Downpipe vs Cat-Back: Which Should You Buy First?

On a G20 M340i, a catless downpipe costs $199.99 and the least expensive AWE cat-back costs $1,845, and the cheap part is the one that adds power. That is the whole downpipe vs cat-back argument in one sentence, and it is why the forum answer for a turbo car is almost always "downpipe first." The longer answer depends on whether you want horsepower, sound, or a car that still passes inspection.

This guide decides which part to buy first on a turbocharged BMW (N54, N55, B58, S55, S58), an Audi B8 or B9 S4 and S5, a MK7 or MK8 GTI and Golf R, and the A90 Supra, what each costs, and how the two work together once you own both. Naturally aspirated cars get one paragraph.

Prices are from our storefront and vary by dealer and tip finish. Gains and weights come from our product pages and the makers' published data (AWE, Akrapovic, APR, Dinan). Owner reports come from SupraMKV, Bimmerpost and Audizine threads.

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The Short Answer

Buy the downpipe first for power on a turbo car: with a supporting tune, our $199.99 B58 catless downpipe is rated at 30 to 40 hp over stock and our $289.99 S58 pair at 30 to 50 hp. Buy the cat-back first for sound, weight and street legality: AWE, Borla and Dinan systems run $1,574 to $2,875 and add 6 to 16 hp at most.

Downpipe vs Cat-Back: What Each Part Actually Does

A downpipe is the pipe bolted to the turbocharger outlet, and on every modern turbo car it also carries the primary catalytic converter, so replacing it removes the single biggest restriction in the exhaust. A cat-back is everything behind that point: mid pipes, resonators, mufflers and tips. It sits downstream of the restriction, which is why it changes sound far more than power.

Open up the pipe at the turbine and the turbo spools sooner, exhaust gas temperature drops, and a Stage 2 map can ask for more boost. Open up the pipe at the back bumper and the turbine never notices.

What a Downpipe Changes on a Turbo Car

A downpipe with a supporting tune is the biggest single power gain per dollar on a B58, S55 or S58, and without the tune it is a louder exhaust that sets a catalyst code. Our product pages rate the B58 4.5 inch catless pipe at 30 to 40 hp over stock with a supporting tune, the S55 3 inch pair at 25 to 40 hp, and the S58 3 inch pair at 30 to 50 hp, with more on upgraded turbos.

Owner numbers line up. A 2021 Supra 3.0 on SupraMKV with a catless downpipe, stock intake and a Stage 2 map on 93 octane put down 443 whp and 522 wtq, close to 20 whp of it from the pipe alone on his own baseline (dyno type not stated).

The costs that come with the pipe are the ones people forget to budget.

• A tune is part of the price. A catless pipe on the stock map sets a catalyst efficiency code, usually P0420 (P0430 for the second bank on an S55 or S58), and a 2024 Supra owner on SupraMKV also had overboost and throttle closing until the car was flashed. bootmod3 lists at $595 and MHD at about $435 with a map pack, so a DIY B58 Stage 2 lands at $660 to $820. Read our B58 Stage 1 vs Stage 2 guide and bootmod3 vs MHD first.
• It smells. A 2023 M3 Competition xDrive owner on Bimmerpost put it plainly: sport cat downpipes cut the smell by about half with barely any power gain, catless pipes add a little power and smell on pump gas, and E85 is fine either way.
• It is louder than people expect. On the stock exhaust a catless B58 pipe stays near stock in Comfort and gets much louder in Sport, with sharper pops on a stock map.
• It is a competition part. The legal section below covers that. On the B58, add the $21.99 crush gasket to the order.

What a Cat-Back Changes: Sound, Weight and Drone

A cat-back changes how the car sounds every day, drops 6 to 18 lb depending on the system, and adds very little power on a turbo car because the factory rear section is not the restriction. The makers publish the numbers and are honest about how small they are.

• Power. AWE rates the M340i Track Edition at 16 hp and 12 lb-ft at the wheels on stock software and the G42 M240i Touring at up to 10 hp and 17 lb-ft. Akrapovic measured 10.9 hp at 6,000 rpm on its own dyno against a stock M4 Competition. Dinan claims 6 hp from its G8X valved axle-back. Owners rarely feel any of it.
• Weight. AWE says the M340i Track Edition saves 11 lb. Akrapovic's G80 titanium Slip-On is 8.3 kg (about 18 lb) lighter, which it quotes as 35 percent. Dinan's G8X axle-back is 38 lb against 44 lb stock. APR's MK8 GTI system is 9 percent lighter than stock.
• Drone. Drone is a low frequency hum at a steady cruise rpm, and it is the one way a cat-back makes a car worse. AWE's Touring Edition uses its 180 Technology to cancel it, the Track Edition deliberately leaves it out, and APR uses a Helmholtz resonator for the same job on the GTI. Resonated versions are quieter at cruise than non-resonated versions of the same system.
• Valves. Factory valved cars (G20, G8X, Supra) keep their valve logic with a Dinan or AWE SwitchPath system, so Comfort stays civil and Sport opens up. A valve controller sets them anywhere in between; one 2022 X5 owner on Bimmerpost runs his at 50 percent.

Which Should You Buy First? A Decision Table

Buy the downpipe first when the goal is power or a staged build, and the cat-back first when the goal is sound on a car you are keeping stock or inspection legal. Here is the call by goal, with our storefront prices.

One exception runs the other way. If your tuning platform will not turn off the rear oxygen sensor check, the downpipe is a code generator, not a power part. COBB removed every map with catalyst codes disabled in March 2021 and settled with the EPA for $2.9 million in September 2024, so a MK7 GTI on an Accessport gets the cat-back.

Fitment and Cost by Chassis

The table below pairs each chassis in this guide with the downpipe and cat-back options we carry, with our prices, so the real gap between the two purchases is visible for your car.

Two notes. The B8 and B8.5 S4 are supercharged, so there is no turbo to spool; the downpipes still carry the primary cats and change the sound, which is why AWE sells a resonated version to tame rasp. And 2025 and newer G20 and G22 cars use a revised B58 pipe our downpipe does not cover. We are an authorized dealer for every brand we carry, and in stock hardware ships fast.

How a Downpipe and a Cat-Back Work Together

A downpipe sets the flow and the volume, and the cat-back decides how much of that reaches the cabin, so choose the two as a pair even if you buy them a year apart. A catless pipe feeding a non-resonated Track Edition is a track car. The same pipe feeding a resonated Touring Edition is a daily driver that still sounds like something at full throttle.

The owners who regret the combination almost always stacked the loudest version of both. The SupraMKV owner in the story below ran an Active Autowerke downpipe and a Remus cat-back on a 2017 M2 and called it "waaaay too much." The fix is cheap: pick the resonated or Touring cat-back when a downpipe is in the plan, or go catted on the downpipe when the cat-back is already loud.

On a naturally aspirated car the debate collapses. There is no turbo to feed, so the cat-back is the exhaust mod and the only question is whose sound you like. Borla, Corsa and MagnaFlow build most of the V8 and NA systems in our cat-back collection, and the Corsa and Borla lines publish a sound level per system so you can pick the volume on purpose.

What Owners Actually Say

Owners who have run both parts agree on the order and disagree on how loud is too loud. Five reports, paraphrased, with the car and the outcome.

Downpipe first, again. A 2023 GR Supra owner on SupraMKV had one budget and a city that tickets loud cars. He had already run an Active Autowerke downpipe with a Remus cat-back on a 2017 M2 and found the pair far too loud, so this time he asked which single part to buy. Nearly every reply said downpipe first: more gain, easier to install and resell, and harder for an inspector to spot than new tips. He ordered the catted AA pipe and left the HKS cat-back for later.

• 2020 GR Supra, HKS cat-back on the stock downpipe, then an AA downpipe added (SupraMKV). The cat-back alone was deeper but not too loud. The downpipe made the bigger difference in volume and tone, loud but good, and still quiet with the valve closed.
• 2024 GR Supra 45th Anniversary, downpipe first (SupraMKV). The downpipe alone "just isn't enough" for this owner, who had a cat-back going in the following week. Right first part; some owners want the second within days.
• 2023 M3 Competition xDrive, full system (Bimmerpost). The most useful negative we read. Sport cat downpipes cut smell in half with barely any power gain, catless adds rasp so resonated is preferred, and the rear section sounds much like stock because the valves bypass the muffler when open. His advice: a single resonated mid pipe and a valve controller first, downpipes for volume, the rear last.
• 2021 GR Supra, off brand cat-back replaced with AWE Touring resonated (SupraMKV). The first system was too loud to live with. The AWE was "just right at cruise" and came alive under throttle; a second 2021 owner running it with an AA downpipe called it louder than stock, not too loud, no rasp.
• Audi 8V A3, AWE 200 cell catted downpipe (Audizine). No code for roughly 35,000 miles, then a P0420 after moving to a newer map with the catalyst check left on. The older tunes had been suppressing it. A 200 cell cat passes the check only when the map ignores it.

Legal and Emissions

This part is sold for off road and competition use only. Removing or defeating a catalytic converter on a vehicle driven on public roads violates the Clean Air Act, and many states inspect for it. Check your state and local laws before buying.

That wording applies to every catless downpipe we sell, including our B58, S55 and S58 pipes. Catted downpipes are sold as emissions legal for your state only with a CARB Executive Order or an EPA verification, EO number named. We are not aware of a B58 or S58 downpipe with a CARB EO; AMS describes its GESI catted Supra pipe as 49 state legal with a race waiver for California, and that is the most compliant option we carry. Replacement converters are a different category: MagnaFlow's direct fit catalytic converters come in EPA and CARB grades, and our MagnaFlow OEM grade vs CARB grade guide explains which your state requires.

A cat-back is legal to install in all 50 states as a rule, because it replaces no emissions equipment, and AWE backs its systems with a No CEL guarantee. Noise is the only limit: California caps exhaust noise at 95 dB measured by the SAE J1492 procedure for vehicles under 6,000 lb GVWR, and many cities write their own tickets on top of that. Non-resonated Track systems are the ones that get measured.

Mistakes We See

The most expensive mistake in the downpipe vs cat-back decision is buying the cat-back for power, and the second is bolting on a catless downpipe without a map. The rest are cheaper, and all six are avoidable before the order goes in.

• Buying a cat-back for horsepower on a turbo car. The makers' own numbers top out around 16 hp at the wheels. That $1,845 buys a downpipe, a gasket, a bootmod3 license and most of a set of tires.
• Running a catless downpipe on the stock map. P0420 is guaranteed, the 2024 Supra owner above also got overboost and throttle closing, and the pops get loud. Order the pipe and the map together.
• Assuming a catted downpipe never codes. The Audizine A3 ran 35,000 miles clean and then coded the day the map stopped hiding it. "High flow" and "no CEL" are claims that depend on the tune, not the pipe.
• Buying the loudest version by default. Track and non-resonated systems exist for owners who want them. Everyone else buys Touring or resonated, especially with a downpipe coming later.
• Stacking catless plus non-resonated. The 2017 M2 story above. A catless pipe adds rasp on an S55 or S58; a resonated mid pipe or Touring rear section takes it back out.
• Forgetting the small parts and the cut. The B58 pipe needs a $21.99 single use crush gasket, S55 and S58 pairs need two. On G8X cars, a vendor in the Bimmerpost thread noted that full cat-backs from AWE, Akrapovic and Remus require cutting the stock system at the factory cut points; a mid pipe plus axle-back does not. Check before ordering for a lease car.

Who This Is For (and Who Should Skip It)

The downpipe is for the owner who is going to tune the car, drives it off road or in competition, and has no annual OBD emissions inspection to pass. The cat-back is for the owner who likes the car's current power, wants it to sound like what it is, and wants to keep the title clean.

Skip the downpipe if you are leasing, if your state plugs into the OBD port every year, if you will not tune the car, or if you cannot stand a fuel smell in the garage. Skip the cat-back if you are chasing a dyno number on a budget under $1,000; put the money into the pipe and the map and let the stock rear section carry the sound for now. Skip both on a 2025 or newer G20 or G22 until fitment is confirmed, because the revised B58 pipe is a different part.

If It Were Our Car

Our pick is downpipe plus tune first on every tuned turbo BMW and Supra, and cat-back first on the supercharged Audi, the COBB tuned Volkswagen, and anything naturally aspirated. The S58 is the one we hedge on, for a reason the M3 owner above explained.

• B58 M340i, M240i or Supra, tuned. Downpipe, gasket and a Stage 2 map first, about $660 to $820 DIY. A year later, the AWE Touring resonated if the car commutes, the Track if it does not. The B58 catless downpipe deep dive has the install.
• S58 G80, G82 or G87. The S58 is already loud and the factory valves do most of the work, so a daily driven M3 gets a resonated mid pipe or the Dinan valved axle-back first, as the M3 Competition owner advised. A competition car gets the S58 3 inch catless downpipes and a tune first; our S58 downpipe guide and S58 Stage 1 vs Stage 2 guide cover the rest.
• S55 F80, F82 or F87. Same answer: downpipes plus tune for a competition car, AWE SwitchPath or Dinan axle-back for a street car. The S55 downpipe guide has fitment by year.
• N54 or N55 335i. Downpipes with a tune are the classic recipe and we do not currently carry a pipe for either engine, so the honest advice is a Dinan or AWE axle-back from us and a downpipe from a maker with a CARB EO if one exists.
• Audi B8 or B8.5 S4. Cat-back first. With no turbo to spool, AWE's Touring Edition at $1,745 to $1,995 does more for the experience than a downpipe does for the dyno. AWE's resonated downpipes come second; they also cure the rattle AWE reports in a few cars running its cat-back on factory downpipes.
• MK7 or MK8 GTI and Golf R. Cat-back first on a COBB car, since the platform will not hide a catalyst code; AWE Track at $945 or APR at $1,079 are the value picks. A downpipe only makes sense on a platform that supports it in a state that will not test for it.
• Anything naturally aspirated. Cat-back, from the brand whose sound clip you liked. Buy the quieter level if unsure, because nobody returns an exhaust for being too civil.

What Pairs With Each

A downpipe pairs with a tune and a gasket; a cat-back pairs with the resonator and valve choices that match the downpipe you have or plan to have. For a leased car, an axle-back keeps the stock pipes uncut. And if the budget allows, the ceiling of the cat-back market is still Akrapovic. The G80 M3 and G82 M4 Slip-On Line Titanium is $7,502.50 plus a carbon tail pipe set, saves about 18 lb, needs no remap, and keeps its ECE type approval on OPF cars until you add a downpipe. We carry it alongside the full AWE Tuning range and the broader exhaust lineup, so there is a stainless answer at a third of the price. Returns follow our refund policy; street legality is not a return reason on a competition part.

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The Bottom Line

Downpipe vs cat-back is a sequencing question, and on a turbo car the sequence is downpipe plus tune first, cat-back second. The pipe is $199.99 to $289.99, adds 25 to 50 hp with a supporting tune depending on the engine, and is a competition part that smells and fails an emissions check. The cat-back is $945 to $7,503, adds 6 to 16 hp on the makers' dynos, drops 6 to 18 lb, and is legal everywhere the noise ordinance allows. Buy the cat-back first if you are leasing, inspected, untuned or supercharged. When you own both, put a resonator somewhere in the line.

Frequently Asked Questions

Should I buy a downpipe or cat-back first?

Downpipe first if you want power from a turbo car and plan to tune it; our B58 pipe is $199.99 and rated at 30 to 40 hp over stock with a supporting tune. Cat-back first if you want sound, are leasing, face an annual OBD inspection, or will not tune the car.

Does a cat-back exhaust add horsepower on a turbo car?

Very little. AWE rates its M340i Track Edition at 16 hp at the wheels on stock software and Akrapovic measured 10.9 hp at 6,000 rpm on a G82 M4, both on the makers' dynos. Owner back to back tests rarely show a difference you can feel.

Do I need a tune with a downpipe?

Yes. A catless downpipe on the stock map sets a catalyst efficiency code, usually P0420, and B58 owners also report overboost and throttle closing until the car is flashed. The 30 to 40 hp figure for our B58 pipe assumes a supporting tune with the rear oxygen sensor check turned off.

Can I run a downpipe with the stock exhaust?

Yes, and most owners do for a while. Our B58, S55 and S58 pipes work with the factory cat-back, and the stock rear section with its valves keeps the car close to stock in Comfort mode. You still need the tune.

Are cat-back exhausts legal in all 50 states?

As a rule yes, because a cat-back replaces no emissions equipment. Noise laws still apply: California limits exhaust noise to 95 dB under the SAE J1492 test for vehicles under 6,000 lb GVWR, and many cities enforce their own limits.

Is a catless downpipe legal for the street?

This part is sold for off road and competition use only. Removing or defeating a catalytic converter on a vehicle driven on public roads violates the Clean Air Act, and many states inspect for it. Check your state and local laws before buying.

Will a cat-back drone on the highway?

A non-resonated or Track system can. AWE's Touring Edition uses 180 Technology to cancel drone at cruise and APR uses a Helmholtz resonator on the GTI, so pick a resonated system if the car spends time at a steady 70 mph.