The Correlation Between Catalytic Converters and Horsepower
Concerns about catalytic converters reducing power are largely unfounded for modern vehicles. While older converters were restrictive, advancements in materials and design mean today’s high-flow catalytic converters cause virtually no loss in engine power when properly sized. If you live in California or the UK, removing your cat will result in a failed inspection, even though you may be seeking maximum power.
In this article, I’ll present dyno-proven facts and practical advice to set the record straight so you can have a clean, legal street car without losing power and trouble with the feds. Let's start by being clear on the reality of catalytic converters vs power restriction.
The Myth vs. The Reality
Myth
“Cat deletes add huge power.” On many car forums, people insist that catalytic converters are a major choke point. They believe taking the cats out or installing a straight pipe will add a bunch of horsepower.
Reality
Modern performance cars can easily make 500-1,000+ hp with full emissions equipment installed. This includes the factory-tuned 1064-hp Corvette ZR1, and even beyond 1,000bhp, we have a plethora of vehicles under manufacturers such as Bugatti and Koenigsegg, all of which meet emissions standards with their catalytic converters intact. Did you honestly think they would leave any available power on the table?
Clearly, well-designed cats can flow enough for extreme horsepower. Real-world dyno tests show that switching from high-flow cats to straight pipes often gains only on the order of 0–2% (or practically nothing) in power. Such tiny differences are within the noise of testing and driver “butt dyno” feel.
Rather than giving you a paragraph comparison and expecting you to take my word for it, I’ll explain why removing or “de-catting” your car can be more of a headache.
How Catalytic Converters Work
A catalytic converter cleans up the harmful exhaust gases. Hot exhaust flows through a honeycomb core coated with precious metals (platinum, palladium, rhodium). These metals trigger chemical reactions that convert pollutants (unburned fuel, carbon monoxide, NOx) into harmless gases (CO₂, H₂O, N₂). For this process to be effective, a large surface area is needed (hence the fine honeycomb) and high heat, but it also must allow exhaust gases to flow through.
Early catalytic converters (1970s) used packed beads or very dense ceramics. Those designs were highly restrictive, having much more of a negative effect on performance. Back then, enthusiasts often “gutted” their cats or fitted test pipes and saw noticeable gains. This history is why cats got a bad reputation.
Modern catalytic converters use a monolithic honeycomb structure that’s much more open and flow-efficient. The honeycomb is often made of ceramic or metallic foil with ultra-thin walls. By using thinner substrates, optimised cell density, and a larger cross-sectional area, today’s high-flow cats present minimal resistance to exhaust. In other words, advances in catalyst design have virtually eliminated the old performance drawbacks.
Keeping the cat at a proper operating temperature is important. Modern engines and ECUs manage fuel mixtures and ignition timing to keep cats working efficiently without robbing power. For example, running slightly rich at full throttle (as many factory tunes do) helps keep the cat cool and healthy, with no significant power loss if tuned correctly.
Why Cats are Often Removed
I'll be real, not everybody is just removing their cats for a louder exhaust. If you're going for volume, the least complicated option is to start from the cat-back. This means removing mufflers and resonators downstream from the catalytic converter.
Cost
If you are getting your ECU tuned and the factory cat is restrictive, the truth is, it's cheaper to get a straight section (also known as a decat or test pipe). Catalysts use precious metals, so they’re expensive to replace.
Theft
This naturally brings me to another reason why people remove cats. I'll admit, this one is rarer, but I have seen it firsthand. I'm talking about the theft of catalytic converters.
Many are relatively accessible to cut off with a simple reciprocating saw or angle grinder. The emissions device is then taken to a recycler who will pay good money for the unit or materials if a serial number is supplied. Once this has happened several times, consumers see no point in replacing the unit and would rather remove it altogether. Toyota Priuses and SUVs are heavily targeted due to the value or accessibility of the emissions device.
Thankfully, there are anti-theft devices available, and manufacturers are often placing them in less accessible areas of the vehicle.
Faults and Failures
Once an emissions device has failed multiple times (usually EGR valves and DPFs), customers are reluctant to replace them, especially if they're not covered under any kind of warranty. The same applies to catalytic converters, but in most cases, if this is occurring, there is an underlying issue causing the blockage in the first place.
Burble, Pops 'n' Bang Tunes
I'll go into the specifics later on, but essentially, anything that introduces excessive heat or post-combustion ignition in the exhaust can damage a catalytic converter. Additionally, those enabling these features know that they are less effective at giving you those crackles and loud pops.
Flames
No unburnt fuel, no fire. I love a good fire show, but it has to be natural, a byproduct. Not designed specifically for that. Otherwise, sacrifices are made somewhere. This means no gimmicks, daft features or devices. Artificial flame-producing strategies usually involve fuelling compromises, and something has to give.
Track Use
Some owners remove their cats because they think a decat makes the car more suitable for track use. In reality, many track day organisers in the UK and Europe expect road cars to keep their emissions equipment fitted, and noise limits are usually the real barrier. A properly sized high-flow cat has no noticeable impact on track performance, and it often helps keep drive-by noise within limits. Removing the cat tends to make the car louder rather than faster, which leads many drivers to be turned away before they even get a session.
Power
This entire article is about the assumption that cats restrict power, so there’s no need to expand much here. The idea is to remove any restrictions and get maximum clean air into the engine as possible. Remove cats (and often other emissions devices along with them, such as DPF & EGR valves).
Dyno Test Evidence: Do High-Flow Cats Rob Power?
To separate fact from forum fiction, let’s look at actual dyno results comparing exhaust setups:
Greg Banish’s Twin-Turbo LS3 (660 whp)
Engine calibration expert Greg Banish tested his 660 horsepower twin-turbo Corvette with and without catalytic converters. With older high-flow 3-inch cats, the car made 651 wheel hp. Swapping to straight pipes (no cats) yielded 660 hp; a gain of only 9 hp (~1.5%). Then he tried new-generation GESI high-flow cats, and power came out exactly the same 660 rwhp as open pipes. Zero loss in power. Properly specified high-flow cats did not reduce peak power at all. The minor difference with the older cats (1.5% drop) was essentially within normal run-to-run variance. Banish also measured exhaust backpressure: the dual cats and mufflers added only ~2.6 psi, versus ~0.5 psi with straight pipes, which is a negligible difference at this power level.
Grassroots Motorsports Miata Test
Grassroots Motorsports magazine tested a modified Mazda Miata (NA 1.6L engine) with various catalytic converters vs. a straight test pipe. The results were eye-opening for those expecting big gains. The test pipe freed at most 1–2 horsepower at high RPM. High-flow performance cats from Jackson Racing, MagnaFlow, and Flyin’ Miata all showed similarly tiny gains of about 1–2 hp over the stock cat in different parts of the rev range. In fact, one high-flow cat even smoothed out some minor dips caused by the straight pipe, resulting in a power curve virtually identical to the straight pipe. The magazine concluded that “modern cats can be used with no negative impact on performance” when chosen correctly. A lightweight MagnaFlow cat was even half a pound lighter than the straight pipe, busting the myth that a cat is always a weight penalty.
Other Examples
Many tuners agree with these findings. It’s not uncommon for dyno tests on modern cars to show no measurable difference between a quality high-flow cat and a decat pipe, especially on turbocharged engines. OEMs themselves prove the point: as mentioned, production supercars and muscle cars exceeding 600–700 hp all run cats without issue.
The only time a catalytic converter will seriously reduce horsepower is if it’s incorrectly specified, damaged, clogged, or melted. A broken converter (e.g. monolith brick collapsed or coated in soot) can block the exhaust path and cause a big power drop. In those cases, replacing the cat is the fix. A healthy high-flow cat, by contrast, flows freely. If you’ve “felt a huge difference” after removing cats on a modern car, chances are your original cats were faulty or other changes (like tuning) accounted for the improvement.
Why such minimal differences? It comes down to exhaust backpressure. Every exhaust component (manifold, turbo, cat, muffler) adds some resistance. On a well-designed system, a high-flow cat contributes only a small fraction of the total backpressure. For instance, in Banish’s test at 660 hp, the turbos themselves accounted for ~22.5 psi of backpressure, whereas the cats+mufflers added only ~2.1 psi. Turbo or not, if the cat is large enough, it’s simply not a significant restriction.
As Banish put it:
“As long as the plumbing into and out of a good catalyst is adequately sized, the cats don’t cause a significant pressure drop or horsepower loss”.
How Backpressure Reduces Volumetric Efficiency
Backpressure reduces engine power by interfering with the way an engine breathes. When exhaust pressure is high, the piston must work harder to push out the exhaust gases, increasing pumping losses and reducing the power reaching the crank. Any exhaust gas that remains in the cylinder also occupies space that should be filled with fresh air and fuel, which lowers the mass of oxygen available for combustion. This drop in fresh charge reduces volumetric efficiency and therefore reduces power. Engines with high VE make more torque and power because each cylinder is filled as completely as possible.
A simple rule to follow: When backpressure rises, VE falls.
This relationship is well documented in engine research, with studies showing a clear decline in VE as exhaust restriction increases. Turbocharged engines can be affected even more because high exhaust pressure reduces the pressure difference across the turbine, slowing response and reducing potential boost. Modern high-flow catalytic converters avoid these issues because their pressure drop is very low compared with the manifold, turbine or mufflers.
Naturally Aspirated vs. Turbo vs. Highly Modified Engines
Naturally Aspirated (NA) Engines
On an NA engine, exhaust backpressure directly opposes the pistons, pushing out exhaust gases. In theory, any reduction in backpressure can free up a bit of power. However, as I’ve already mentioned, with modern low-restriction cats, the added backpressure is so small that power loss is negligible. For example, the Miata’s ~1–2 hp gain without a cat was only at the very top of the rev band. For most of the power curve, there was no advantage. If an NA engine is relatively stock or mildly tuned, the stock cat likely flows enough that you won’t see measurable gains by removing it. Only when an NA engine is heavily modified (high RPM cam, big displacement, etc.) might the factory cat become a bottleneck. In those cases, the solution is usually to fit a larger high-flow cat rather than running open pipes.
Turbocharged Engines
Turbo motors are a special case because the turbocharger itself creates a lot of exhaust restriction (the turbine needs pressure to spin). This means the turbo is the dominant factor in exhaust backpressure, not the catalytic converter. A high-flow cat downstream of the turbo has a minor impact compared to the turbo’s turbine housing and wheel. In Banish’s twin-turbo test, removing the cats gained only 9 hp on a 660 hp setup. He noted that upgrading to bigger turbos would yield far more gain than deleting cats.
It’s not uncommon for turbocharged engines to need some exhaust tuning for optimum response. But too many people throw out the cats and oversize their exhaust. An overly large, catless exhaust can reduce low-end torque or create undesirable noise. High-flow cats often provide a nice balance by keeping some exhaust velocity and heat for the turbo without choking the top end.
Highly Modified / High-Output Builds
What if you’re chasing horsepower? In those cases, you must ensure the cats are sized for the job. A small OEM cat from a 200 hp car won’t support a 600 hp engine without causing a pressure drop or overheating. Fortunately, the aftermarket offers performance catalytic converters in various sizes, rated for very high power. For instance, GESI (Global Emissions Systems) manufactures EPA-compliant high-flow cats for different power levels. Here is a general guideline for catalytic converter sizing vs. power:
| Catalytic Converter Core Size (diameter) | Recommended Max Engine Power |
|---|---|
| ~3–4 inch (76–101 mm) “High-Flow” Cat (typical OEM or HO cat) | Up to ~500 HP |
| ~4.5 inch (114 mm) Ultra High Output Cat | Up to ~850 HP |
| ~5 inch (127 mm) Motorsport Cat (high-flow Gen2 type) | Up to ~1000 HP |
| ~6 inch (152 mm) Motorsport Cat (largest cores) | Up to ~1200+ HP |
Table: Examples of catalytic converter sizing recommendations for various power levels (based on GESI G-Sport performance cats). Always match the cat size to your engine’s output and exhaust tubing diameter for best results.
Ensuring Your Sports Catalytic Converter Isn’t Restrictive
If you decide to upgrade to a sports cat (high-flow performance catalytic converter), how can you be sure it won’t hold back your engine? In this section, I’ll explain what makes a “sports cat” different and how to size and select one that will not be a bottleneck.
What is a Sports Catalytic Converter?
A “sports cat” is an aftermarket catalytic converter designed for higher flow and performance. It typically uses a lower cell count (cells per square inch, CPSI) in its honeycomb substrate than a standard OEM cat. Most stock catalytic converters have a dense substrate around 400–600 CPSI, whereas sports cats often use 200 CPSI or even 100 CPSI to reduce backpressure. Fewer cells means larger passages for exhaust gas, which makes the converter less restrictive. Many sports cats also use high-efficiency metallic substrates instead of traditional ceramic; the metallic core can be made with thinner walls, allowing lower CPSI (and thus higher flow) without completely sacrificing catalytic surface area.
Cell Count (CPSI) and Flow vs. Emissions: Cell count directly affects both flow and emissions performance. A 400 CPSI catalyst contains 400 tiny reaction cells per square inch; as this number goes down, there are fewer physical cells obstructing the exhaust, so backpressure drops and flow improves. However, a lower cell count also means less surface area coated with precious metals to clean the exhaust gases. Manufacturers consider a 200 CPSI sports cat a good compromise: it significantly improves flow over stock while still providing enough catalyst area to oxidise pollutants in most cases. Dropping to an ultra-low 100 CPSI (or removing the cat entirely) further reduces restriction, but the catalytic conversion may become insufficient. If the cell count is too low, the exhaust gases don’t stay in contact with the catalyst material long enough to be fully treated, and conversion efficiency plummets. This can trigger a check-engine light for low catalyst efficiency and result in emissions test failures. In essence, not every “high-flow” cat will guarantee emissions compliance – flow and emissions cleaning have an inverse relationship, so one must strike a balance.
Do high-flow cats pass emissions tests?
It depends on the quality/design of the cat and the test standards. Many modern OEM cats are already relatively high-flow (thanks to improved substrates and coatings) while still meeting regulations. Reputable sports cats (often 200 CPSI metallic units) can typically pass inspections if they’re properly warmed up, since a hot catalyst works more efficiently. But a sports cat is less forgiving of cold starts or poor engine tuning. Enthusiast reports show that 200-cell cats sometimes only just meet MOT (UK emissions) limits, and can fail if not heated thoroughly. For example, one driver’s car with a 200 CPSI sports cat initially failed a fast-idle emissions test with CO ~1.76% (limit 0.20%) and HC 621 ppm (limit 200), which is a huge exceedance. The cat had been “rather cold” during that test, and the consensus was that getting it red hot with a spirited drive before re-testing was required to have any chance of passing. Many owners find that a hard run (high revs, motorway driving) immediately before the test is necessary to heat a sports cat sufficiently.
To answer the question, a high-flow cat can be emissions-compliant, but only if it’s of good quality and fully up to operating temperature. You should never assume that bolting on any aftermarket “sports” cat will guarantee you pass emissions. You should check whether the unit is approved for road use, and be prepared to make sure it’s hot during testing.
Exhaust Flow Requirements and CFM per Horsepower
To ensure your sports cat isn’t a restriction, consider the exhaust flow (volume per minute) it can support relative to your engine’s output. A useful rule of thumb from industry experts is that an engine requires roughly 2.2 CFM (cubic feet per minute) of exhaust flow per brake horsepower to avoid power loss from backpressure. At this flow capacity, the exhaust system’s added backpressure is almost negligible (on the order of 0.2 psi). Essentially, this means if your engine produces 300 bhp, the exhaust (including the catalytic converter) should flow about 300 × 2.2 ≈ 660 CFM or more with minimal restriction. Likewise, a 500 bhp engine would need on the order of 1100 CFM flow capacity for the exhaust system. Manufacturers of quality high-flow catalytic converters will often provide a CFM rating or horsepower rating for their units.) This figure can tell you if a given cat can handle your engine’s output. Always use engine (crank) horsepower for these comparisons, not wheel horsepower, since that correlates directly with exhaust volume produced.
To put the numbers in perspective, 1 CFM ≈ 28.3 L/min of gas flow. So 660 CFM is nearly 18,700 L/min of exhaust gas throughput. Engines with forced induction or high RPM ranges push even larger volumes, so ensuring adequate flow capacity in the cat is critical. If the cat’s flow capacity is below the ~2.2 CFM per hp guideline, it will become a bottleneck. Exhaust backpressure will rise, and you may lose power at high load. On the flip side, if the cat (and overall exhaust) can flow at or above this benchmark, it will essentially perform as if you had an open test pipe in terms of power.
To briefly pull the previous paragraphs together:
- Check the CFM or horsepower rating of the sports cat and make sure it meets or exceeds your engine’s needs with a safety margin.
- If no data is provided, be cautious: consider choosing a larger or better-designed unit, or measuring pressure drop across the cat in real operation to verify it’s not choking the engine.
Sizing the Catalytic Converter (Diameter and Dual vs. Single)
The physical size (cross-sectional area) of the catalytic converter should match the intended flow. A sports cat typically comes in diameters to match common exhaust tubing (e.g. 2.5-inch, 3-inch, 4-inch, etc.). As a rule, the cat’s inlet/outlet diameter shouldn’t be smaller than the rest of your exhaust piping – otherwise it becomes a constriction point. The flow capacity of a converter scales with its cross-sectional area (all else being equal). For instance, a straight 3-inch diameter exhaust pipe has about 6.5 in² of area and can flow roughly 747 CFM, which corresponds to about 339 bhp worth of engine output per pipe. In comparison, a 2.5-inch pipe (area ~4.43 in²) flows ~509 CFM (sufficient for ~232 bhp). If your car has a dual exhaust with one cat on each bank, the flow splits between two converters, effectively doubling the flow capacity (two 2.5-inch cats can flow roughly 1018 CFM combined). This is why many V8 vehicles with dual 2.5" high-flow cats can support around 450+ bhp in total without restriction, whereas the same engine with a single 2.5" cat would be badly choked. Below are some approximate flow capacities for different pipe/cat sizes:
| Pipe/Cat Diameter | Single Pipe Flow (est. CFM) | Supports Up To (Single) | Supports Up To (Dual) |
|---|---|---|---|
| 2.5-inch (63.5 mm) | ~509 CFM | ~232 hp | ~463 hp |
| 3-inch (76.2 mm) | ~747 CFM | ~339 hp | ~679 hp |
| 3.5-inch (88.9 mm) | ~1029 CFM | ~468 hp | ~935 hp |
| 4-inch (101.6 mm) | ~1440 CFM* | ~655 hp* | ~1310 hp* |
Table: Estimated exhaust flow capacity and approximate horsepower supported by various pipe diameters (assuming ~2.2 CFM per hp). Dual exhaust systems roughly double the capacity compared to a single pipe of the same size.
*: 4-inch values extrapolated from formula; a 4" pipe has ~12.6 in² area.)
From the above, you can see that one 4-inch cat is roughly equivalent to two 3-inch cats in flow potential. In contrast, two 2.5-inch cats have less flow capacity than a single 4-inch system. In practical terms, if you’re running a big single exhaust (for example, on a turbocharged car), you may need a large-diameter sports cat (e.g. 4") to ensure the high output isn’t restricted. If your vehicle has dual smaller cats, each one only needs to handle about half the total CFM, so smaller diameters can suffice for a given total horsepower. Always factor in the number of cats and their diameters when calculating flow capacity. It’s the total effective flow area that matters: two smaller converters in parallel share the load, whereas a single converter must flow it all. Make sure the combined cross-sectional area (and corresponding CFM capacity) of your catalytic converters is adequate for your engine’s bhp. If in doubt, err on the larger side or consult flow data from the manufacturer.
Pops, Bangs, and Tunes
We’ve established that a good catalytic converter doesn’t kill power. However, certain tuning choices and driving habits can kill the converter. A popular fad in the car scene is adding “pops and bangs” or crackle maps via ECU tuning. This typically works by injecting extra fuel on deceleration and retarding ignition timing so that fuel ignites in the exhaust, causing loud crackles or backfires. It’s a fun sound, but it’s very hard on your exhaust and cats.
When unburned fuel detonates in the exhaust system, it creates extreme heat and pressure spikes inside the catalytic converter. The cat’s ceramic or metallic substrate can overheat and melt or break apart from this abuse.
Experts warn of aggressive burble/pops & bangs tunes that:
“result in rapid deterioration of exhaust components... [and] extreme heat in the exhaust has a big impact on the integrity of the manifold and catalytic converter, with rapid wear and premature damage likely.”
To be blunt: dumping fuel that later explodes or even significantly increases heat in the cat will shorten its life, if not destroy it outright, including other related components.
Some examples of risky strategies include:
- “Burble” or crackle tunes on factory cars, even if factory-calibrated, overuse (constant crackling on overrun) heats up the cat unnecessarily. Aftermarket burble tunes often go further and can definitely toast a cat.
- Launch control and two-step rev limiters (especially on turbo cars) that fire fuel/spark to keep the turbo spooled often shoot flames. Those flames pass through (or originate in) the cat, which can overheat it.
- Running overly rich “anti-lag” mixtures - rally-style anti-lag systems are notorious for destroying catalytic converters (which is why race cars using ALS often run without them).
- Misfires or failing ignition - unburnt fuel from a misfiring cylinder will burn in the hot cat, potentially melting it. A severe misfire can cause a cat meltdown in short order.
If you love the pop-and-bang sound, be aware you’re effectively treating your cat like a small incendiary bomb absorber. Over time, the core may crack or clog. Once a cat is melted/clogged, it does become a huge blockage (and then you will indeed lose power until it’s fixed!). For longevity, it’s best to avoid excessive crackling tunes on street cars with cats. Manufacturers that offer gentle “burble” modes typically calibrate them to stay within safe limits. But many aftermarket tunes push it further for louder bangs, so proceed with caution.
Legal and Emissions Considerations
Beyond the technical reasons, remember that catalytic converters are there for emissions, and removing them is illegal on road-going vehicles in most jurisdictions.
Based on my experience working at a tuning garage, this is often ignored. People seem to prefer putting the factory cat back in, just for inspection time. Others will likely know someone who can fudge the inspection. But inspectors are cracking down on this, especially in the UK, now that specific testing equipment is being mandated, making it harder to fake an emissions test.
Each country has their own legislation regarding emissions, or in some cases, a lack of it.
United States (EPA Regulations)
The U.S. Clean Air Act prohibits tampering with emissions equipment on any vehicle used on public roads. This includes removing or gutting catalytic converters. It is a federal offence, subject to hefty fines. The EPA has pursued shops and individuals for “tampering events”. Violators can face civil penalties up to about $4,500 per instance. Remember COBB’s $2.9M Civil Penalty from 2024? Many states also have annual inspections or OBD-II smog checks that will fail a car if the cats are missing or if the O₂ sensors/catalyst monitors indicate a problem. California is especially strict: any modification must use CARB-approved cats, and visual inspections will flag removed or non-compliant cats. In practice, legitimate exhaust shops “won’t even touch” a cat delete because of the liability and fines.
United Kingdom & EU
In the UK and Europe, it’s similarly illegal to run a road car without the required catalytic converters. Since about 1993, petrol cars must have a catalytic converter to meet MOT (emissions test) standards. A de-catted car will fail an MOT test for emissions and is not road legal. In a recent landmark case, a UK tuning shop was prosecuted for removing a car’s cat and reprogramming its ECU; they were fined ~£7,200, and the case highlighted that any modification causing a car to no longer meet its type approval emissions is an offence. So if you do live in the UK, you can upgrade to high-flow type-approved sports cats, but running without any cat on the street is unlawful.
“It’s an offence to use on a road a vehicle that has been modified in such a way that it no longer complies with the air pollutant emissions standards it was designed to meet.”
Elsewhere
Most countries have equivalent rules. Australia, Canada, etc., all require the emission control devices to remain on passenger vehicles. Penalties vary, but the theme is the same. It’s permissible to decat a dedicated race car or off-road-only vehicle, but the moment you drive on public roads, you’re breaking the law. Also, insurance companies may void coverage if you’ve illegally removed emissions equipment.
Why Emissions Regulations Exist
Without cats, your car will emit far more hydrocarbons, carbon monoxide, and NOx; the stuff that causes smog and health problems. Beyond legality, it’s worth noting that your car will smell and pollute more without a cat. Catalytic converters do a very effective job of cleaning up the exhaust (often removing 90+% of certain pollutants). If you’ve ever followed an older or one wth a cat-delete, you’ll notice the acrid exhaust smell. So keeping cats is not just about following rules; it genuinely improves air quality and the driving experience.
Best of Both: Keep Cats and Power
After examining the evidence, we can easily come to the conclusion that catalytic converters do not significantly reduce engine horsepower in a properly designed setup. You just need to do a bit of research: ensure they are sized correctly and meet the CFM to maintain maximum flow. The theory that “huge gains” can be made from removing cats is outdated. Modern high-flow cats are engineered so well that the difference in performance is minimal, as demonstrated by dyno tests showing virtually identical power with or without cats.
In contrast, the downsides of removing catalytic converters are greater than the negligible horsepower difference:
- Your car will emit more pollution
- Usually smells bad (more noticeable when sitting in traffic)
- Breaks the law (with associated fines or VI’s (police mandated Vehicle Inspection at your cost)
- Unless updated in the ECU (check-engine lights)
- Increased drone unless other adaptations are made to the exhaust system
For street-driven vehicles, the smart choice for performance and fewer headaches is to run quality catalytic converters. Sure, if you want your pops and bangs for the street takeovers to impress the boys, your cat will be destroyed.
The “cats vs no cats” debate is one where science and real-world testing deliver a conclusion based on reality and simplicity for you. Experience from tuners and engineers with years in the field reinforces the trustworthiness of this conclusion. If one were to focus on proper exhaust sizing (e.g. follow the ~2.2 CFM per hp guideline) and a good cat, and you can have your cake and eat it too: that is, enjoy every bit of your engine’s horsepower and pass emissions.
Not convinced? I’m well aware that many of my readers may not care about emissions and want a (subjectively) simpler option. Especially when reaching the stage of custom engine setups with big turbos. This was simply an exercise to debunk a myth that has circulated for far too long since effective alternatives became apparent. Now it's all cleared up with a definitive answer; enthusiasts like you can use this to make your own informed decision.
