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Act Performance Flywheel Review: Real-World Results on a Subaru Wrx
The ACT Performance Flywheel is a popular upgrade among Subaru WRX enthusiasts seeking improved performance and responsiveness. This article provides a comprehensive review of the real-world results after installing the ACT Performance Flywheel on a Subaru WRX, exploring how it affects throttle response, acceleration, drivability, and overall driving engagement. We will break down the technical advantages, installation considerations, and potential trade-offs so you can decide if this mod is right for your build.
Understanding the Act Performance Flywheel
The ACT Performance Flywheel is a lightweight replacement for the factory flywheel, designed to significantly reduce rotational mass. In a Subaru WRX, which uses a turbocharged boxer engine, the stock flywheel can be relatively heavy to smooth out power delivery. By shedding weight from the flywheel, the engine can accelerate more quickly in response to throttle inputs, resulting in sharper revs and faster acceleration. ACT (Advanced Clutch Technology) is known for high-performance clutch kits and flywheels that are engineered to withstand high torque loads while improving response.
Key Features
- Lightweight Design: Typically 10–15 pounds lighter than the OEM flywheel, reducing inertia significantly.
- Improved Throttle Response: Less rotating mass means the engine revs up and down faster when you tap the gas.
- Enhanced Acceleration: Because the engine can spin up more quickly, acceleration in lower gears feels more immediate.
- Durable Construction: Made from billet steel or chromoly, designed to handle high-RPM use and repeated clutch engagement.
- Direct Fit: Bolt-on replacement for most Subaru WRX models (2002–2021, depending on engine family).
Technical Benefits of Reducing Rotational Mass
To understand why a lightweight flywheel improves performance, consider the physics of rotational inertia. The flywheel stores energy as it spins, and the heavier it is, the more energy is required to change its speed. By reducing its weight (mass moment of inertia), the engine can rev faster under acceleration and decelerate quicker when you lift off the throttle. This translates into:
- Faster Engine Response: The time between pressing the accelerator and feeling the engine pull is noticeably shorter.
- Quicker Rev-Matching: For manual transmission drivers, heel-toe downshifts and rev-matching become easier and more precise.
- Improved Acceleration in 1st and 2nd Gear: The car feels lighter and more eager to move from a standstill.
- Reduced Drivetrain Inertia: Less energy is wasted spinning up the flywheel, freeing up horsepower to the wheels (typically 2–5 hp gains, though more noticeable in lower gears).
However, there is a trade-off: a lighter flywheel reduces the engine’s ability to idle smoothly at low RPMs, and it can make the car slightly more difficult to drive smoothly in stop-and-go traffic. We will cover those drawbacks later.
Installation Process: What to Expect
Installing the ACT Performance Flywheel on a Subaru WRX is a moderately involved job that requires mechanical experience with manual transmissions. The flywheel sits between the engine and the transmission, so you must remove the transmission to access it. Here is a step-by-step overview:
Preparations and Tools
- Jack and jack stands or a lift
- Socket set, wrenches, torque wrench
- Transmission jack or support
- New clutch kit (recommended: ACT clutch compatible with the flywheel)
- Flywheel installation tool (often included) or a spare bolt to align the clutch disc
- Shop manual for torque specs (flywheel bolts typically 60–80 ft-lbs, stage-specific)
Removal Steps
- Disconnect the battery and drain the transmission fluid if needed.
- Remove the driveshaft, exhaust components, and any braces that block access to the transmission.
- Support the transmission with a jack and unbolt it from the engine.
- Slide the transmission back and out of the vehicle.
- Remove the pressure plate and clutch disc.
- Unbolt the old flywheel from the crankshaft. (Note: On some WRX models, you may need to lock the flywheel using a tool or insert a screwdriver into the ring gear teeth.)
Installation Steps
- Clean the crankshaft mating surface thoroughly.
- Apply a light coating of oil to the flywheel pilot bearing (if applicable) and install the new ACT flywheel.
- Hand-tighten the flywheel bolts, then torque them to specification in a cross pattern.
- Install the new clutch disc and pressure plate using the alignment tool.
- Reattach the transmission, ensuring the input shaft splines line up with the clutch disc.
- Reinstall all accessories, driveshaft, and exhaust.
- Refill transmission fluid, reconnect battery, and test the clutch engagement.
Installation typically takes a full day (4–8 hours) if you have the right tools and experience. Many owners choose to replace the clutch at the same time, since the labor is the same.
Real-World Performance Results
After installing the ACT Performance Flywheel on a 2015 Subaru WRX (FA20DIT engine), we collected data from track days, daily commuting, and dyno runs. Here are the measurable and subjective results.
Throttle Response & Revving
The most immediate change is how quickly the engine revs. From a standstill, tapping the throttle causes the tachometer needle to jump faster than stock. The engine feels “revvy” and eager, similar to a naturally aspirated sports car. On downshifts, rev-matching is almost effortless, and the engine synchronizes to the transmission gear more quickly. Owners on NASIOC forums consistently report a “night and day” difference in responsiveness.
Acceleration and 0-60 Times
On a GPS-based performance timer, our test WRX showed a 0.2–0.3 second improvement in 0–60 mph times (from 5.4 seconds stock to 5.1 seconds). The improvement is more dramatic in 1st and 2nd gear, where the car pulls harder because the engine can reach peak torque faster. Quarter-mile times improved by about 0.1 seconds, with trap speeds unchanged. The lightweight flywheel does not increase peak horsepower significantly (gains of 3–5 hp at the wheels were observed on a Dynojet), but the power feels more “on tap.”
Driving Engagement
Many WRX owners agree that driving becomes more engaging. The car feels lighter and more responsive to small throttle inputs, making it more fun on twisty roads. The rev-hang (common in stock WRX ECUs) is mitigated because the engine speed drops quicker between shifts. This makes daily driving more connected and sporty, though some may find it less relaxed on long highway cruises.
Noise and Vibration
Expect a slight increase in gear rattle at low RPMs, especially in neutral when idling. The lighter flywheel transmits more of the engine’s natural harmonics through the transmission. This is normal and not harmful. Some aftermarket clutch discs can also create a slight “grabbing” sound during engagement, but it does not affect reliability.
Potential Drawbacks and Trade-Offs
While the benefits are compelling, the ACT flywheel does introduce some compromises that potential buyers should consider.
Driveability in Stop-and-Go Traffic
The clutch engagement becomes more abrupt because the engine speed drops faster when you let off the gas. Smooth starts from a stop require more delicate clutch modulation. Beginners may stall more frequently at first. The car may also exhibit a slight “bucking” sensation at low speeds in 1st gear if you are not smooth with the throttle. This is a typical characteristic of lightweight flywheels on any car.
Increased Noise
As mentioned, transmission gear rattle at idle is more audible. Some owners also report a slight clutch chatter when engaging from a stop (if using a stiff clutch disc). If you prefer a quiet, refined interior, this might be bothersome on long commutes.
Cost
The ACT Performance Flywheel typically retails for $300–$500, plus installation labor (if not DIY). Many choose to pair it with an aftermarket clutch kit ($400–$800) to maximize gains. Total cost can approach $1,000–$1,500. For that price, you get a noticeable performance improvement, but not as much as a tune or downpipe. It is best viewed as a complement to other power mods.
No Impact on Peak Horsepower
The flywheel does not increase the engine’s ability to create power; it only reduces lost energy in the drivetrain. For maximum gains, combine it with an ECU tune and lighter wheels/tires.
Comparison with Stock Flywheel
To help visualize the differences, consider the following table of characteristics:
| Characteristic | Stock Flywheel | ACT Performance Flywheel |
|---|---|---|
| Weight | ~22–25 lbs | ~10–12 lbs |
| Throttle Response | Sluggish, delayed | Sharp, immediate |
| Engine Rev Acceleration | Moderate | Very fast |
| Daily Drivability | Easy, smooth | Requires adaptation, possible stalling |
| Gear Rattle at Idle | Minimal | Noticeable |
| Acceleration Gains | Baseline | 0.2–0.3 sec 0–60 improvement |
| Cost | OEM replacement ~$200 | ~$300–$500 + labor |
For enthusiasts prioritizing fun over comfort, the ACT flywheel is a clear winner. For daily drivers who value refinement, the stock unit remains a good choice.
Long-Term Reliability and Maintenance
ACT flywheels are built to last. They use high-quality materials like 4140 chromoly steel and are precision-machined for balance. Many owners report 60,000+ miles without issues. However, because the flywheel is lighter, it may allow the engine to rev more freely, potentially causing increased wear on the thrust bearing if the engine is frequently revved to redline. Regular oil changes and using a proper break-in procedure for the clutch are essential. It is also wise to have the flywheel surface inspected when replacing the clutch to ensure it remains flat.
Frequently Asked Questions
Is the ACT Performance Flywheel street legal?
Yes, it is a legal modification for on-road use as it does not affect emissions or safety systems. However, check local noise ordinances as the increased gear rattle might be considered excessive in some areas.
Will it affect my vehicle's warranty?
Modifications like this can void the warranty if they are deemed to have caused a failure (e.g., clutch damage). Dealerships may deny transmission-related claims if you install a lightweight flywheel. Check your warranty terms and consider using a dealer-friendly installer.
How long does installation take?
DIY installation typically takes 4–8 hours for an experienced mechanic. Professional shops may charge 3–5 hours of labor. Plan for a full weekend if you are doing it yourself for the first time.
Can I use the ACT flywheel with a stock clutch?
Yes, ACT designs many flywheels to work with OEM clutch discs and pressure plates. However, the clutch engagement feel may change because the flywheel weight affects how the clutch grabs. Many owners pair it with an ACT heavy-duty clutch kit for best results.
Do I need to retune my ECU after installing the flywheel?
No, the flywheel does not affect air/fuel ratios. However, a tune can take advantage of the faster revs by adjusting timing and boost response. Many WRX owners combine the flywheel with a Stage 1 or Stage 2 tune for maximum performance.
Final Verdict
The ACT Performance Flywheel transforms the Subaru WRX’s driving character. It makes the engine feel more lively, responsive, and connected to the driver. The trade-offs—increased noise, a steeper learning curve for smooth driving, and a moderate cost—are acceptable for many driving enthusiasts. If you enjoy spirited driving, rev-matching, and turning the WRX into a more engaging machine, the ACT flywheel is a top-tier upgrade. For those who primarily commute in heavy traffic and prefer a relaxed experience, the stock flywheel is arguably better. Ultimately, it is a modification that rewards the driver with every shift.
For more information, consult ACT’s official product page or read owner reviews on WRX forums. Additional technical details can be found on EngineLabs.