Introduction: Why Turbocharge Your Honda Grom?

The Honda Grom (MSX125) has earned a cult following for its small stature, low seat height, and go-kart-like agility. Straight from the factory, the 125cc single-cylinder engine produces around 6 horsepower, which is perfectly adequate for beginners and urban commuters. However, the Grom chassis can handle far more power, and experienced riders often find themselves craving a broader powerband and punchier acceleration. Adding a mini turbo system—combined with supporting modifications—can more than double the output, bringing you to around 15 horsepower or even higher. This transforms the Grom into a pocket rocket that feels faster, snappier, and far more engaging to ride without sacrificing its signature fun-factor. Below you will find ten essential upgrades that work together to safely boost your Grom from 6 to 15 hp.

1. Complete Turbocharger Kit

The single most effective modification for major horsepower gains is a purpose-built turbocharger kit. These kits are engineered specifically for the Honda Grom and include the turbocharger itself, a mounting bracket, an oil feed and return system, and all necessary hoses and clamps. A well-designed mini turbo kit can increase the engine output from 6 hp to approximately 12–15 hp while also providing a noticeable torque increase across the entire rpm range.

What to Look For

  • Proper fitment: Kits from established Grom tuners like DHM Racing or Drow Sports are machined to fit the stock frame and engine without clearance issues.
  • Oil system: A good kit includes an oil line kit that taps into the engine's lubrication system to keep the turbo reliable.
  • Complete hardware: Look for a kit that includes a high-flow air filter, intake piping, and a blow-off valve to protect the turbo.

Performance Expectations

With a properly tuned turbo system, you will see a dramatic improvement in mid‑range and top‑end pull. The Grom will accelerate noticeably harder from 40 mph to 65 mph, making highway merging safer and more confident. However, a turbo alone cannot reach its full potential without the other upgrades listed below.

2. High‑Performance Fuel Injectors

Stock fuel injectors are sized for a naturally aspirated 6‑hp engine. Once you introduce forced induction, the engine requires more fuel to maintain the correct air‑fuel ratio. Upgrading to a high‑flow injector (typically around 220–250 cc) ensures that the mixture stays rich enough under boost to prevent detonation and maximize power.

Installation Considerations

  • Compatibility: Many turbo kits come with a recommended injector size. Verify that the injector matches your ECU tuning solution.
  • Atomization: High‑quality injectors provide better fuel atomization, which promotes more complete combustion and reduces the risk of hot spots in the cylinder.

3. Performance Exhaust System

A restrictive stock exhaust creates back‑pressure that robs power, especially when the engine is producing higher exhaust volumes under boost. Replacing it with a full exhaust system—header and muffler—designed for the Grom allows exhaust gases to exit quickly. This reduces turbo lag and increases peak horsepower.

  • Yoshimura AT2: A popular stainless steel system that provides a clean sound and solid power gains.
  • Two Brothers Racing: Offers an aluminum muffler that saves weight and improves flow.
  • Zoom Brew: A budget‑friendly option that still delivers measurable performance.

Note: A full exhaust system works best when combined with an ECU tune and a high‑flow air filter. Together they form the foundation of a properly breathing engine.

4. ECU Tune (Flash or Piggyback Module)

No matter how many mechanical upgrades you install, the engine management system must be recalibrated to take full advantage of the increased air flow. An ECU tune adjusts fuel maps, ignition timing, and—if equipped—boost control parameters. For the Honda Grom, popular options include a mail‑in flash from DHM Racing or a plug‑and‑play module like the Aracer RCPower X.

Why This Matters

  • Air‑fuel ratio: A proper tune keeps the mixture safe under all load conditions, preventing lean conditions that can damage pistons or valves.
  • Throttle response: Crisp throttle tip‑in makes the bike feel more lively and precise.
  • Boost control: If you are running a turbo, some tuners can adjust the wastegate duty cycle to regulate boost pressure.

5. Upgraded Air Filter (High‑Flow)

A stock paper air filter restricts the volume of air the engine can draw in. Switching to a high‑flow oiled cotton or foam filter (such as K&N or BMC) reduces intake restriction and allows the turbo to spool more freely. This is a simple, inexpensive upgrade that complements every other modification on this list.

Maintenance Tip

High‑flow filters require periodic cleaning and re‑oiling. For a street‑driven Grom, cleaning the filter every 3,000–5,000 miles is sufficient to maintain optimal airflow.

6. Lightweight Flywheel

The stock flywheel is designed for smoothness and idle stability. Replacing it with a lightweight unit (typically around 1.5–2 pounds lighter) reduces rotational inertia. This allows the engine to accelerate and decelerate faster, improving throttle response and making the bike feel more eager to rev. While a lightweight flywheel alone does not add peak horsepower, it dramatically improves the perceived performance, especially when paired with a turbo that spools quickly.

Trade‑Offs

  • Idle stability: A very light flywheel can make the engine less stable at idle; most Grom riders find a 1.5‑pound reduction is the sweet spot.
  • Stalling risk: Take extra care when launching from a stop; the engine will lose revs faster if you dump the clutch.

7. Performance Camshaft

The factory camshaft is ground for low‑end torque and fuel economy. A performance camshaft with higher lift and longer duration allows the engine to breathe more efficiently at higher RPMs. Combined with a turbo, this can extend the powerband and increase the peak horsepower ceiling. Look for a cam from recognized Grom tuners like DHM or BDL.

Installation Advice

Upgrading the camshaft requires removing the valve cover and timing chain, so it is a moderately involved job. Always replace the valve shims and seals while you are in there. This upgrade works best when paired with stiffer valve springs to prevent valve float at higher RPM.

8. Turbo Wastegate Upgrade (Electronic Boost Controller)

The wastegate on a small turbocharger controls the amount of exhaust gas that bypasses the turbine, which regulates boost pressure. The stock wastegate on most budget Grom turbo kits uses a simple spring mechanism. Upgrading to an electronic boost controller or a higher‑quality manual wastegate gives you finer control over boost levels. This allows you to run safe, consistent boost without spiking or dropping off.

Why Upgrade?

  • Precision: Eliminate boost creep (unwanted boost increase at high RPM).
  • Adjustability: Dial in boost from 5 psi to 8 psi depending on fuel quality and engine health.
  • Safety: An electronic controller can act as a boost cut if pressure exceeds a set threshold.

9. Intercooler Installation (Air‑to‑Air or Air‑to‑Water)

Compressing air heats it up, reducing its density and increasing the risk of detonation. An intercooler cools the compressed air before it enters the engine, increasing intake density and allowing a safer, more powerful tune. For the Honda Grom, space is limited, so most builders use a compact air‑to‑air intercooler mounted behind the radiator or in the lower fairing area.

Cooling Benefits

  • Lower intake temperatures: Reduces the chance of pre‑ignition.
  • Increased power: Cooler air is denser, meaning more oxygen per combustion cycle.
  • Consistency: Power does not drop off as much during sustained hard riding.

10. Performance Tires and Sprocket Upgrades

Once your Grom is making 15 horsepower, the stock tires will struggle to put the power down. Upgrading to sticky, performance‑oriented tires—such as the Michelin Pilot Street or Pirelli Diablo Rosso—significantly improves traction and cornering confidence. Additionally, changing the final drive ratio (sprockets) can help you use the extra power more effectively. A 15‑tooth front sprocket (instead of 14) reduces low‑end wheelspin and allows the bike to pull longer in each gear.

Tire Recommendations

  • Michelin Pilot Street Radial: Excellent wet and dry grip, long tread life.
  • Pirelli Diablo Rosso Scooter: Soft compound for maximum corner grip.
  • Bridgestone Battlax BT39: Lightweight and predictable.

Sprocket Tips

For a turbo Grom, a 15/34 sprocket setup (front/rear) is a common starting point. Adjust based on your riding style: more teeth on the rear gives quicker acceleration but lower top speed; fewer teeth on the rear favors top speed and cruising.

Putting It All Together: The 15‑HP Build Path

To reach a reliable 15 horsepower, you do not need to install all ten upgrades at once. A practical build order is:

  1. Start with the basics: High‑flow air filter, performance exhaust, and an ECU tune. These will get you to about 8–9 hp.
  2. Add the turbo kit and the accompanying fuel injector. This step is where the big power jump occurs, taking you to 12–15 hp with a proper tune.
  3. Fine‑tune the breathing: Wastegate control and intercooler ensure the turbo operates efficiently and consistently.
  4. Strengthen the driveline: Lightweight flywheel, camshaft, and performance tires allow the engine to rev freely and put the power to the pavement safely.

It is strongly recommended to have a professional dyno tune after the turbo installation. A safe, conservative tune will preserve engine longevity, while an aggressive tune can quickly lead to overheating or mechanical failure on a small‑displacement engine.

Reliability Considerations

Pushing a 125‑cc engine to three times its stock output requires careful attention to maintenance. Use high‑quality synthetic oil and change it every 500–1,000 miles, as the turbo places additional thermal stress on the lubricant. Monitor spark plug condition and fuel quality—low‑octane fuel combined with high boost can cause knock. Many Grom owners also install a cheap oil temperature gauge to keep an eye on heat buildup during long rides.

If you are planning to track your bike or ride aggressively for extended periods, consider upgrading the radiator to a larger unit (thicker core) and adding an oil cooler. These additions help keep engine temperatures within a safe range and reduce the risk of detonation.

Cost vs. Value: Is a Turbo Grom Worth It?

A complete turbo build for the Honda Grom typically costs between $1,500 and $3,000 depending on the quality of parts and whether you do the installation yourself. This may seem high compared to a naturally aspirated big‑bore kit, but the turbo offers a more dramatic power increase and a unique sound that turns heads. The Grom also remains highly reliable if the tune is conservative and the supporting mods are installed correctly.

For riders who enjoy hands‑on modification and want a truly one‑of‑a‑kind mini bike, a turbocharged Grom is an incredibly rewarding project. It combines the light, playful nature of the stock bike with enough power to surprise larger motorcycles and keep you grinning every time you twist the throttle.

External Resources

Conclusion

Transforming the Honda Grom from a 6‑hp commuter into a 15‑hp pocket rocket is one of the most satisfying projects in the mini‑moto world. By carefully selecting and installing the right combination of a turbocharger, supporting fuel and intake mods, a proper ECU tune, and improved tires and gearing, you can achieve a bike that is quicker, faster, and far more exciting to ride. Always prioritize reliability and safety—a well‑built turbo Grom will deliver years of enjoyment without sacrificing the playful character that makes the bike so beloved in the first place.