The Smart Roadster’s 698cc three-cylinder engine is a remarkable piece of engineering, but the stock Garrett 1238S turbocharger is a limiting factor for anyone serious about extracting real performance. A smart roadster turbo upgrade is one of the highest-impact modifications you can make — but it is also one that demands careful planning. Get it right and you can transform the car’s character dramatically. Get it wrong and you risk damaging a powertrain that is increasingly difficult and expensive to replace. This guide covers every realistic turbo upgrade path, what each one actually delivers, the supporting modifications you will need, and the honest trade-offs involved.
Understanding the Stock Garrett 1238S Turbocharger
Before you spend a penny on an upgrade, you need to understand exactly what you are starting with. The stock turbocharger fitted to all Smart Roadster 452 variants is the Garrett 1238S, a compact fixed-geometry unit with a pneumatic wastegate actuator. It is physically tiny by modern standards, which is appropriate for an engine displacing less than 700cc, but that compactness also defines its ceiling.
The standard boost pressures across the range tell the story clearly: the 45kW Lite runs at 0.89 bar, the 60kW standard car at 1.09 bar, the 66kW SB2 Brabus at 1.33 bar, and the full 74kW Brabus at 1.43 bar. The same physical turbocharger produces those different outputs purely through boost pressure calibration and ECU mapping. This is both encouraging and cautionary — encouraging because it proves the unit has headroom, cautionary because the Brabus figures represent close to the safe ceiling for the 1238S compressor map.
If you are experiencing inconsistent boost or suspect your actuator is already worn, it is worth reading about wastegate actuator failure symptoms before committing to an upgrade, since many cars that feel down on power simply have a failing pneumatic actuator rather than a turbocharger that has reached its limits.
The 1238S flows adequately for around 80–85 horsepower at the crank before it enters efficiency islands on the compressor map that generate excessive heat and begin to hurt rather than help power. Understanding this ceiling is the foundation of all sensible upgrade planning.
Route One: Optimising the Stock Turbocharger
For the majority of Smart Roadster owners, the most cost-effective turbo upgrade is not a turbo upgrade at all — it is maximising what the stock unit can do with better calibration and supporting hardware. A properly executed ECU remap on a healthy 1238S can bring a standard 60kW car to around 90–100 horsepower reliably, which represents a transformative real-world improvement for a car weighing under 800kg.
The key here is supporting the turbo rather than fighting its limitations. Upgraded intercooling, a less restrictive induction kit, and a freer-flowing exhaust all reduce thermal and backpressure loads on the compressor, which means it operates more efficiently at any given boost level rather than simply spinning harder and generating heat.
If you have not already explored what a calibrated remap can extract from your stock setup, our detailed breakdown of Stage 1 power gains sets realistic expectations and explains why mapping always comes before hardware changes. Attempting to upgrade hardware without a corresponding map change is a common and costly mistake — the ECU will not exploit the improvement and in some cases will actively work against it through fuel trimming and boost control.
For owners on the 45kW Lite variant, the calculus is different. That model was built without an oil cooler, which becomes a critical limitation as soon as you increase thermal output. Understanding why the 60kW engine includes an oil cooler is essential reading before pushing a Lite beyond its factory calibration.
Route Two: Larger Turbocharger Options
If your goal is genuine power above 100 horsepower at the crank, a physically larger turbocharger becomes necessary. The Smart Roadster community has experimented with several options over the years, and the consensus around a handful of units has solidified through real-world testing.
Garrett GT1241z and GT1446
The GT1241z is the most commonly fitted upgrade, offering a larger compressor wheel than the 1238S while retaining broadly similar turbine housing dimensions. This allows fitment without extensive exhaust manifold modification, which matters enormously in the tight packaging of the mid-rear engine bay. Expected gains over a remapped stock turbo are in the region of 15–20 horsepower, bringing totals to around 110–125 horsepower depending on supporting mods. Spool time increases slightly but remains acceptable for road use.
The GT1446 is a step further and is favoured by owners targeting 130 horsepower or beyond. It requires more significant pipework changes and benefits from a matching exhaust manifold. Turbo lag becomes more noticeable below 3,000 rpm, which on a car with the Softouch automated gearbox — with its characteristically slow shift execution — can make low-speed progress feel agricultural. This is a track or track-biased road build, not a daily driver upgrade.
Mitsubishi TD02 and TD025 Family
Because the Smart Roadster’s engine is Mitsubishi-derived, several TD02 and TD025 variants from the Mitsubishi parts bin offer excellent compatibility with minimal fabrication. These are popular in mainland European build scenes and offer good compressor maps for the 90–110 horsepower target range. Sourcing good-condition used units is easier than for Garrett alternatives, and OEM Mitsubishi quality is well documented.
Whichever turbo you choose, pairing the hardware with complementary camshaft upgrades designed specifically for the 698cc engine will unlock substantially more of the new turbo’s potential by improving cylinder filling at higher boost levels.
Supporting Modifications: What a Turbo Upgrade Actually Requires
A larger turbocharger without proper supporting modifications is a recipe for disappointment at best and engine damage at worst. The following are not optional extras — they are prerequisites.
Fuelling
The stock fuel injectors and fuel pump are sized for the factory power outputs. Beyond approximately 100 horsepower at the crank, fuel delivery becomes the limiting factor. Upgraded injectors — typically 310cc or 360cc units — are required for any build targeting over 110 horsepower, alongside a fuel pressure regulator that maintains consistent rail pressure under the higher demand. A wideband lambda sensor and associated logging capability are essential for tuning validation.
Intercooling
The stock intercooler is adequate for Brabus-level output but insufficient for sustained high-boost running with a larger turbo. A front-mounted intercooler conversion or a substantially larger side-mount unit will be required. Charge temperature has a direct and significant effect on knock resistance, so this is not an area to economise on.
Engine Internals
For builds targeting 120 horsepower and above, attention to the cylinder head becomes important. Flow work on the ports and combustion chamber, combined with uprated valve springs to cope with higher lift profiles, materially reduces the risk of heat-related failure. Cylinder head porting on the 698cc unit is a specialist job but one that returns genuine, measurable improvements in flow efficiency at high boost.
Boost Monitoring
Running a larger turbocharger without real-time boost monitoring is unwise. The Bosch MEG 1.1 ECU can be read for MAP sensor data through an OBD interface, but a dedicated boost gauge gives you immediate visual feedback and will alert you to overboost conditions before the ECU’s safety strategies intervene. Setting up proper boost monitoring should be on your list before you start the engine after any turbo change.
Smart Roadster Turbo Upgrade: Realistic Power and Cost Expectations
Honesty matters here. The Smart Roadster community has a history of optimistic claims, and separating marketing from measured results is important before you commit budget.
A remap-only build on a healthy 60kW car with stock turbo: 85–100 horsepower, cost £300–£700 depending on mapping provider, highly reliable, retains tractability.
GT1241z with remap, upgraded fuelling and intercooler: 110–120 horsepower, total cost £1,500–£2,500 depending on labour and parts sourcing, requires careful calibration.
GT1446 with full supporting build including head work and camshaft: 125–140 horsepower, total cost £3,000–£5,000+, this is a serious project car build.
Weight context is everything here. At approximately 790kg, even 100 horsepower gives the Smart Roadster a power-to-weight ratio that embarrasses many sports cars costing ten times as much. The gains from a remap alone are genuinely significant in the real world — a larger turbo build is for those who understand they are entering diminishing-returns territory and are happy to pay the premium for bragging rights as much as outright pace.
From a reliability perspective, the 60kW variant is always the correct base for a turbo upgrade project. Its oil cooler and slightly more robust calibration make it a meaningfully better starting point than the 45kW Lite, and the price premium for a good 60kW car is easily justified by the reduced risk and higher upgrade ceiling.
Installation Considerations and Common Mistakes
The Smart Roadster’s mid-rear engine bay is compact and access is restricted. Professional installation of a turbocharger by someone who has not worked on a 452 before frequently results in extended labour time and avoidable errors. Seek out a specialist with documented Smart Roadster experience, or at minimum someone familiar with Mitsubishi three-cylinder applications.
Common errors include reusing old oil feed and return lines, which can starve the new turbo of lubrication on first start and cause immediate failure; failing to pre-oil the new turbocharger before installation; and neglecting the coolant lines that feed the turbo’s water-cooled bearing housing on later variants. A turbocharger that is not properly primed with oil before first start will fail within minutes.
Exhaust manifold cracking is a known weakness on high-boost Smart Roadster builds. Inspect the manifold before installation and consider a stainless steel replacement manifold for any build over 1.2 bar of boost. The stock cast manifold will crack under repeated thermal cycling at elevated boost levels, and a failed manifold will destroy your tune and potentially your catalytic converter.
For a broader understanding of how the 698cc engine’s architecture responds to tuning, the comprehensive guide to the 698cc three-cylinder covers the block and head design details that underpin all upgrade planning. Understanding the engine’s architecture is as important as understanding the turbocharger sitting on top of it.
A smart roadster turbo upgrade is the most impactful single modification available for the 452, but it must be approached methodically. Begin with a quality ECU remap on a healthy base car — our performance mapping packages offer a calibrated route from 90 to 125 horsepower depending on your goals. If you then decide a hardware upgrade is right for you, build the supporting systems before you bolt on the turbo. The result, in a car this light, will redefine what you thought a 700cc engine could feel like.
