Sea-Doo RXT-X 300 Performance Guide
Sea-Doo RXT-X 300 performance upgrades for the 1630 ACE (300HP and 325HP) touring platform. GT40 Stage 1 System-3 builds, dual-exhaust fitment, FAQ. Engineere
The GT40 build map for the touring-aggressive 1630 ACE 300 — same engine as the RXP-X, different hull mission, the same reliability story to unwind.
The Ski
Sea-Doo RXT-X 300 performance upgrades follow the same engine playbook as the RXP-X — different hull, identical 1630 ACE failure points. The RXT-X 300 is what happens when Sea-Doo takes the 1630 ACE 300/325 and bolts it to the ST3 touring hull instead of the racing platform. You get the same supercharged powerplant, the same 300HP rating, the same intercooler architecture — but in a hull built for endurance rides, long-haul comfort, and two-up touring without sacrificing top-end pull. That makes the RXT-X 300 the most-used 300HP Sea-Doo in real-world hours. It also makes it the platform that sees the most service-curve consequences from the same factory weaknesses we map on the RXP-X 300 build map.
This is the GT40 build map for the RXT-X 300. Same engine playbook as the RXP-X with the hull-specific notes that matter for buyers who actually log 200+ hours a season. Start with the PWC Performance Glossary if the terminology is new, and the Closed-Loop vs Open-Loop Cooling guide for why cooling rebuilds keep showing up on the high-hour skis.
Stock Specs
| Spec | Stock RXT-X 300 (2019+) |
|---|---|
| Engine | Rotax 1630 ACE — 1,630 cc, 4-stroke, 3-cylinder DOHC |
| Induction | Supercharged, intercooled |
| Rated horsepower | 300 HP (manufacturer claim) |
| Peak HP RPM | ~7,900 RPM |
| Stock top speed | 67–70 mph (touring hull is slightly slower than RXP-X at equal power) |
| Fuel | 91 octane minimum (premium) |
| Hull | ST3 touring — wider, more buoyant, designed for stability and endurance |
| Dry weight | ~882 lbs |
| Fuel capacity | 18.5 gal (larger than RXP-X for endurance use) |
| Seating | 1–3 person rated |
Year evolution — what actually changed
- 2019: First model year of the modern 1630 ACE 300 in the RXT-X. ST3 touring hull, ECO/Sport/Touring modes, launch control, full instrumentation suite.
- 2020: Iron-grey graphics update, minor IBR refinements. Engine and hull carryover.
- 2021: 9" touchscreen first introduced on Limited variants. Powertrain unchanged.
- 2022–2023: Sea-Doo Tech Package (audio, premium nav) added. Color refreshes. Engine and hull unchanged.
- 2024: RXT-X 325 introduced alongside as the higher-output variant. The 300 carries forward in parallel as the price-anchored 300HP touring option.
- 2025–present: Continues as the production 300HP touring ski. Hull, engine, intercooler architecture unchanged from 2019.
Same engineering story as the RXP-X 300 — from 2019 forward, you're working on the same engine, same supercharger, same ribbon-cooled intercooler. Aftermarket fitment is consistent across the model run.
Stock weaknesses — the honest assessment
The 1630 ACE in 300HP / 325HP trim shares its weaknesses with the RXP-X 300 because it's the same engine. The hull-specific consequences differ:
1. Ribbon-cooled intercooler corrosion. Same engine, same intercooler design, same failure path. The touring use case (longer runs, more saltwater exposure over a season) actually accelerates ribbon-corrosion symptoms vs. the racing-hull RXP-X. 2. Supercharger ceramic washer wear. Same supercharger, same wear curve. The touring-hull use case typically logs more total hours per season, which means owners hit the 100-hour ceramic washer inspection cadence sooner in calendar time. 3. Cast J-pipe exhaust outlet. Same exhaust path, same corrosion vulnerability. Touring use means more time in saltwater for many owners, which sharpens the corrosion timeline. 4. Hull-specific: the ST3 touring hull has more wetted surface area than the RXP-X racing hull. At equal power, the touring hull gives back ~2–3 mph in top speed but adds significantly more cornering stability and two-up handling. Performance modifications on the RXT-X tend to focus on usable mid-range and sustained-WOT power rather than peak top-speed numbers.
GT40 builds address every engine-side weakness identically to the RXP-X. The hull-specific consideration is mostly about which exhaust kit fits (RXT-X uses the dual-rear-exhaust path, RXP-X uses the single-rear-outlet path on 2016-2019).
Performance Ceiling
| Build | Approximate Crank HP | Notes |
|---|---|---|
| Stock | 300 | Manufacturer rating, no margin built in |
| Bolt-on (no tune) | 310–320 | Intake + tubing + ribbon delete + catch can |
| Stage 1 (tune optional) | 320–335 | Same bolt-ons, conservative calibration if added |
| Stage 2 (tune + pulley + fuel) | 335–355 | Matched calibration unlocks bolt-on potential plus pulley overdrive |
| Stage 3 (race exhaust + open loop + injectors + premium tubing) | 360–385 | Sustained-WOT capable; mandatory cooling and exhaust upgrades |
| Beyond Stage 3 (internals) | 400+ | Ported supercharger, blower spacer, custom fuel system. Custom-build territory. |
Top-speed gains on the touring hull are slightly more compressed than on the RXP-X racing hull. The same GT40 Stage 3 System build that puts an RXP-X 300 into the high-70s will put an RXT-X 300 into the mid-70s. The mid-range and acceleration gains are equivalent between hulls.
For touring use, the practical Stage that matters most is GT40 Stage 2 System — the calibration-and-pulley step that gives back meaningful mid-range torque for two-up loading and sustained cruise speeds without committing to the GT40 Stage 3 System cooling-and-exhaust overhead.
The GT40 Stage Progression
GT40 Stage 1 System — The Foundation
What it does: unrestricts the breathing path and protects the parts that fail at stock power. The same GT40 Stage 1 System build map as the RXP-X 300, with hull-specific fitment notes for the exhaust outlet path.
Parts:
- Carbon-fiber cold-air intake
- Catch can (oil-vapor separator)
- Intercooler charge tubing with 50mm BOV
- Ribbon delete kit (recommended Stage 1, mandatory Stage 2+)
Power gain: approximately +10 to +15 HP, sharper throttle response, top-speed gain of 1–3 mph.
Difficulty: DIY with hand tools, ~3–4 hours bench time.
Price band: roughly $1,200–$1,600 in parts.
ECU tune: not required at GT40 Stage 1 System.
GT40 Stage 2 System — Calibration Unlocked
What it does: unlocks the GT40 Stage 1 System flow potential through matched calibration, pulley overdrive, and fuel-system support. For RXT-X owners this is often the practical stopping point — the right balance of power, reliability, and touring-friendly service intervals.
Parts (additive on top of GT40 Stage 1 System):
- Supercharger pulley upgrade
- Matched ECU calibration (via Magic Module or equivalent)
- Fuel system upgrade (higher-flow injectors or supplemental capacity)
Power gain: approximately +25 to +40 HP over stock, total system output in the 335–355 HP range. Mid-range torque is the most-felt gain for touring use.
Difficulty: pulley install is the technical step (puller required); calibration is plug-and-play. ~6 hours bench plus tune session.
Price band: roughly $2,000–$2,800 in GT40 Stage 2 System components on top of GT40 Stage 1 System.
GT40 Stage 3 System — Race Prep
What it does: removes the last factory restrictions, adds open-loop cooling capacity, and supports the full GT40 Stage 2 System+ power envelope at sustained WOT. For touring use, GT40 Stage 3 System is typically chosen by owners who want full-throttle confidence during long-haul rides, not just for top-speed peaks.
Parts (additive on top of GT40 Stage 1 System + GT40 Stage 2 System):
- Stage 3 stainless rear exhaust kit (dual-outlet path for RXT-X)
- Open Loop Cooling kit (mandatory above 280HP sustained)
- Premium charge-air tubing
- High-flow injectors matched to Stage 3 calibration
- Refined ECU map (Stage 3 calibration)
Power gain: approximately +60 to +85 HP over stock, total system output in the 360–385 HP range.
Difficulty: dealer-level work. Exhaust sealant cure, open-loop conversion (irreversible), dyno calibration confirmation. 8–10 hours across two sessions.
Price band: roughly $3,800–$5,200 in GT40 Stage 3 System components on top of GT40 Stage 1 System + GT40 Stage 2 System.
Beyond GT40 Stage 3 System
Custom-build territory. Ported supercharger housing, blower spacer, custom fuel system, billet impeller. Not catalog. Talk to us if you're at GT40 Stage 3 System and want to push further.
What GT40 currently stocks vs. roadmap
GT40 Stage 1 System, GT40 Stage 2 System, and GT40 Stage 3 System components are fully cataloged for the RXT-X 300. The dual-rear-exhaust path for the RXT-X is a different SKU than the single-outlet RXP-X exhaust — confirm fitment by hull HIN year and model when ordering Stage 3.
Common Failure Points & How To Address Them
Ribbon intercooler corrosion
Same engine, same intercooler architecture, same failure path as the RXP-X 300. Internal corrosion takes hold from saltwater operation and standing condensation; symptoms progress from gradual cooling loss to coolant migration into the intake side.
GT40 solution: Ribbon Delete Kit — eliminates the ribbon entirely. Recommended at GT40 Stage 1 System, mandatory GT40 Stage 2 System+.
Supercharger ceramic washer wear
Same supercharger as the RXP-X 300, same wear pattern. Touring-use owners typically log more hours per season and hit the 100-hour ceramic-washer inspection threshold sooner.
GT40 solution: GT40 supercharger service kit at 100-hour inspection cadence. Critical for GT40 Stage 2 System+ builds.
Cast J-pipe + dual-rear-exhaust outlet failure
What fails: the RXT-X uses a dual-outlet rear exhaust path. Both the J-pipe (cast iron, coated) and the dual through-hull outlets degrade in saltwater operation. Symptoms: cosmetic rust progressing to structural rust, occasional exhaust-side water seepage at the outlet seal.
Why: cast iron in marine service is a cost-effective OEM compromise. The dual-outlet path multiplies the surface area exposed to saltwater corrosion.
GT40 solution: GT40 Stage 3 System dual rear exhaust kit — replaces the cast J-pipe with stainless steel and the dual outlets with corrosion-immune stainless components.
ECU limp mode from over-rev
Same factory ECU calibration as the RXP-X 300, same conservative rev-limiter triggers. Touring use cases (rougher water, wake encounters) are common triggers for over-rev faults that drop the ski into limp mode.
GT40 solution: GT40 Stage 2 System calibration includes a re-mapped rev limiter and a relaxed over-rev fault response.
Hull-specific: dual-exhaust outlet seal wear
What fails: the dual through-hull exhaust outlets use rubber seals at the hull penetration point. Saltwater operation and UV exposure age the seals; symptoms include minor water intrusion into the engine bay during planing transitions.
Why: standard OEM design, accelerated by saltwater + UV duty.
GT40 recommendation: inspect outlet seals annually. Replace as part of any GT40 Stage 3 System exhaust install. Stainless exhaust outlets ship with new seal hardware.
Install Difficulty by Stage
| Stage | Difficulty | Tools | Time |
|---|---|---|---|
| Stage 1 | DIY-friendly | Hand tools (10/13/17mm sockets, T25/T30 Torx, hose-clamp pliers, torque wrench) | 3–4 hours |
| Stage 2 | Intermediate | Stage 1 tools + supercharger pulley puller + Magic Module or equivalent | 6 hours + tune session |
| Stage 3 | Dealer-level | Stage 1+2 tools + coolant drain pan + 2-hour exhaust sealant cure | 8–10 hours across two sessions, dyno mandatory |
The RXT-X engine bay layout is slightly easier to work in than the RXP-X — the touring hull's larger footprint gives more elbow room for the catch can install and intercooler tubing routing. GT40 Stage 1 System first-timers tend to finish slightly faster on an RXT-X than an RXP-X for this reason.
Every kit ships with the GT40 install guide for that specific SKU.
Tune Strategy
GT40 Stage 1 System — no tune required. Factory ECU adapts within heat cycles.
GT40 Stage 2 System — tune mandatory. Pulley overdrive without calibration risks limp mode and worse. For touring use, a pump-91 Stage 2 map is the sweet spot — power gain without aggressive service-curve consequences.
GT40 Stage 3 System — dyno mandatory. Real numbers under load required.
Pump fuel vs. E85. Most RXT-X owners run pump 91 at every stage. E85 is rare on the touring platform — touring use prioritizes fuel-range and corrosion-care simplicity over peak power.
Reliability trade-off by power level. GT40 Stage 1 System trades nothing. GT40 Stage 2 System with pump 91 is the dominant touring build — strong, reliable, manageable service curve. GT40 Stage 3 System is for owners who want full-throttle confidence on long rides.
Maintenance Schedule
| Hours | Service |
|---|---|
| Every 25 hours | Oil + filter + spark plug inspection. Catch-can drain. Visual on all clamps and joints. |
| 50 hours | Full service: oil + filter + plugs replace, fuel filter, air filter clean-or-replace, all hose-clamp re-torque, exhaust sealant inspection. |
| 100 hours | Supercharger inspection. Ceramic washer replacement if running Stage 2+. Dual-exhaust outlet seal inspection. |
| 200 hours | Comprehensive teardown — supercharger rebuild kit, intercooler tubing inspection, full cooling-system flush (vinegar pass if saltwater operation), full exhaust inspection. |
GT40 service kits match the standard intervals. Touring-use owners who log 100+ hours per season hit the 200-hour interval within 2 seasons — plan the teardown accordingly.
FAQ
Will GT40 Stage 1 System void my warranty? Magnuson-Moss Warranty Act protects against blanket voiding for aftermarket bolt-ons unless the manufacturer can directly tie a failure to a specific aftermarket part. Stage 1 is bolt-on only with no ECU work — the lowest warranty risk possible.
Do I need a tune for GT40 Stage 1 System? No. Stage 1 is designed to run on the factory calibration.
What's the best Stage for two-up touring use? GT40 Stage 2 System with pump 91. The mid-range torque gain transforms two-up acceleration without committing to GT40 Stage 3 System service overhead.
Can I run GT40 Stage 3 System with two-up loading? Yes — Stage 3 power benefits two-up use cases significantly. Just plan for the more aggressive service interval at higher hours.
How does the RXT-X 300 differ from the RXP-X 300 for performance modifications? Same engine, same GT40 Stage 1 System/2/3 component map on the engine side. The GT40 Stage 3 System exhaust kit differs (RXT-X uses dual-outlet, RXP-X uses single-outlet on 2016-2019). Otherwise the modification map is identical.
Will GT40 Stage 3 System reduce my fuel range? Yes. Stage 3 power means more fuel burn at equivalent throttle position. Plan for ~15–20% lower fuel range at Stage 3 vs. stock under similar use.
Should I install the open-loop cooling kit even at GT40 Stage 2 System? Recommended for owners who sustain WOT or run in warm water. Not strictly required at Stage 2 power levels in typical recreational duty cycles.
Can I install the dual-rear-exhaust kit by myself? DIY-capable for experienced mechanics — the dual-outlet path adds complexity over the single-outlet RXP-X exhaust. First-timers should plan extra time or use a marine technician.
What's the top speed at each Stage on the RXT-X? GT40 Stage 1 System: 68–71 mph. GT40 Stage 2 System: 71–74 mph. GT40 Stage 3 System: 74–77 mph. Touring hull gives back 2–3 mph vs. equivalent RXP-X build.
Does GT40 build skis for customers? Yes — full GT40 Stage 1 System, GT40 Stage 2 System, and GT40 Stage 3 System builds at our facility. Contact support@gt40marine.com to schedule.
Will GT40 Stage 2 System modifications affect my insurance? Generally no for GT40 Stage 1 System and Stage 2 bolt-ons. Some insurance carriers require declaration for GT40 Stage 3 System race-spec modifications. Check with your carrier before declaring power changes.
Is the 325HP RXT-X variant a better starting point than the 300? Mechanically the 325 is the same 1630 ACE with a different pulley and ECU calibration. From a GT40 Stage 1 System/2/3 build path, both arrive at similar final outputs. The 325 starts ~25 HP higher; the GT40 Stage 3 System endpoint is similar. Pick on price, color, and feature set rather than performance ceiling.
Related GT40 Builds & Reading
- RXP-X 300 Performance Guide — same engine, racing-hull build map.
- [Sea-Doo
GT40 Stage Systems — Quick Compare
The same platform builds, side-by-side. Pick the depth that matches how you ride and how much downtime you can absorb.
| Feature | GT40 Stage 1 System | GT40 Stage 2 System | GT40 Stage 3 System |
|---|---|---|---|
| HP Gain | +25–30% over stock | +45–50% over stock | +65–75% over stock |
| Fuel | 91 octane | 91 / 93 | 91 / 93 (E85 optional) |
| Reliability | Excellent — same service intervals as stock | Strong — supercharger service tightens to ~100 hrs | Race-focused — 75–100 hr supercharger service |
| Skill Level | Beginner DIY | Intermediate | Advanced / dealer |
| Cooling Upgrades | OLC included | OLC included | OLC + intercooler bypass |
| ECU Tune | Yes — included | Custom dyno tune | Custom dyno tune |
| Install Time | ~3 hours | ~6 hours | ~10+ hours |
| Price | $899.99 | $2,499.99 | $4,999.99 |
Why GT40
- Built and tested in the USA — Bonney Lake, WA. Every Stage System gets bench-built, water-tested, and re-verified before it ships.
- Real rider-tested setups — every spec on this page was validated on actual skis under real-world load. No bench math, no copy-paste forum data.
- Performance-focused support — when you call or email, you're talking to people who ride the RXT-X 300/325HP touring platform and build the kits. Not a call-center script.
- Carefully matched component packages — no random Amazon-grade brackets, no surplus injectors. Every part in the bundle is sourced to work with the others.
- 12 photo-step install guides + 1-on-1 install help when you need it. We don't sell a part and disappear.