Can I install a downpipe myself?

Yes, with basic mechanical skills and tools. Allow 3–5 hours and ensure proper torque specs are followed.

Does a downpipe add horsepower?

Yes, typically 30–50 whp when combined with a tune. Without tuning, gains are minimal (5–15 whp).

Is a catless downpipe illegal?

Yes, in most countries. Removing a catalytic converter violates federal emissions laws in the U.S., EU, Canada, and Australia.

Will a catted downpipe pass inspection?

It depends. High-quality catted pipes from reputable brands often pass if the rest of the emissions system is healthy.

What size downpipe is best for a stock Supra?

A 3.5” or 4.5” catted downpipe is ideal. Larger diameters suit tuned engines; smaller ones preserve low-end response.

What Is an A90 Supra Downpipe and How Does It Improve Performance?

If you own a 2020–2024 Toyota GR Supra (A90) with the B58 engine, upgrading the downpipe is one of the most effective modifications to increase performance, enhance turbo response, and improve exhaust flow. A high-flow a90 supra downpipe reduces backpressure by replacing the restrictive factory unit, often resulting in measurable gains in horsepower and torque—especially when paired with an ECU tune. However, choosing between catted and catless designs, understanding emissions legality, and ensuring compatibility with your tuning setup are critical decisions that impact both drivability and compliance.

Why Upgrade the Downpipe on an A90 Supra?

The factory downpipe on the B58-powered MKV Supra includes a close-coupled catalytic converter designed to meet strict emissions standards. While effective for regulation compliance, this component restricts exhaust gas flow from the turbocharger outlet to the rest of the exhaust system. By replacing it with an aftermarket B58 downpipe, you reduce turbulence and backpressure, allowing the twin-scroll turbo to spool faster and operate more efficiently.

Users commonly report:

⚡ Smoother and quicker turbo spool-up
📈 Gains of 30–50 wheel horsepower with a proper tune
🔊 More aggressive exhaust note with pops and crackles (especially catless)
🧼 Reduced risk of heat soak due to improved gas velocity

However, these benefits come with trade-offs, including potential check engine lights (CEL), inspection failures, and warranty implications—all of which must be weighed before installation.

Catted vs. Catless Downpipes: What’s the Difference?

One of the first decisions when shopping for an a90 supra downpipe is whether to go catted (with catalyst) or catless (no catalyst).

Catted Downpipes ✅

A catted downpipe retains a high-flow catalytic converter just after the turbo flange. These units are engineered to prevent oxygen sensor errors and avoid triggering a P0420 code (catalyst efficiency below threshold).

Pros:

Legal for street use in most regions
No need for a tune to prevent CEL (though tuning maximizes gains)
Passes emissions testing in states like California (if CARB-compliant)
Reduces sulfur smell associated with raw exhaust gases

Cons:

Slightly less power gain than catless (typically 10–15 hp less)
Higher cost due to inclusion of premium catalyst material
Limited long-term durability if low-quality catalyst is used

Popular brands offering catted versions include Active Autowerke, MST, and VRSF. Some manufacturers claim their catted pipes are "CEL-free" without requiring a tune, but results vary based on sensor age and engine condition.

Catless Downpipes ❗

A catless downpipe removes the catalytic converter entirely, providing maximum exhaust flow. This design is favored in racing or off-road applications where emissions compliance isn’t required.

Pros:

Highest potential power gains
Best turbo spool characteristics
Deepest, rawest exhaust tone with pronounced burbles
Lower cost compared to high-end catted units

Cons:

Triggers check engine light on nearly all stock ECUs
Produces strong sulfur/rotten egg odor under load
Illegal for public road use in most U.S. states and EU countries
Fails emissions inspections universally

Despite performance advantages, many owners revert to catted setups after experiencing discomfort from exhaust fumes or failing state inspections. Forums such as r/Supra and SupraMKV frequently discuss real-world experiences with catless operation, noting that even short drives can produce noticeable smells inside the cabin1.

Performance Gains: How Much Power Do You Actually Get?

While marketing materials often claim "up to 70 whp gains," actual results depend heavily on supporting modifications and calibration.

In independent dyno tests:

Stock A90 Supra (B58): ~335 whp
With downpipe only (no tune): +5 to +15 whp
With downpipe + basic flash tune: +30 to +50 whp
With full bolt-ons (intake, intercooler, exhaust) + custom tune: +60+ whp

The key takeaway? A downpipe alone won’t unlock massive gains. Its primary benefit is improving turbo efficiency, which becomes fully realized only when the ECU adjusts fueling, boost targets, and timing maps to take advantage of reduced backpressure.

Without a tune, excess exhaust flow can lead to lean conditions or erratic boost behavior, potentially harming the engine over time. Always pair a downpipe upgrade with at least a conservative stage 1 or stage 2 tune from a reputable tuner.

Tuning Requirements After Installation

After installing any non-OEM downpipe—especially catless—you’ll likely need to modify the vehicle’s software to maintain safe operation.

Oxygen Sensor Management

The B58 uses two oxygen sensors: pre-cat (S1) and post-cat (S2). When the catalytic converter is removed or replaced with a high-flow unit, the S1 sensor detects unchanged exhaust chemistry, leading the DME (engine computer) to flag inefficiency.

Solutions include:

🔧 Flash tuning to disable catalyst monitoring (via COBB, APR, GIAC, etc.)
🔌 Simulated O2 signal devices (not recommended; unreliable and detectable)
🔄 Replacement with a catted pipe designed to pass OEM thresholds

Reputable tuners offer maps specifically calibrated for downpipe-equipped vehicles, adjusting lambda targets and knock correction strategies accordingly.

Danger of Running a Catless Downpipe Without a Tune

Some users attempt to run a catless downpipe without tuning, hoping to avoid costs. However, this can result in:

⚠️ Persistent check engine light affecting resale value
📉 Suboptimal fuel trims causing rough idle or hesitation
🔥 Increased exhaust gas temperatures (EGTs) under load
🛑 Long-term damage due to uncorrected air/fuel imbalances

Reddit threads document cases where prolonged untuned operation led to premature turbo wear or sensor failure2. For reliable performance, always remap the ECU after modifying exhaust components.

Emissions Legality and Regional Regulations

The legality of an a90 supra downpipe varies significantly by region.

United States

In the U.S., removing or bypassing a catalytic converter violates the Clean Air Act (42 U.S.C. § 7522). Enforcement varies:

📍 California (CARB): Strictest rules; requires EO-numbered parts for legal modification
📍 Other states: Some perform visual or sniff tests; others rely on OBD-II scans
📍 No-inspection states: Texas, Florida, Colorado allow easier compliance avoidance

Even if you pass inspection now, future registration renewals may require retesting. Additionally, dealerships can refuse service if modifications are detected.

European Union

In the EU, removing a catalytic converter invalidates type approval and insurance coverage in most countries. MOT-style tests measure hydrocarbon, CO, and NOx levels—catless systems will fail outright.

Some European owners use temporary 'test pipes' during inspection, then reinstall performance sections afterward—a practice technically illegal but widely practiced.

Canada & Australia

Both nations follow strict emissions frameworks similar to the U.S. Environment Canada and Australian Design Rules prohibit tampering with emission control devices.

Bottom line: If you plan to drive on public roads regularly, choose a CARB-compliant or equivalent catted downpipe unless local enforcement is known to be lax.

Material Quality and Construction: What to Look For

Not all supra mk5 downpipes are built equally. Key construction factors include:

Pipe Diameter

Most aftermarket units use 3.5” or 4.5” tubing. While larger diameter reduces restriction, it can hurt low-end torque if mismatched to engine output.

⚙️ 3.5”: Balanced flow, good for daily-driven builds
⚙️ 4.5”: Max flow, ideal for high-horsepower or track-focused cars

Flange Type and Fitment

The turbo-side flange must align perfectly with the turbine housing. CNC-machined 304 stainless steel flanges (like those from VRSF or AMS Performance) offer better sealing and longevity than cast alternatives.

Mandrel Bending vs. Crush Bending

Mandrel bending maintains consistent inner diameter through curves, minimizing flow disruption. Avoid crush-bent pipes, which narrow internally and create turbulence.

Weld Quality

TIG welding provides stronger, cleaner joints than MIG. Inspect product photos for uniform bead patterns and absence of porosity.

Brand	Type	Diameter	Price Range	Notes
VRSF	Catted / Catless	4.5"	$299–$479	Full mandrel bend, CNC flange
AMS Performance	Race (Catless)	4.5"	$630	Stainless steel, competition-only
Active Autowerke	Catted (Signature)	4.5"	$1,140–$1,800	High-flow catalyst, CEL-free claim
Boost Logic	Stainless Steel	4.5"	$450	Racing use only
CTS Turbo	Race Downpipe	4.5"	$280	Budget option, mixed reviews

Installation Process and Tools Needed

Replacing the downpipe on an A90 Supra is a moderate DIY job (~3–5 hours) for experienced enthusiasts.

Tools Required

🧰 Socket set (10mm, 13mm, 18mm)
🔧 Torque wrench
📦 Jack stands and floor jack
📎 O2 sensor socket
🛢️ Anti-seize compound
🌬️ Shop towels (exhaust soot cleanup)

Key Steps

Lift and secure the vehicle safely
Disconnect battery ground terminal
Unplug front O2 sensor connector
Remove heat shield bolts
Loosen turbo-to-downpipe and downpipe-to-exhaust connections
Install new unit with fresh gaskets
Reconnect O2 sensor and test for leaks

Common pitfalls include stripped bolts (due to heat cycling) and misaligned flanges. Applying anti-seize during reassembly helps prevent future issues.

Common Misconceptions About A90 Supra Downpipes

Several myths persist in online communities:

Misconception #1: “More Pipe Diameter = Always Better”

While larger diameter reduces restriction, oversizing can reduce exhaust velocity, hurting low-end response. The B58 responds best to well-balanced flow, not just big pipes.

Misconception #2: “You Don’t Need a Tune With a Catted Downpipe”

Even with a high-flow catted unit, tuning unlocks full performance. Factory maps aren’t optimized for altered backpressure.

Misconception #3: “All Catless Pipes Sound Aggressive”

Sound depends on downstream components (mid-pipe, muffler, resonators). A catless downpipe alone won’t guarantee loud crackles without additional exhaust mods.

Resale and Warranty Implications

Installing an aftermarket downpipe exhaust upgrade for 2020–2024 Toyota Supra A90 may affect:

🚗 Factory warranty: Dealers can deny powertrain claims if modifications are linked to failure
📉 Trade-in value: Check engine lights or visible modifications reduce appraisal offers
🌐 Digital logs: BMW/TNGA platforms store tuning history in multiple modules (DME, TCU, ICV)

Some tuners offer "stock restore" functions, but evidence of modification may persist in freeze frame data.

Final Recommendations: Who Should Upgrade?

An a90 supra downpipe makes sense if:

You’re pursuing stage 2+ modifications
You accept the legal and environmental trade-offs
You plan to tune the ECU immediately after install
Your region allows modified vehicles on public roads

It’s not recommended if:

You prioritize reliability and warranty protection
You live in a strict emissions state (e.g., California, New York)
You want minimal maintenance and hassle-free ownership

Frequently Asked Questions

Can I install a downpipe myself?: Yes, with basic mechanical skills and tools. Allow 3–5 hours and ensure proper torque specs are followed.
Does a downpipe add horsepower?: Yes, typically 30–50 whp when combined with a tune. Without tuning, gains are minimal (5–15 whp).
Is a catless downpipe illegal?: Yes, in most countries. Removing a catalytic converter violates federal emissions laws in the U.S., EU, Canada, and Australia.
Will a catted downpipe pass inspection?: It depends. High-quality catted pipes from reputable brands often pass if the rest of the emissions system is healthy.
What size downpipe is best for a stock Supra?: A 3.5” or 4.5” catted downpipe is ideal. Larger diameters suit tuned engines; smaller ones preserve low-end response.