Add American V8 preset, carb fuel transport, and adaptive desktop audio

README.md

@@ -144,6 +144,59 @@ physical cell diameter and length. Compare exhausts with `--exhaust racing`
 this option. See [YZ125 verification](verification/YZ125_BASELINE.md) for the
 geometry, assumptions, speed sweep and remaining unloaded-RPM limitation.
 
+## 1980s American V8
+
+Choose **1980s American 5.0 V8** in the desktop menu, or run:
+
+```sh
+./build-desktop/fluidsim_desktop --scenario v8 --scene field
+```
+
+This is an estimated naturally aspirated 302-class cross-plane V8: eight
+101.6 x 76.2 mm cylinders, 7:1 compression, mild pushrod-style valve timing,
+1-5-4-2-6-3-7-8 firing order, 750 RPM idle target and 7000 RPM limiter
+(6750 RPM recovery). Crank/flywheel inertia is 1.1 kg m², versus the original
+.28. Eight unequal .60--.78 m headers feed two independent 1.5-litre bank
+collectors: cylinders 1--4 and 5--8. Each collector has its own 50 mm outlet
+pipe (1.8 / 1.95 m), and each outlet has a separate spatial audio emitter. A
+nominal 3000 kg five-speed car uses the existing clutch/gear controls.
+The V8 clutch has 1100 Nm capacity to shorten launch slip with the heavy
+flywheel and car; engagement remains progressive rather than forcibly locking.
+Final drive is approximately 3.624:1 (6.16 / 1.7), making gearing 1.7x taller
+than the previous setup: 1.7x road speed at the same RPM and selected gear.
+Throttle opening uses a 65 ms response and 2.3 travels/second limit; closing
+uses 160 ms and 1.4 travels/second for a slower release.
+The V8 uses the shared ideal EFI fuel dose with ECU throttle/idle control,
+ignition, overrun fuel cut and rev limiting. Its earlier carb setup has been
+removed. Independent exhaust banks retain their uneven cross-plane pulse
+spacing rather than blending both banks in a shared collector. Numerical
+diffusion in the pipe solver and audio filtering still limit high-frequency rasp.
+Dimensions and ignition/cam calibration are design estimates;
+the output is not a validated period engine dyno curve.
+
+The intake uses one 70 mm inlet duct, one 65 mm throttle body and a shared
+4-litre plenum feeding eight 38 mm diameter, 12 cm runners (previously 30 cm).
+The revised mesh has 19 pipes and 190 cells, down from 310. Runner cells are
+6 cm long and exhaust cells approximately 5--6.5 cm long. This coarsening trades
+some high-frequency resolution for more real-time headroom.
+
+The earlier lift-off slowdown came from the CFL timestep falling below the
+22.7 microsecond audio interval: the whole fluid network advanced almost twice
+per sample, doubling boundary solves and cell updates. The revised 16-second
+rev/coast benchmark stayed at one substep per sample and took .51--.57 wall
+seconds per simulated second here, including coast-down (previously up to
+1.28). It returned to 759 RPM with finite audio and zero pipe positivity
+repairs. The CFL limit remains enforced; other operating points may still
+require subdivisions. The desktop retains the 4x playback gain.
+
+Benchmark or capture with:
+
+```sh
+./build-desktop/fluidsim_scenario_benchmark 8 transient v8
+./build-desktop/fluidsim_scenario_benchmark 20 coast v8
+./build-desktop/fluidsim_audio_capture --scenario v8 --seconds 6 --throttle .3 --out /tmp/v8
+```
+
 ## Twin with a VZ21-size turbo
 
 Choose **600 cc twin + VZ21-size turbo** in the startup menu, or bypass it:
@@ -228,26 +281,50 @@ thermodynamics, crankcase and atmosphere state, profiled-pipe geometry,
 inertive and closed boundaries, gearing and clutch momentum transfer, finite
 runs for every scenario, and finite audio/reverb output.
 
-## ECU and throttle control
+## Fuel and throttle control
+
+Desktop playback has automatic gain enabled by default for every scenario.
+Press **G** to toggle it; the dashboard shows its state. It tracks stereo RMS
+and peaks after DC removal, lifting quiet idle audio gradually (1.5-second
+gain-rise time constant) and reducing gain faster for loud revs (30 ms).
+Extra gain is capped at 12x above the existing fixed 4x playback gain, with
+linked stereo peak protection. Silence does not request additional boost and
+underruns freeze the detector. Disabling smoothly returns to fixed playback
+gain. Simulation pressures and WAV capture levels are unaffected.
 
 Both desktop engines use `Ecu` (`include/FluidSim/Ecu.hpp`). The desktop and
 CLI throttle values are pedal requests; the actual throttle is advanced every
 simulation sample, independently of rendering. Opening and closing use an
 80 ms first-order actuator response and a maximum travel rate of two full
-travels per second. An ideal butterfly's projected opening determines orifice
-area continuously; shaft obstruction and edge leakage are not modeled.
+travels per second. The four-stroke uses an ideal butterfly projection; the
+two-stroke uses the circular-segment aperture uncovered by a translating slide.
+Shaft obstruction and edge leakage are not modeled.
 
-The ECU uses ideal sensors and direct access to trapped fresh-air mass at
-intake-valve closure to calculate each cylinder's fuel dose. This explicitly
+The four-strokes use ideal sensors and direct access to trapped fresh-air mass
+at intake-valve closure to calculate each cylinder's fuel dose. This explicitly
 permitted idealization replaces injector flow/duty-cycle and fuel-film models.
-The nominal AFR is 14.7, enriched progressively toward 12.8 above 70% throttle.
+Their nominal AFR is 14.7, enriched progressively toward 12.8 above 70% throttle.
 A PI idle controller targets 1800 RPM with filtered RPM feedback, bounded idle
 authority and anti-windup. Overrun fuel cut engages above 3500 RPM on a closed
 pedal and restores fuel below 2400 RPM to catch idle. The twin's rev limiter cuts fuel
-and spark at 10000 RPM, recovering below 9750 RPM. The two-stroke has no rev
-limiter; its idle, overrun and sensor-fault controls remain active. Invalid sensor readings
-inhibit fuel. A simple RPM-based ignition advance map is provided. An already
-burning charge completes combustion when a cut begins.
+and spark at 10000 RPM, recovering below 9750 RPM.
+
+The two-stroke uses a simulated 38.1 mm slide carburetor instead of the EFI
+dose. Effective pilot, needle and main restrictions draw liquid fuel from a
+finite float bowl with the orifice relation `m_dot = Cd*A*sqrt(2*rho*delta_p)`.
+The jet pressure signal comes from simulated mass flow, local venturi dynamic
+pressure and the measured static pressure drop. A wall film deposits and
+evaporates fuel during transients. Delivered fuel is added downstream of the
+carb as a conserved species and travels through the plenum, reed, crankcase,
+transfer duct and cylinder. Combustion is therefore limited by the fuel and
+oxygen actually trapped, rather than receiving a cylinder-side AFR dose.
+
+The two-stroke has no rev limiter and deliberately does not use electronic
+overrun fuel cut. On a high-RPM throttle lift, wet-wall and pilot fuel can pass
+through unlit cycles if the trapped mixture cannot ignite. Spark remains at
+the CDI timing; there is no scheduled overrun firing pattern. Unburned fuel
+is transported out through the exhaust without pipe combustion. No sound
+samples are mixed in.
 
 All ECU parameters are accessible calibrations, not measured engine maps.
 There is no starter motor, automatic restart, knock sensor, lambda sensor,
@@ -255,11 +332,14 @@ catalyst, cold-start temperature model or injector electrical model. Idle
 control can still stall under excessive load and does not impose crankshaft
 speed or add artificial sustaining torque. Existing combustion/thermal models
 remain approximations; ECU changes do not establish their physical accuracy.
-The two-stroke doses fuel after the last scavenging port closes, targets
-3000 RPM idle, and uses idle ignition trim in addition to throttle control. Its
+The two-stroke targets 3000 RPM idle and uses idle ignition trim in addition to
+throttle control. Its
 reed is a damped lumped cantilever model with finite opening time and seating
 contacts. Cylinder and crankcase moving-volume work use adiabatic energy changes.
-Reed material, combustion and controller parameters still require calibration.
+Jet dimensions are effective flow calibrations rather than measured Mikuni
+parts. Fuel uses the gas model after wall-film release; droplet size,
+atomization, evaporation heat and oil are not modeled. Reed material,
+combustion and controller parameters still require calibration.
 C# trace parity is intentionally changed by these control and physics changes.
 
 VS Code's default build task builds the desktop through CMake. Use