaguar2Device.cpp
index 5eb26f6..25706be 100644
--- a/src/jaguar2/RtlJaguar2Device.cpp
+++ b/src/jaguar2/RtlJaguar2Device.cpp
@@ -249,6 +249,13 @@ void RtlJaguar2Device::bring_up(SelectedChannel channel) {
   if (_cfg.tuning.disable_cca)
     SetCcaMode(true);
 
+  /* ACK window (DEVOURER_ACK_TIMEOUT_US): one library default on every
+   * generation — see the DeviceConfig field doc. */
+  _device.rtw_write8(0x0640, static_cast<uint8_t>(
+      _cfg.tx.ack_timeout_us > 255   ? 255
+      : _cfg.tx.ack_timeout_us < 1 ? 1
+                                     : _cfg.tx.ack_timeout_us));
+
   /* DEVOURER_XTAL_CAP — apply the crystal-cap trim once the AFE is up
    * (issue #217, the narrowband CFO lever). */
   if (_cfg.tuning.xtal_cap)
diff --git a/src/jaguar3/RtlJaguar3Device.cpp b/src/jaguar3/RtlJaguar3Device.cpp
index 89b1f9b..e443ff5 100644
--- a/src/jaguar3/RtlJaguar3Device.cpp
+++ b/src/jaguar3/RtlJaguar3Device.cpp
@@ -878,6 +878,14 @@ void RtlJaguar3Device::InitWrite(SelectedChannel channel) {
   else
     _txpkt_img.store(0, std::memory_order_relaxed);
   apply_dpdt_route_8822e(); /* 8822E DPDT/eFEM pin-mux (post-coex) */
+  /* ACK window (DEVOURER_ACK_TIMEOUT_US): one library default on every
+   * generation, replacing the halmac per-bandwidth REG_ACKTO defaults
+   * init_wmac_cfg just wrote (the 128 default covers the slowest
+   * narrowband ACK) — see the DeviceConfig field doc. */
+  _device.rtw_write8(0x0640, static_cast<uint8_t>(
+      _cfg.tx.ack_timeout_us > 255   ? 255
+      : _cfg.tx.ack_timeout_us < 1 ? 1
+                                     : _cfg.tx.ack_timeout_us));
   apply_replay_wseq(); /* DEVOURER_REPLAY_WSEQ golden-init replay (debug) */
   if (_cfg.debug.bb_dump) {
     /* Full MAC+BB dump (0x000..0x4ffc — MAC plane, then BB incl. the RF
diff --git a/src/kestrel/RtlKestrelDevice.cpp b/src/kestrel/RtlKestrelDevice.cpp
index a153416..a4983e8 100644
--- a/src/kestrel/RtlKestrelDevice.cpp
+++ b/src/kestrel/RtlKestrelDevice.cpp
@@ -254,6 +254,13 @@ void RtlKestrelDevice::InitWrite(SelectedChannel channel) {
    * re-asserts explicitly (idempotent) when the knob is set. */
   if (_cfg.tuning.disable_cca)
     SetCcaMode(true);
+  /* ACK window (DEVOURER_ACK_TIMEOUT_US): one library default on every
+   * generation — byte0 of R_AX_RSP_CHK_SIG (the field the vendor's
+   * narrowband path scales); see the DeviceConfig field doc. */
+  _device.rtw_write8(0xCC00, static_cast<uint8_t>(
+      _cfg.tx.ack_timeout_us > 255   ? 255
+      : _cfg.tx.ack_timeout_us < 1 ? 1
+                                     : _cfg.tx.ack_timeout_us));
   _tx_mgmt_ep = _device.nth_bulk_out_ep(0); /* B0MG -> BULKOUTID0 */
   _tx_data_ep = _device.nth_bulk_out_ep(3); /* ACH0 -> BULKOUTID3 */
   if (_tx_mgmt_ep == 0) {