Sea-Doo 300 Cooling Kit: Open Loop Conversion

The 300 HP 1630 ACE — the engine in the RXP-X 300, RXT-X 300, GTX Limited 300, and Wake Pro 230's higher-output sibling — leaves the factory with a closed-loop cooling system. Coolant circulates inside the engine, exchanges heat with raw water in a heat exchanger, and returns. The architecture works fine on a stock craft in moderate conditions.

The moment you add boost, drop the pulley, or push extended WOT in warm water, that architecture becomes the limiter. Coolant temperatures climb, the ECU pulls timing to protect the engine, and the horsepower you paid for evaporates inside a 15-minute session.

A **Sea-Doo 300 cooling kit** — specifically an open loop conversion — replaces the closed-loop coolant path with a direct raw-water cooling circuit through the heads. More cooling capacity, lower equilibrium temperatures, and the supercharged calibration the engine was actually designed to run.

GT40Marine builds the open loop cooling kit at $384.99 — engineered to fit both the 230 and 300 HP 1630 ACE platforms, billet aluminum hardware, and the same marine-grade hose stack we use on every cooling product. This page walks through what the kit changes, who needs it, and how it compares to the rest of the market.

What the Sea-Doo 300 cooling system actually does (stock)

The 1630 ACE 300 leaves the factory with a closed-loop cooling system:

• Coolant (a 50/50 antifreeze mix) circulates inside the engine through the heads and block.
• A water pump pulls raw water from below the hull, routes it through a heat exchanger, and discharges it overboard.
• The closed-loop coolant exchanges heat with the raw water in that exchanger, then returns to the engine.

The system is sealed, predictable, and works inside its design envelope. The design envelope is a stock engine producing stock boost in moderate ambient water temperature.

When you push past that envelope — and 300 HP supercharged in 85°F water is past it — the heat exchanger becomes the bottleneck. Coolant temps climb above the threshold where the ECU starts protecting the engine. You lose timing advance. You lose boost target. You lose horsepower.

What an open loop cooling conversion does

A **Sea-Doo 300 open loop cooling kit** (often called OLC) routes raw water from the same intake the heat exchanger uses — directly through a billet cover that mounts in place of the closed-loop coolant outlet, then through the cylinder head water passages, and overboard.

The result:

• **No closed-loop coolant in the heads.** The heads are cooled directly by the lake or ocean the boat is running in.
• **Significantly higher cooling capacity** — raw water flow into the heads is dramatically larger than what the closed-loop side could deliver through the heat exchanger.
• **Lower equilibrium coolant temperature under sustained load.** The ECU's heat-protection routines stay out of the picture across a wider operating envelope.

The trade-offs you should know:

• **Raw water in the engine.** In saltwater, this requires a disciplined post-ride flush. Most owners flush every session anyway; the OLC kit makes it a hard requirement.
• **The closed-loop coolant system is bypassed**, not removed. The OLC kit replaces the coolant path with a raw-water path. Reverting to stock is a kit swap.
• **The engine runs slightly cooler than its design temperature.** This is generally good for power and longevity, but it does require the ECU calibration to be sane about the colder operating point. Most aftermarket Stage 2+ tunes already account for it.

The honest version: open loop cooling is the right answer for performance Sea-Doo 300 builds. It is not the right answer for a daily-rider GTX cruising at half-throttle who flushes their boat once a month.

Who should buy a Sea-Doo 300 cooling kit

Buy the OLC kit if:

• Your 300 HP Sea-Doo is supporting a Stage 2 or Stage 3 build (intake + intercooler + tune + exhaust).
• You ride in warm-water environments and pull timing under sustained WOT.
• You run a smaller supercharger pulley or any calibration that increases boost target.
• You're chasing a top-speed program where the last 3–5 MPH lives in the calibration headroom that heat steals.

Don't buy the OLC kit if:

• You ride a bone-stock craft in cool freshwater. The stock cooling system is adequate.
• You can't or won't commit to post-session raw-water flushing in saltwater. The kit will work fine; the engine internals will not.
• You're shopping for a "boat-and-trailer" recreational craft. Save the money.

Comparison: Sea-Doo 300 cooling kit market

The open loop cooling kit category has four players worth comparing. Real differences. Honest table.

The honest read: there are three legitimate open-loop conversion kits in the market plus the OEM closed-loop part. The category-defining brands all use billet aluminum, marine hoses, and serve the same platform. The differentiation is price, support model, and how cleanly the kit drops into the engine bay.

Where GT40Marine wins: **price-to-spec ratio**. $384.99 for a premium billet kit lands $35 under RIVA and roughly half the Fizzle SuperCooler. Manufacturer-direct fulfillment with engineer support.

Where the others may win: dealer-channel coverage if you want a local installer relationship. RIVA's dealer network is the deepest in PWC. Fizzle has the most aggressive marketing.

Real-world expectations from a Sea-Doo 300 OLC install

We won't publish a fabricated dyno number on this page. The gain from an OLC kit depends entirely on (a) how heat-limited your build was before, (b) ambient water temperature, (c) duration of WOT runs, and (d) the rest of the stack.

What you can expect to feel:

• **Sustained top speed holds longer.** Where a stock 300 starts losing 2–4 MPH after 60–90 seconds of WOT in warm water, an OLC-equipped 300 holds peak speed substantially longer.
• **Acceleration consistency on hot days.** Bay or ocean runs where the engine bay was heat-soaking now feel the same in the third run as the first.
• **Higher ceiling on Stage 2+ tunes.** Calibrators can map for the cooler operating temperature, releasing timing and boost that the stock heat envelope wouldn't allow.

The OLC kit is a cooling capacity unlock, not a horsepower additive. It releases the horsepower you bought when you spent $20,000+ on a 300 platform and discovered the cooling system was the leash.

Pairing the Sea-Doo 300 OLC kit with the rest of your stack

The OLC kit lives inside a complete Stage 2+ build. The components that pair with it natively:

**Ribbon Delete Kit.** The OEM ribbon plays a role in noise and water management but introduces airflow restriction. A [ribbon delete](/learn/sea-doo-ribbon-delete-kit) removes that restriction in a way that complements the cooling and airflow improvements from the rest of the stack. Together they're often sold as a paired upgrade for serious 300/325 builds.

**Sea-Doo 325 Stage 1 Performance Kit.** Our [325 Stage 1 kit](/products/sea-doo-325-stage-1-performance-kit) is the matched starting point for buyers stepping into the platform. OLC + Stage 1 + a calibrator tune is the highest-ROI three-component package on the 300/325 platform.

**Intercooler upgrade.** An upgraded intercooler drops post-supercharger intake air temperature; the OLC kit drops engine coolant temperature. Together they address the two thermal limits that hold back a hard-driven 300.

**Titanium exhaust.** Removes the exhaust-side thermal mass that contributes to engine bay heat-soak. A [titanium exhaust](/learn/sea-doo-titanium-exhaust) is the natural Stage 3 follow-on after the OLC kit is in place.

Install considerations

The OLC kit is a moderate-difficulty job. Most owner-installers complete it in 2–3 hours with hand tools, a drain pan, and a clean workspace. Professional install at a marine shop is typically 1.5–2 hours.

What we won't publish: torque specs on the head cover fasteners, clamp sequencing on the marine hoses, or step-by-step routing for the raw-water feed. Those values come from the BRP service manual or directly from GT40Marine's engineering team. A wrong torque value on a head-adjacent fastener is a head-gasket failure waiting to happen.

What we will tell you:

• The kit includes every clamp and fitting needed for a clean install on the supported platforms.
• Marine-grade hoses are pre-cut to length for the standard routing.
• Engineer support is direct — call or text us during the install if you need a value or a sanity check.
• Post-install, run the boat at idle on a hose flush and verify raw water is flowing through the OLC circuit before launching.

Saltwater operating discipline

If you ride in saltwater, the OLC kit elevates the importance of post-session flushing from "good practice" to "mandatory." Raw saltwater is now circulating through the head passages.

Standard discipline:

• Flush every session with the OEM hose flush port.
• Run the engine on the flush for the manufacturer-recommended duration.
• For long storage or end-of-season, follow the OEM winterization or anti-corrosion procedure.

Followed properly, the kit is durable for years in saltwater service. Skipped, the engine will tell you about it on a timeline of months, not years.

Frequently asked questions

Bottom line

The 300 HP 1630 ACE is a serious engine running a cooling system designed for the constraints of a 2018-era recreational craft. If you ride the boat the way it can be ridden, the cooling system becomes the limiter before the engine does.

The GT40Marine Sea-Doo 300 cooling kit is the right answer for that build — billet aluminum, marine hardware, fits 230 and 300 platforms, manufacturer-direct at $384.99, and engineered to live inside the rest of the GT40 stack.