Why Consider Smart Roadster Cylinder Head Porting?
The Smart Roadster 452 is a remarkable machine — 698cc, three cylinders, a Mitsubishi-derived turbocharged engine mounted mid-rear, and yet capable of delivering genuinely entertaining performance when properly developed. But the factory cylinder head is a compromise, designed for mass production tolerances rather than optimal gas flow. Smart Roadster cylinder head porting is the process of reshaping, smoothing and enlarging the intake and exhaust ports to reduce turbulence, lower pumping losses and increase the volume of charge entering and leaving the combustion chamber. For a small-displacement engine relying heavily on forced induction, every incremental improvement in volumetric efficiency matters. This article covers what porting involves, what gains are realistic, how it interacts with your turbo and ECU, and whether the investment makes sense for your specific build.
Understanding the 698cc Head: Stock Limitations
The three-cylinder engine in the Smart Roadster shares its lineage with Mitsubishi’s 3B2 family, and whilst it is a robust unit, the cylinder head casting reflects the priorities of a city car rather than a sports machine. Port cross-sections are modest, surface finish inside the ports is rough from casting, and the transition between the port and valve seat is rarely optimised for smooth flow. The combustion chamber itself is generally well shaped for a wedge design, but gasket mismatch between the head and inlet manifold is common — a misaligned step here acts as a significant flow obstruction. Valve sizes are conservative: the head was designed to work across a wide torque band at low revs rather than to breathe freely at higher engine speeds. These limitations become more apparent as boost pressure rises, because the turbocharger can only pressurise charge air to the point where the head itself becomes the bottleneck. If the ports cannot accept the volume being offered, you are building pressure in the manifold unnecessarily.
What Cylinder Head Porting Actually Involves
Port Matching
The most cost-effective first step is port matching — aligning the inlet manifold gasket face to the head ports so there is a continuous, flush transition. Any step or lip at the joint causes flow separation and turbulence. This requires a gasket as a template, careful scribing, and removal of material with a die grinder. It is achievable by a careful home mechanic with the right tooling and patience, and the improvement in flow can be significant relative to the effort involved.
Inlet Port Work
Beyond port matching, inlet port work involves blending the port shape to remove casting marks, enlarging the port cross-section in areas where the casting is thick, and improving the radius of curvature at the short-side turn — the inside of the bend as the port curves towards the valve. This is where flow separation is most likely to occur. Material removal here must be controlled: removing too much shortens the port and changes the resonance characteristics. The goal is a smooth, slightly enlarged port with good velocity through the valve curtain area. Do not port match to the valve seat diameter itself — maintaining a slight narrowing just before the valve is intentional and aids atomisation.
Exhaust Port Work
Exhaust ports benefit from similar treatment, though the priority here is removing obstructions and smoothing rather than maximising cross-section. Exhaust gases are hot and moving fast; large cross-sections at low flow velocities can actually reduce scavenging efficiency. A moderate clean-up, valve guide boss blending, and smooth transition to the exhaust manifold gasket face is the appropriate approach for a road-going engine. Aggressive exhaust porting is more relevant to naturally aspirated engines than to turbocharged applications.
Valve Seat Cutting and Valve Work
To realise the full benefit of port work, the valve seats should be re-cut with a multi-angle profile — typically a three-angle cut creating a narrow primary seat, a wider back cut, and a throat cut. This improves flow coefficient across the valve at all lift points and is particularly effective at low lift, where the Smart Roadster’s relatively modest cam profile operates most of the time. Having the valves refaced or replaced at the same time is sensible economics when the head is already off.
Flow Bench Testing: What Numbers to Expect
A cylinder head flow bench measures airflow in CFM (cubic feet per minute) at a standardised test pressure — commonly 28 inches of water — across the full range of valve lift. The stock Smart Roadster head flows modestly compared with dedicated performance engines, which is expected given the small port sizes. A well-executed port and polish on the 3B2-derived head can improve inlet flow by 10–18% at peak lift, with larger gains at lower lift points where the stock geometry is least optimised. Exhaust flow improvements are typically smaller — 5–10% — which is appropriate given the turbocharger’s role in managing exhaust backpressure. If you do not have access to a flow bench, a reputable engine builder can flow the head before and after work, giving you documented evidence of improvement. Tracking boost pressure throughout your build is equally valuable — a properly flowed head combined with appropriate boost will show a measurable reduction in intake manifold pressure required to achieve the same charge mass, which is a direct indicator of improved volumetric efficiency.
Integration with Turbo and ECU Remapping
Cylinder head porting does not exist in isolation. The Garrett 1238S turbocharger fitted to the Smart Roadster is a fixed-geometry unit with a pneumatic wastegate, and it is already working reasonably hard in standard 60kW trim. Improving the head’s flow characteristics means the turbo reaches target boost with less effort and at a slightly lower threshold, which can improve spool-up response. However, the full benefit of head work will only be unlocked if the ECU mapping is updated to take advantage of the improved breathing. Running a modified head on a standard map leaves considerable power on the table, because the fuelling and ignition tables are calibrated around the stock engine’s volumetric efficiency curve. A proper ECU remap calibrated to your engine’s actual state of tune is therefore not optional if you want to see the gains from head work reflected in real-world performance. The wastegate actuator should also be inspected and confirmed serviceable before any power increase, since a tired actuator will allow boost to creep unpredictably once the engine is breathing more freely.
It is also worth noting that the Softouch automated manual gearbox has its own tolerance for torque increases. A cylinder head build combined with a remap is a meaningful power step, and the gearbox actuator calibration should be considered as part of a holistic build plan. If the gearbox is hesitating or hunting under load, address that before adding power. You can learn more about how the Softouch actuator responds to torque increases to understand what to expect and plan your build accordingly.
Costs, Realism and Build Priority
Cylinder head porting on a 698cc engine is labour-intensive relative to the displacement involved. A competent engine builder who has worked on small-bore three-cylinder heads will typically charge 4–8 hours of labour for a full port and polish including valve seat cutting. Expect to pay £300–£600 in the UK for thorough work, plus valve and seal components. If you are also having the head skimmed for flatness — which is advisable whenever the head is removed — add another £60–£100. This is not cheap for a car that may have cost you £8,000–£12,000, and you must be honest about the return on investment. Porting alone, on a stock turbo and standard map, will deliver a modest seat-of-the-pants improvement that is difficult to isolate from other variables. As part of a complete build — new turbo or uprated actuator, remap, intercooler, and free-flowing exhaust — head work contributes meaningfully to the overall package and should not be overlooked. For anyone building towards 100hp or beyond, it is close to essential. For a lightly modified daily driver, boost and mapping changes deliver more measurable improvement per pound spent. Understand the full specification of your car before committing — checking your VIN to confirm your exact build variant will clarify whether you have the oil-cooled engine (60kW and above) necessary to support sustained high-performance use.
Practical Advice Before You Start
- Remove the head carefully. The Smart Roadster engine is mid-rear mounted; access requires removing the engine cover, disconnecting ancillaries and unbolting the head from the rear of the car. Allow a full day for a first-time removal.
- Check for cracks. Have the head pressure-tested before spending money on porting work. A cracked head is scrap.
- Do not over-port. Removing too much material weakens the head, changes port velocity unfavourably, and can break through into coolant passages. Less is often more.
- Document everything. Photograph port shapes before and after. If you have access to a flow bench, use it at both stages.
- Use a proper gasket set. Replace all head gaskets, valve stem seals and head bolts (which are torque-to-yield and must not be reused).
- Torque correctly. The 3B2-derived head is sensitive to uneven clamping. Follow the torque and angle-tightening sequence precisely.
Is Smart Roadster Cylinder Head Porting Worth It?
If you are building a serious Smart Roadster performance engine — targeting 100hp or more from the 698cc unit — then smart roadster cylinder head porting is a genuinely worthwhile investment that unlocks headroom no amount of additional boost can fully recover. The work improves the fundamental breathing of the engine, reduces intake restriction, and allows the turbocharger and ECU to work more efficiently. Approached as part of a comprehensive build rather than a standalone modification, ported head work will contribute to a measurable improvement in throttle response, mid-range torque and top-end power. Do it properly, remap afterwards, and you will have an engine that punches considerably harder than the factory ever intended from this diminutive three-cylinder.
