ARC-TUNING — Rev / Speed Limiter

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Scarecrow Briefing — Rev / Speed Limiter

The two safety nets at the top end. RPM Limit is a hard fuel-cut + spark-cut at a given RPM to protect the engine from over-rev. Speed Limit is a fuel-cut at a given vehicle speed (factory stop on tire-rated cars). Both ship enabled on every GM cal. Tuners adjust them; rarely remove them entirely.

(1) Soft + Hard limit pair. Modern GM PCMs use two RPM thresholds: a soft limit ~50–100 RPM below the hard limit where timing is pulled (you feel the power fall off), and a hard limit where injectors and spark cut. The soft limit is the "buffer" — gives the driver a moment to lift before the hard cut. (2) Limit must respect valvetrain. Don't raise the limit above what your valve springs / pushrods can handle. Stock LS1 valvetrain limit is ~6500 RPM. PAC 1218 springs — ~6800. PAC 1219 — ~7200. Above that, dual-spring or beehive setups required. (3) Speed limit on tire-rated cars. Stock C5 Z51 = 168 mph (Z-rated tires). Stock C6 = 198 mph. Stock LS-Vans = 100 mph. Track use: raise or remove. Street use: leaving stock keeps you legal-safe and protects against a tire blowout. (4) DBW vs cable limiter behavior. DBW limiters can also close the throttle plate at the limit (smoother). Cable can only fuel-cut/spark-cut (jolts). The DBW closure is set in a separate scalar.

Page is short by design — limits are simple scalars. Get the values right, ship.

3.1 Hard RPM Limit (Fuel + Spark Cut) 📍 LOCATE

HPTEngine → Engine Limiters → RPM Limiter → Hard

— Set to valvetrain ceiling

The injector/spark cut RPM. Engine cannot pass this value. Defaults: LS1 6500, LS6 6700, LS3 6600, LS7 7100, LS9 6600. Raising requires valvetrain confirmation. Stock LS1 valvetrain at 6800+ floats valves and risks contact. PAC 1218 to 6800. PAC 1219/LS7 dual to 7200. Above that → custom valvetrain.

The RPM where the engine cuts fuel and spark to keep itself from blowing up. Don't raise it past what your valve springs can handle.

3.2 Soft RPM Limit (Timing Pull Window) 📍 LOCATE

HPTEngine → Engine Limiters → RPM Limiter → Soft

— ~50–100 RPM below hard limit

The "buffer zone" before the hard cut. Spark advance is pulled progressively in this band — the driver feels the power fall off as a cue to lift. Without a soft limit, the engine pulls hard up to redline then crashes into the hard cut — abrupt and rough. With a properly-tuned soft limit, the rev hits a wall smoothly. Typical: hard 6500, soft 6450, with timing pull starting at the soft and reaching ~10° pulled by the time hard fires.

A "warning zone" 50–100 RPM below the hard limit where the engine pulls timing so power falls off smoothly. Easier on the driver and on the engine than just slamming into the hard cut.

3.3 Vehicle Speed Limiter 📍 LOCATE

HPTEngine → Engine Limiters → Vehicle Speed Limiter

— Tire rating dependent

Vehicle speed cut value. Factory values are tire-rated (Z = 149+ mph nominal, raised in some cals to 168/186/198). Tuners commonly raise or remove for track use. Street use: leaving stock value is the safe call — a tire blowout above tire-rating speed is catastrophic. Race only / closed course → freely adjusted.

Top speed cut. Factory limits are based on what your tires can handle. Don't raise it without rated-for-it tires.

3.4 DBW Throttle Closure at Limit (Gen IV / DBW only) 📍 LOCATE

HPTEngine → Throttle → DBW → Limiter Behavior

— DBW-only feature

On DBW (Drive-By-Wire) cars, the limiter can additionally CLOSE the throttle plate at the limit, on top of cutting fuel and spark. Gives a smoother limit feel — engine just "stops accelerating" instead of "punches in the chest." On cable-throttle cars there's no equivalent — the plate is mechanically tied to the pedal.

On newer DBW LS cars, the throttle plate can close itself at the rev limit so it feels smooth instead of sudden. Old cable-throttle cars don't have this.

3.5 Verification Loop — Drive to limit, verify behavior

Brief verification log: pull to redline once or twice in 2nd or 3rd, confirm hard cut at expected RPM, soft cut produces feelable torque rolloff at expected band, no overshoot. Speed limit verification only on closed course.

Drive up to the rev limit once and confirm it cuts where you set it. Speed limit only test if you're at a track.