Why I Added a Setrab Oil Cooler to My Mazda MX‑5 – and Got 8 hp for It

I’ve owned my Mazda MX‑5 (NB) for three seasons, and like any enthusiast, I keep looking for honest power gains without forced induction. After reading about the effects of heat soak on the BP‑series engine, I decided to install a Setrab oil cooler. The result: a dyno‑verified 8‑horsepower increase, lower oil temperatures, and noticeably sharper throttle response. This article covers the exact costs, component choices, installation process, and the science behind the gain. If you are considering an oil cooler for your Miata, this real‑world breakdown will help you decide.

Why Oil Temperature Matters More Than You Think

Most MX‑5 owners pay close attention to coolant temperature but ignore oil temp until it’s too late. Engine oil serves as a lubricant, a coolant for bearings and pistons, and a hydraulic fluid for Variable Valve Timing (if equipped). As oil heats above 120 °C (248 °F), its viscosity drops significantly. Thinner oil cannot maintain a stable film between moving parts, leading to increased boundary friction and metal‑to‑metal contact. That friction robs power.

On a stock NB Miata with the 1.8‑liter BP‑Z3, oil temperatures can hit 130 °C after 20 minutes of aggressive back‑road driving, especially on a warm day. At that point, the ECU may also pull ignition timing if intake air temps are high. The combination of thin oil and retarded timing costs 6–12 hp compared to operating at 90–100 °C. A properly sized oil cooler brings that temperature back under control, allowing the engine to run more efficiently and safely.

External resources explain this in more detail. Flyin’ Miata offers thorough guides on oil cooler selection for track‑driven NBs, and 949 Racing discusses the thermostat and fitting layout you need for daily driving.

Why Setrab? A Quick Brand Breakdown

Setrab is a Swedish manufacturer known for high‑quality bar‑and‑plate oil coolers. Bar‑and‑plate cores transfer heat more efficiently than cheaper tube‑and‑fin designs because the oil flows through a series of parallel plates with turbulators. This creates more surface area and better heat rejection per square inch. Other brands like Mocal and Earl’s perform well, but Setrab’s stamped end tanks and welded construction tend to resist fatigue under high pressure. I chose a Setrab 25‑row 619 (width 285 mm, height 175 mm, depth 52 mm) because it fits in the lower grill area of an NB without cutting the bumper support.

For my setup, I paired it with:

  • Thermostatic sandwich plate (Setrab 50‑170‑61) – essential for street driving to avoid over‑cooling during winter.
  • ‑10 AN braided stainless hoses – flexible, heat resistant, and much easier to route than hard lines.
  • Remote oil filter adapter – optional but helpful because the sandwich plate can push the filter into the subframe on some NBs.
  • Racing brackets (2x rubber‑isolated mounts) to keep the cooler off the condenser.

Full Cost Breakdown (August 2024 Prices)

Prices fluctuate, but here is what I paid for a complete system with no reused parts. Note that shop labor is often $100–$200 extra; I did the work myself.

ComponentCost (USD)
Setrab 25‑row oil cooler (619‑3258)$205
Thermostatic sandwich plate (Setrab 50‑170‑61)$79
‑10 AN braided hose (12 ft)$60
‑10 AN push‑lock fittings (6 pieces)$45
Mocal remote filter mount$38
Racing brackets + hardware$22
Oil (5W‑30, 2 qt extra for refill)$12
Total (no labor)$461

If you go without a thermostat and reuse existing hoses, you can bring the total closer to $300. But I recommend including the thermostat plate; a fully open cooler in cold weather can keep oil below 75 °C, causing excessive engine wear and poor fuel economy. The $79 piece is cheap insurance.

Step‑by‑Step Installation for NB MX‑5

Below is the method I used. The same process applies to NA models with minor bracket modifications.

Tools and Supplies

  • Socket set (10 mm, 12 mm, 14 mm, 17 mm)
  • Torx bits (T25, T30 for under‑tray)
  • Oil drain pan
  • Razor blade for old gasket removal
  • Hose cutter or sharp knife
  • Thread sealant (Loctite 567) on pipe threads
  • Zip ties and split loom

Step 1 – Drain Oil and Remove Stock Filter

Warm the engine slightly, then drain oil. Remove the oil filter. If you are using a remote filter mount, now is the time to bolt it to a solid chassis location. I attached mine to the driver‑side frame rail behind the bumper support.

Step 2 – Install the Thermostatic Sandwich Plate

Clean the block surface where the filter normally seats. Apply a thin layer of oil to the O‑ring on the sandwich plate, then screw it on. Torque to about 18 lb‑ft. The plate has two ‑10 AN ports: one for the cooler feed (out), one for return (in). Follow the arrow markings on the base.

Step 3 – Mount the Cooler

Position the Setrab cooler in the lower mouth of the nose. The ideal location is centered between the license plate bracket and the lower tie‑bar. Use two brackets to secure it to the chassis rails. I placed rubber bushings between the brackets and the cooler to reduce vibration. Ensure the cooler does not interfere with the hood latch or A/C condenser lines.

Step 4 – Route the Hoses

Measure hose lengths loosely; it’s better to cut twice than too short. Route the feed hose from the sandwich plate to the lower port on the cooler, and the return hose from the upper port back. Keep hoses away from hot exhaust components and sharp edges. Use split loom where they pass near the radiator core support. Secure with zip ties every 6 inches.

Step 5 – Connect and Refill

Push the hoses onto the AN fittings and tighten with a wrench (push‑lock: no clamps needed if hose is correct size). Reinstall the drain plug and fill with fresh oil. Run the engine for 30 seconds, then top off oil.

Step 6 – Leak Check and Bleed

Inspect every connection while the engine idles. Rev to 2500 rpm for a few seconds. Check the sandwich plate gasket. Drive gently for 5 minutes, then re‑torque fittings if necessary. Most leaks happen at the sandwich plate threads. A dab of sealant on the adapter’s NPT threads prevents seepage.

The 8‑Horsepower Gain – Dyno Evidence

I made a baseline pull on a Dynojet before the install: 116 whp (stock NB2, 151 000 km, intake only). After the cooler and a 20‑minute cool‑down, the oil had stabilized at 98 °C versus the previous 125 °C. The dyno then showed 124 whp. The 8‑hp gain came from two combined effects:

  • Reduced friction: Oil viscosity remained in the 9–11 cSt range (instead of 5–6 cSt), lowering internal drag on bearings and piston rings.
  • Stable ignition timing: The ECU no longer pulled timing due to high intake air temps (the cooler also lowered under‑hood temps slightly).

A third factor: the cooler oil helps maintain VVT actuation on NB2 engines. When oil is too thin, the VVT spool valve can’t advance timing fully, costing low‑end torque. After the install, I felt a stronger pull from 3000 to 4500 rpm. This aligns with data from EngineLabs’ article on oil temperature and viscosity.

Real‑World Driving Impressions

On the street, the biggest difference is on‑ramps and mountain passes. Previously, after three hard corners the oil temp would creep toward 130 °C and the engine felt flat. Now it stays at 90–95 °C even in summer. The car pulls harder exiting slow corners. On a 34 °C day at the track, oil temp never exceeded 110 °C. That gives me confidence to push harder for longer sessions.

One unexpected benefit: the engine stops pinging on 91‑octane fuel. Lower oil temp means lower coolant temps (through better heat rejection), which helps the knock sensor. I no longer see any knock retard on the OBD‑II scanner.

Potential Downsides and How to Avoid Them

Over‑Cooling in Winter

Without a thermostat, oil may struggle to reach 80 °C in cold weather. This causes slow engine warm‑up, increased fuel consumption, and possible condensation buildup. The thermostatic plate solves this. I notice in 5 °C weather my oil takes 15 minutes to reach 75 °C, but the thermostat bypasses the cooler until it fully opens at 82 °C. On a short commute, the oil never needs active cooling.

Extra Point of Failure

More hoses and fittings mean more potential leak points. Use quality AN fittings and double‑check torque. After 12 months and 10 000 km, my installation has zero leaks. Carry a spare O‑ring for the sandwich plate if you track the car.

Reduced Ground Clearance

If you mount the cooler too low, it may hit speed bumps. I installed mine just behind the front bumper lip, roughly 5 inches above the ground. No issues with normal driving, but caution on steep driveways.

Is an 8‑hp Oil Cooler Worth It?

For track‑day use or aggressive canyon runs, absolutely. The gain is repeatable and not dependent on premium fuel or aggressive tuning. Combined with a Setrab cooler’s build quality, the system will outlast your engine. For a pure street car that rarely sees high loads, the gain may be less than 8 hp because oil temps rarely spike to dangerous levels. Still, even a 4–5 hp gain and reduced engine wear justify the $460 investment.

If you decide to install one, take your time with routing and always test for leaks. The DIY process is satisfying, and hearing the engine pull harder without heat‑soak is addictive. For more Miata‑specific tips, I recommend the Miata.net Garage article on oil cooler installation.

The Setrab oil cooler is not a bolt‑on miracle that will double your horsepower. But it is one of the smartest reliability‑ and performance‑mods you can make. My MX‑5 is faster, smoother, and (most importantly) happier.