A smart roadster NA conversion – removing the turbocharger and running the 698cc three-cylinder on natural aspiration – is technically possible but yields roughly 35–42 bhp, far below any standard variant. The conversion offers no practical benefit and is almost universally considered a retrograde modification by engineers and experienced owners.
The question comes up more often than you might expect. A damaged turbocharger, a confusing fault code, or a misguided belief that ‘simpler is more reliable’ leads some owners to wonder whether the Smart Roadster 452 could be run without its forced induction system entirely. This article examines the engineering reality of a smart roadster NA conversion, what power you would actually get, what components would need to change, and – crucially – why the turbo is not the fragile liability many owners fear. By the end, you will have a thorough, honest answer to every aspect of this question.
Understanding the 698cc Engine’s Relationship With the Turbo
The Smart Roadster’s M160 698cc three-cylinder is a Mitsubishi-derived unit that was engineered from the outset around forced induction. Its compression ratio sits at approximately 8.8:1 across the turbocharged variants – deliberately low to accommodate boosted charge pressure safely. A naturally aspirated engine of this displacement would typically run a compression ratio of 10.5:1 to 12.0:1 to extract useful power. This immediately identifies the first major obstacle of any smart roadster NA conversion: the pistons are the wrong specification for naturally aspirated operation.
On boost, the 60kW standard variant produces 61 PS and 100 Nm. Remove the turbo, retain the standard 8.8:1 pistons, and you are looking at a specific output typical of a late-1990s city car engine – around 40 bhp at best. The engine will run, but performance would be worse than a first-generation Smart City Coupé. On a two-seat sports car weighing roughly 790 kg, the result would be barely driveable on anything other than flat urban roads.
What a Smart Roadster NA Conversion Would Actually Require
For the conversion to even approach reasonable naturally aspirated power, the following changes would be mandatory:
- Higher-compression pistons: Standard forged or cast units suitable for NA operation at 10.5:1 or above. These are not off-the-shelf items for this engine; custom manufacture or sourcing from related Mitsubishi applications is required.
- Revised camshafts: The standard cam profiles are optimised around turbo response and mid-range torque under boost. NA operation benefits from higher lift and longer duration profiles. If you want to understand the scope of that challenge, our detailed look at what aftermarket camshaft options actually exist for this engine makes sobering reading.
- Complete intake redesign: The intercooler, boost pipes, turbo inlet, and associated plumbing all become redundant. A new throttle body arrangement, intake manifold and air filter assembly would be needed.
- Full ECU remap: The Bosch MEG 1.1 ECU contains boost maps, fuelling tables, and ignition advance curves all calibrated around forced induction. Without a turbo, every table would need rewriting from scratch. This is not a simple remap; it is essentially a clean-sheet calibration exercise.
- Exhaust manifold change: The current manifold feeds the Garrett 1238S turbine housing. An NA exhaust manifold would need to be fabricated or sourced.
The combined cost of parts, machining, fabrication, and ECU development would comfortably exceed £3,000–£5,000 for a competent execution – to end up with a car producing less power than the cheapest turbocharged standard car in standard form.
The Turbo Is Not the Reliability Problem Owners Think It Is
Much of the motivation for exploring a smart roadster NA conversion stems from a belief that the turbocharger is a fragile, expensive, and troublesome component. The reality is more nuanced. The Garrett 1238S fitted to the Roadster is a compact, fixed-geometry unit running at relatively modest pressure ratios by modern standards. It is not a high-strung performance turbo; it is a durable, conservatively rated piece of engineering.
The most common turbo-related complaints on the Smart Roadster are usually not turbo failures in the mechanical sense. They are boost control faults – typically a failed pneumatic actuator or a cracked boost pipe – which are cheap and straightforward to fix. Owners who believe their turbo has failed often discover the actuator diaphragm has split, a £15–£40 component. True turbo bearing or compressor wheel failures are comparatively rare on well-maintained, oil-change-compliant cars.
If your interest in going NA is driven by a boost fault, address the fault. The cost and outcome are incomparably better than a conversion.
Smart Roadster NA Conversion: The Power Deficit in Numbers
To illustrate precisely what would be lost, here is a comparison of real-world power figures:
- 45kW Lite (standard): 61 PS, 0.89 bar boost
- 60kW standard: 82 PS, 1.09 bar boost
- 66kW SB2 Brabus: 90 PS, 1.33 bar boost
- 74kW full Brabus: 101 PS, 1.43 bar boost
- NA conversion (estimated): 38–42 PS, 0 bar boost
The relationship between boost pressure and the torque the engine develops is direct and well-documented for this engine family – you can explore how boost pressure translates into torque on the 698cc to appreciate just how much work the turbo is doing. Without it, you are left with a tiny displacement engine producing power commensurate with that tiny displacement – roughly what a modern 650cc motorcycle makes in touring trim, but in a car chassis.
Are There Any Legitimate Reasons to Consider Going NA?
Intellectual honesty requires acknowledging that edge cases exist. Possible reasons someone might still consider a smart roadster NA conversion include:
Track-Specific Regulations
Certain club motorsport categories enforce NA-only classes for small-displacement vehicles. If you are building a dedicated track car for such a series and cannot run a turbo legally, the conversion makes contextual sense. Even then, you would need the high-compression piston and cam work described above to be competitive.
Experimental or Engineering Interest
Some owners pursue conversions purely for the technical challenge and the knowledge gained. This is a completely valid motivation. Tuning a small-capacity NA engine on a standalone ECU is a rich learning exercise. Just enter it with realistic expectations.
Catastrophic Turbo Failure With No Replacement Available
If the Garrett 1238S were ever to become genuinely unobtainable – which it currently is not – running NA temporarily could keep a car mobile. In practice, remanufactured and new-old-stock units remain available from multiple suppliers, so this scenario is largely hypothetical.
For any other motivation – reliability, simplicity, cost, performance – the conversion makes no engineering sense whatsoever.
What to Do Instead: The Real Alternatives
If you are dissatisfied with your Roadster’s performance and are tempted by a radical modification, the answer almost certainly lies in the opposite direction: making better use of the turbo you already have, not removing it.
Before exploring mapping and hardware upgrades, it is worth understanding how air-fuel ratio interacts with power and safety under boost – this explanation of lambda and fuelling on a turbo triple gives the theoretical foundation. For owners on the 45kW Lite variant, there is also a well-documented path to bring the car up to full 60kW specification, including the oil cooler that the Lite shamefully omits. Both routes preserve the turbo, work with the engine’s design intent, and deliver genuine improvements at a fraction of the cost and complexity of an NA conversion.
The Smart Roadster’s turbo is not a liability to be removed. It is the single component that transforms a 698cc city car engine into something worth fitting to a sports car. Appreciate it, maintain it, and – if you want more – tune it properly.
Frequently Asked Questions
How much power would a naturally aspirated Smart Roadster produce?
With the standard 8.8:1 compression pistons and no other changes, an NA Smart Roadster would produce approximately 38–42 PS. With custom high-compression pistons and revised camshafts, perhaps 50–55 PS could be achieved, but at very high cost and complexity.
Can the Bosch MEG 1.1 ECU be remapped for NA operation?
Technically yes, but it requires a near-total recalibration of fuelling, ignition advance and all boost-related tables, which are currently set to zero. There is no existing NA base map for this ECU. A skilled tuner with full write access could develop one, but it is a significant undertaking.
Is the Garrett 1238S turbo difficult to source as a replacement?
No. Remanufactured Garrett 1238S units and new-old-stock examples remain available from turbo specialists across Europe. Prices for remanufactured units typically range from £250 to £500 fitted, making replacement far more sensible than converting the car to NA.
Would a smart roadster NA conversion pass an MOT or IVA test?
In the UK, the conversion would need to comply with emissions regulations calibrated to the original turbocharged specification. An NA car running a clean-sheet ECU calibration may struggle to meet the original CO and HC thresholds at the original test parameters, potentially causing MOT failures on emissions grounds without careful development work.
