Sea-Doo Open Loop Cooling Explained

Sea-Doo factory cooling is a closed-loop architecture: coolant circulates inside the engine, sheds heat to raw water through a heat exchanger, and returns. It's the right architecture for a manufacturer optimizing for emissions, serviceability, and worst-case environmental contamination across millions of stock craft worldwide.

It is not the right architecture for a 300 HP supercharged 1630 ACE running sustained WOT in 85°F water.

**Sea-Doo open loop cooling** — usually abbreviated OLC — is the aftermarket conversion that replaces the closed-loop coolant path through the heads with a direct raw-water cooling circuit. More cooling capacity, lower equilibrium temperatures, more headroom for the calibration. It's the most important supporting upgrade on serious 1630 ACE builds, and it's the upgrade that makes a tuned Sea-Doo actually feel tuned in real-world conditions.

This page covers the engineering — what closed loop is, what open loop changes, the trade-offs, and how to think about whether your craft needs it. If you've decided you need it, see our [Sea-Doo 300 cooling kit pillar](/learn/sea-doo-300-cooling-kit) for the GT40Marine product detail.

How Sea-Doo factory cooling works

The 1630 ACE leaves the factory with two cooling loops:

**The closed loop (engine coolant).** A 50/50 antifreeze mix circulates inside the engine — through the cylinder block, the heads, and the thermostat — driven by an internal coolant pump. This is the loop that actually carries heat away from combustion.

**The open loop (raw water).** A water pump driven off the engine pulls raw water from the hull intake. That raw water flows through a heat exchanger where it absorbs heat from the closed-loop coolant. Then it routes through the exhaust manifold to absorb exhaust-side heat, and discharges overboard through the pump outlet.

The two loops never mix inside the engine. The heat exchanger is the thermal interface.

The design optimizes for several things at once: predictable operating temperature regardless of ambient water condition, no risk of mineral or salt contamination inside the cylinder block, and a serviceable architecture that any BRP dealer can work on. It's a sound design for the 90% of Sea-Doo owners running stock craft in moderate conditions.

It has one weakness: cooling capacity. The heat exchanger is a bottleneck. The closed-loop coolant can only shed heat as fast as the exchanger transfers it to the raw-water side. On a stock craft, that capacity is correct. On a tuned craft with elevated boost, increased timing advance, and sustained WOT in warm water, the exchanger becomes the limiter.

How open loop cooling changes the architecture

A Sea-Doo open loop cooling conversion removes the closed-loop coolant from the head cooling path and routes raw water directly through the head water passages.

In practice:

• A billet aluminum cover replaces the OEM closed-loop coolant cover on the cylinder head.
• Raw water from the existing hull intake is routed through the new billet cover, through the head passages, and overboard.
• The closed-loop coolant system in the block is bypassed (the kit re-routes the coolant flow so it doesn't dead-head).

The thermal result:

• **Cooling capacity increases substantially.** Raw water flow rate through the heads is materially higher than what the heat exchanger could transfer.
• **Equilibrium head temperature drops** under the same load.
• **The ECU's heat-protection routines** — timing pull, boost limit, fuel enrichment — stay out of the picture across a wider operating envelope.

The architectural result:

• **Raw water now touches the heads.** In saltwater, this requires disciplined post-session flushing.
• **The closed-loop coolant reservoir** in the block still exists but no longer participates in head cooling.
• **The block runs slightly cooler than its OEM design temperature.** Modern aftermarket calibrations for Stage 2+ builds account for this.

Why this matters: thermal limits define horsepower

A Sea-Doo's published horsepower number — 230 / 300 / 325 — is the engine's output under defined test conditions. Real-world output is whatever the ECU lets the engine produce after the heat-protection logic runs.

The protection logic does several things, in order:

1. As coolant temp climbs above target, the ECU pulls ignition timing. 2. As intake air temp climbs, the ECU lowers boost target. 3. As exhaust gas temp climbs, the ECU enriches the fuel mixture (which lowers efficiency). 4. If any of those climb past the safety threshold, the ECU enters a limp mode.

A stock 1630 ACE in 70°F water rarely sees any of these. A tuned 1630 ACE in 85°F water at sustained WOT can see all four in a 15-minute session. Every degree above target is horsepower the calibration is being forced to hide.

Open loop cooling addresses the root cause of (1) and indirectly improves (3). The intercooler addresses (2). Combined, they release the calibration to actually deliver the power the engine was built to make.

Trade-offs you should know

Open loop cooling is the right answer for serious builds. It comes with operating discipline you need to commit to:

**Saltwater post-session flushing is mandatory.** Raw saltwater is now in the head passages. The kit uses marine-grade billet aluminum, the head passages are designed for water flow, and the architecture is durable — but only with disciplined flushing. Skip it and you'll see corrosion artifacts on a timeline of months, not years.

**Freshwater is more forgiving.** The discipline is still worth it (mineral content, biological growth), but the consequences of skipping a flush are smaller.

**Engine warm-up is slightly slower in cold water.** Without the closed-loop thermostat regulating temperature ramp-up, the engine takes a little longer to reach operating temperature on a cold-start in cold water. For competition starts this matters; for recreational riding it doesn't.

**Some warranty conversations get harder.** BRP's stated position is that aftermarket parts only void coverage for components proven to be damaged by the modification. In practice, any future cooling-system warranty discussion involving an OLC-converted craft is a case-by-case conversation with your dealer.

**Reversion to stock is a parts swap, not a permanent commitment.** The kit replaces a cover, not the head castings or block. If you sell the craft and the new owner wants factory cooling back, the OEM cover and coolant routing go back in.

Who should run open loop cooling

Run OLC if:

• You have a Stage 2 or Stage 3 build on a 230 / 300 / 325 platform.
• You ride in warm water, ride hard, and want sustained top speed.
• You're working with a calibrator who can map for the cooler operating point.
• You can commit to disciplined post-session flushing.

Don't run OLC if:

• Your craft is bone-stock and you ride at half-throttle on weekends.
• You can't or won't flush after saltwater sessions.
• You're shopping for a recreational cruiser, not a performance build.

Open loop cooling: market overview

There are three legitimate OLC kit manufacturers for the 1630 ACE family. Real differences below.

The three kits are functionally comparable — they all convert the head cooling to raw-water flow, they all use billet aluminum covers, and they all serve the same supported HP variants. The honest differentiators are price (GT40Marine is the value position at $384.99), support model (direct vs dealer-channel), and the rest of the ecosystem the kit lives inside.

Install considerations

The OLC conversion is a moderate-difficulty job:

• **Owner install:** typically 2–3 hours with hand tools, a drain pan, and an organized workspace.
• **Professional install:** 1.5–2 hours at a marine performance shop.
• **Tools needed:** standard metric socket set, torque wrench, hose-pliers, drain pan.

The job involves working near the cylinder head. The kit ships with fitment notes specific to your platform; engineer support is direct (call, text, email) if you hit a question.

We don't publish the head-cover torque values or the clamp sequencing on this page. Those values come from BRP's service manual or directly from GT40Marine engineering. A wrong torque value on a head-cover fastener can cost a head gasket. We supply those numbers to buyers, vetted against the specific craft.

After install:

1. Verify raw water is flowing through the OLC circuit at idle on a hose flush. 2. Run the craft at idle on the flush for the manufacturer-recommended duration to confirm no leaks. 3. Take a short shakedown ride at moderate throttle before committing to a hard session. 4. Establish the post-session flush habit from day one.

How OLC pairs with the rest of the stack

Open loop cooling is most valuable inside a complete build:

**With an aftermarket [intake](/learn/sea-doo-air-intake):** the intake removes airflow restriction. The OLC removes the thermal limit that the new airflow exposes.

**With a [ribbon delete kit](/learn/sea-doo-ribbon-delete-kit):** the ribbon delete cleans up the intake tract. The OLC gives the engine the cooling capacity to handle the elevated boost the delete enables.

**With a Stage 2+ ECU calibration:** the calibration maps for the cooler operating point. Without the cooler operating point, the calibrator has to write a conservative map that protects the engine from heat.

**With a [titanium exhaust](/learn/sea-doo-titanium-exhaust):** removes exhaust-side heat from the engine bay. Combined with OLC, the engine bay thermal envelope drops substantially.

Frequently asked questions

Bottom line

Open loop cooling is the architectural change that lets a 1630 ACE deliver the horsepower it's calibrated for under real-world conditions. It is the most important supporting upgrade on serious 230 / 300 / 325 HP builds.

The GT40Marine OLC kit is the direct path to that conversion — premium billet hardware, marine-grade hoses, manufacturer-direct support, and a price point ($384.99) below the dealer-channel norm.