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B58 Racing

Buying guide · Gleb Blekher

Catted vs Catless B58 Downpipe: Sound, Power, Check-Engine Lights and What Else Changes

Short answer: A high-flow catted downpipe swaps the dense factory converter for a freer-flowing 200-cell cat. A catless (race) downpipe has no converter at all. Both need a tune that supports the pipe. The catless pipe is louder, smells of raw fuel, and on a stock map it will set a catalyst code. The catted pipe is quieter and cleaner, and makers sell several that they say won't set a catalyst light. We found no maker that publishes a same-car, same-tune dyno test of its own catted and catless pipes, so any power gap between them isn't proven. Pick catless only for a dedicated competition car. Everyone else should start with a catted pipe.

Parts and tunes in this guide are for off-road / competition use only. Not legal for use on public roads in the United States, including California.

I'm Gleb Blekher, owner of B58 Racing in Dallas, TX. Both of my B58 cars run a downpipe. This guide sticks to what the makers publish and what owners consistently report. Where nobody has published real data, I'll say so.

Catted vs catless at a glance

High-flow catted downpipeCatless (race) downpipe
Catalytic converterYes, usually 200-cell (factory is a dense 700-cell, per Fabspeed)None
SoundDeeper and louder than stock, still muted by the catLoudest. Raspier under load, louder cold starts
Highway droneLow with the factory exhaust (CTS says none)More likely, especially with a non-resonated cat-back
SmellClose to stock once warmRaw-fuel smell at idle, cold start and in the garage
Catalyst code on a stock mapVaries by pipe. Some makers claim no lightYes, P0420 is common
Tune neededYes for the gains. Some makers say not for the lightYes, a tune written for a race pipe
PowerMaker claims around 20 hp, more with a tuneNo published same-car comparison against catted
Cat durabilityCat can be damaged by misfires or crackle mapsNothing to damage
Emissions equipmentKeeps a converterRemoves it. Competition cars only

What a catalytic converter actually does here

The catalytic converter cleans up hydrocarbons, carbon monoxide and NOx before they leave the tailpipe. On the B58 the factory converter sits in the downpipe, right behind the turbo. Fabspeed describes it as a 700-cell ceramic unit and calls that dense core one of the biggest restrictions after the turbo.

A high-flow catted pipe keeps a converter but uses a much more open core. The ones we carry are mostly 200-cell: the CTS Turbo 4.5" high-flow cat, the Fabspeed G20 catted downpipe with its HJS cat, the Scorpion 5" downpipe and the Spool 5" 200-cell catted downpipe. Active Autowerke and AMS use GESI high-flow cats.

A catless pipe, like the Spool 4.5" race downpipe or Spool 5" race downpipe, is just a pipe. That is the whole difference, and everything below follows from it.

Sound and drone

The converter also works as a muffler. Take it out and the car gets louder.

  • Catted: louder and deeper than stock, but the cat takes the edge off. CTS says its high-flow cat stays subtle at cruise, with no drone on the factory exhaust.
  • Catless: owners report it's close to stock at idle and steady cruise in Comfort, then much louder and raspier under load. Cold starts are noticeably louder.
  • Drone usually comes from the combination. A catless pipe with a straight-through, non-resonated cat-back is where most drone complaints come from. If you want a race pipe and a quiet highway, keep a resonated or valved rear section.
  • Bigger pipes change the tone. Spool says the larger 5" diameter of its pipes gives a deeper, richer tone. That's a sound claim, not a power claim.

Our M340i and M440i mods guide and A90 Supra mods guide say the same thing for those cars: any downpipe means a louder car and some drone on the highway.

Power: what the makers claim

Treat every number here as a maker's claim on its own dyno, not our data.

  • Fabspeed (catted, F22 M240i): Fabspeed says its Sport Cat downpipe made peak gains of 21 hp at 6,000 rpm and 29 lb-ft at 3,700 rpm on its in-house DynoJet. It doesn't say whether the test car was tuned.
  • Active Autowerke (catted, A90 Supra): Active Autowerke says its catted downpipe with its Signature exhaust gained 18 whp on the stock tune, and 53 and 94 whp with its Stage 1 and Stage 2 tunes. That's downpipe plus exhaust plus tune, not the downpipe alone.

What we could not find: a maker publishing a back-to-back test of its own catted and catless B58 pipes on the same car and tune. Third-party guides and forums disagree. Some say the gap is small on the stock turbo. Others say catless always makes more. Nobody shows controlled data. My read is that on a stock turbo, the tune matters far more than whether the pipe has a modern high-flow cat in it.

On upgraded turbos, check the cat's power rating. AMS, for example, rates its GESI cat for more than 800 hp.

Check-engine lights, O2 sensors and the tune

The B58 has an O2 sensor before the cat and one after it. The engine computer compares them. A working converter evens out the rear sensor's signal. When the front and rear signals look alike, the DME decides the converter isn't doing its job and sets a catalyst-efficiency code, usually P0420.

  • Catless on a stock map: the rear sensor reads like the front one, so you'll get a catalyst code. Owners also report boost or throttle cuts until the car is tuned. A race pipe needs a tune written for it.
  • Catted: depends on the pipe. Active Autowerke says its GESI pipes won't set a check engine light. Fabspeed guarantees no cat-related light, provided the engine and its software are healthy and properly tuned. CTS says its high-flow cat needs Stage 2 software to avoid one. Wagner says a fault code may show with its kit.
  • Either way, tune it for the gains. The makers' bigger numbers all come with a tune. Load one that matches your pipe before you drive the car hard.

Exhaust leaks and loose O2 connectors can set codes too. Our downpipe install guide covers both.

Smell

A converter burns off the unburned fuel that makes it out of the engine. Without one, you smell it.

Owners of catless B58s consistently report a rich, raw-fuel smell at idle, on cold starts and in a closed garage. A tune doesn't remove it. A good high-flow cat usually smells like stock once it's warm. Expect some smell on a cold start with either pipe while the engine runs rich to warm up.

Heat and cat durability

  • The cat is the fragile part. Misfires and aggressive pops-and-crackles maps send unburned fuel into the converter, where it burns and can overheat the core. CTS says it doesn't cover cats damaged by crackle tunes. Wagner says its cats aren't meant for burble maps.
  • Warranties differ. Active Autowerke's GESI pipe for the G20/G22 has a longer warranty on the shell than on the catalyst core. Spool, Scorpion and others sell their pipes for off-road use, so read each warranty before you buy.

Gen 1 vs Gen 2 vs Supra: fitment differences

Catted or catless, the pipe has to fit your engine and your car.

  • Some pipes fit both generations. CTS, Scorpion's 5" and Spool list Gen 1, Gen 2 and (for Spool) the Supra. Others are made for one family, like the Active Autowerke F-chassis AFlow pipe and the G-chassis version. Don't assume. Read the fitment line. Not sure which engine you have? See our B58 Gen 1 vs Gen 2 guide.
  • GR Supra. Some pipes are Supra-specific, like the AMS Street downpipe with GESI cat, which AMS says fits US-market cars only. Euro-spec Supras with a particulate filter (OPF) need an OPF-specific pipe.
  • OPF cars. CTS and Wagner both say their pipes fit only cars without a GPF/OPF.
  • 2025+ M340i/M440i (B58N). These use a different downpipe with a particulate filter. Active Autowerke sells separate catted pipes for them, and the install is a much bigger job. Send us your VIN first.
  • xDrive. Many pipes, like CTS's, are one part for RWD and xDrive.

How to choose

  1. Dedicated competition car, never on the street: a catless race pipe is the simplest and loudest option. Budget for a tune written for it, and accept the smell.
  2. Everyone else: start with a high-flow catted pipe. On a G20 or G22, I'd stick with a catted downpipe. It's quieter, it smells better, and it's kinder to your neighbors.
  3. Big-turbo build: look at pipe size and the cat's power rating, not just catted vs catless.
  4. Then match the tune to the pipe. Tell your tuner exactly which pipe you installed.

Shop all B58 downpipes, pair one with an exhaust, and see where it fits in the best B58 mods, in order. Unsure what fits? Send us your VIN.

FAQ

Is a catted or catless downpipe better for a B58?

For most owners, catted. It's quieter, smells close to stock and keeps a converter. Catless only makes sense on a dedicated competition car. We found no maker data proving a meaningful power gap on the stock turbo.

Will a catless downpipe throw a check engine light on a B58?

Yes, on a stock map. The rear O2 sensor no longer sees a converter working, so the DME sets a catalyst-efficiency code, usually P0420. A race pipe needs a tune written for it.

Do I need a tune for a catted B58 downpipe?

For the gains, yes. Makers differ on the light: Active Autowerke says its GESI pipes won't set one, while CTS says its high-flow cat needs Stage 2 software to avoid one. Check the listing for your pipe.

How much horsepower does a B58 downpipe add?

Fabspeed claims 21 hp and 29 lb-ft for its catted M240i pipe on its own dyno. Active Autowerke claims 18 whp for its catted Supra pipe plus exhaust on the stock tune, and more with its tunes. These are maker claims.

Does a catless downpipe drone?

It can, especially with a straight-through cat-back. Owners report little drone with the factory exhaust. CTS says its catted pipe doesn't drone with the factory exhaust.

Does a catless downpipe smell?

Yes. Owners report a raw-fuel smell at idle, on cold starts and in the garage. A tune doesn't remove it. A good high-flow cat smells close to stock once warm.

Sources