Hollow Out A Catalytic Converter: Power, Risks, Choices

As an automobile engineer with over five years of experience modifying performance vehicles, enthusiasts frequently ask me how to hollow out a catalytic converter safely. While removing the catalytic properties significantly reduces exhaust backpressure for horsepower gains, it also tampers with emissions control systems. This carries complex legal and mechanical risks to weigh.

Important Headings

This guide will explore proper methods for hollowing out catalytic converters and their impacts on engine efficiency and the environment. With technical diligence and situational awareness, converters can be adapted for racing applications without introducing roadworthiness issues. However, alternatives like high-flow replacement parts better suit street vehicles.

Overview of Catalytic Converters

Catalytic converters act as exhaust emissions control devices, transforming toxic byproducts of internal combustion into less hazardous compounds. Inside a maze-like structure, precious metal catalysts facilitate chemical reactions without being consumed to convert hydrocarbons and nitrogen oxides into safer carbon dioxide, nitrogen, and water vapor before expelling from the tailpipe.

Are you looking for the right type of catalytic converter? Here is our guide on what type of catalytic converter I should buy.

Performance enthusiasts hollow out converters to remove this internal substrate and enlarge pathways. The objective is to reduce exhaust backpressure for amplified engine horsepower, throttle response, and sound. However, without catalytic properties, increased toxic emissions enter the atmosphere. Bypassing chemical conversion also triggers diagnostic trouble codes prompting check engine lights.

Hence, hollowed-out converters become motorsports-only components that are no longer street-legal. However, through thoughtful planning and procedure, engines can benefit from reducer exhaust restrictions without safety or mechanical compromise.

If you have tried out several DIY methods but are not able to remove your catalytic converter? Read our guide on how to remove a catalytic converter without breaking it.

Tools & Materials Needed for Hollow Out A Catalytic Converter

Before hollow catalytic converter modification, assemble the required equipment to access and open the housing unit safely. Improvising tools risk damage, injury, or lackluster results.

Safety Gear

Exhaust temperatures reach 1500°F, making protective equipment necessary. Wear:

  • Leather gloves – Protect hands from sharp metal and hot surfaces
  • Respirator mask – Avoid inhaling residual fiberglass and metal particulate
  • Eye goggles – Shield eyes from debris when drilling
  • Ear protection – Deafening noise levels drilling steel
  • Proper clothes – No loose-fitting garments that could snag

Cutting Instrumentation

The converter outer shell, thick 16+ gauge steel or stainless requires aggressive cutting implements:

  • Diamond-grit hole saw bits
  • High-torque electric drill (corded/battery)
  • Extension bars for reach spots
  • Die grinder with assorted bits
  • Plasma cutter (for larger openings)
  • Air hammer + chisel (for stubborn areas)

Also, have soapstone or markers handy to denote cut lines.

Measurement & Mapping Tools

Analyze factory converter internal dimensions to determine optimal hollowed-out sizing. Use:

  • Measuring tape
  • Inspection mirrors
  • Mechanics stethoscope
  • Exhaust backpressure gauges
  • Smoke testing equipment

Mapping flow dynamics first prevents overzealous drilling from weakening the structure.

Cleaning Agents

Post-procedure, wash all metal shavings and particulate from new passages to prevent downstream damage. Have isopropyl alcohol, brake cleaner, metal prep solutions, and rags available.

Thinking of removing your catalytic converter? Before doing that, please read our article on the impacts of removing a catalytic converter.

Now equipped with proper hollow catalytic converter modification tools, select an application-specific part to modify.

Choosing a Suitable Catalytic Converter Unit

Not all catalytic converters make sound candidates for hollowing procedures. Consider physical design, vehicle integration, and intended use before acquiring an extractor unit.

Converter Types

Catalytic converters come in two main configurations – dual bed and three-way – using different catalyst materials.

Dual Bed

Dual bed converters contain two separate substrate sections housing different chemical coatings tailored to transform specific compounds. The initial stage catalyst reforms hydrocarbons and carbon monoxide, while the downstream catalyst filters nitrogen oxides.

Advantage: Graduated emissions conversion stages

Disadvantage: A more extended unit with higher backpressure


It integrates all catalyst layers into a single substrate section, allowing simultaneous conversion of all three toxic emissions types – hydrocarbons, nitrogen oxides, and carbon monoxide- into one structure.

Advantage: Shorter component with lower restrictions

Disadvantage: No dedicated reaction steps

For hollowing, compact three-way converters are easier to gut and deburr.

Location & Position

Converters are either mounted inline with the exhaust piping or engineered inside mufflers. Inline configurations are simpler to remove and modify externally:

Muffler internal designs require cutting larger openings to access the inner honeycomb.

Determine converter orientation – engine-side vs tailpipe-side – as temperature gradients vary across unit lengths.

Intended Application

Hollowed catalytic converters can no longer chemically treat emissions, making straight-pipe exhaust unsuitable for registered road use in most regions due to environmental laws.

However, racing vehicles operate exclusively on closed courses where emissions regulations do not apply. Hollowed converters pose no legal issues and allow engines to exploit maximum power potential by removing restrictive exhaust bottlenecks.

Consider application before converter modification.

Step-by-Step Process For Hollow Out A Catalytic Converter

With the right tools and suitable converter unit chosen, follow these steps to hollow out internals carefully:

Safety Prep

Park on level ground. Disable fuel to prevent ignition sources and let cool completely before handling exhaust components. Severe burn hazards exist.

Expose Internal Casing

Cut open the converter outer housing at separation seams or weld lines using a plasma cutter/die grinder to reveal the interior substrate. Removing just one end cap often provides adequate access.

Map Internals

Inspect interior dimensions using mechanics mirrors and measuring implements like feeler gauges. Determine the thickness of outer walls and the sizes of inlet, outlet, and interior passages, noting any irregularities.

Blueprinting existing configurations allows for calculating optimal hollow area ratios. Over-expansion risks weakening structures to dangerous degrees, risking cracking, disconnects, or detachment.

Calculate the Drill Hole Limit

Using measurements, compute drill pattern span and density factors that maximize opened area without exceeding safe thresholds for structural stability.

Recommend keeping ~0.20″ casing thickness and ~20% existing substrate in key reinforcement areas. Hole diameters typically range from 0.40″ “to -0.75” based on available commercial drill bit sizes.

Transfer Hole Layout to Metal

Using soapstone chalk or markers, denote calculated drill site centers atop the catalytic converter casing. Outline areas intended to become through channels alongside reinforced regions where the substrate remains intact.

Drill Holes

Don appropriate safety gear. Then, using a diamond-grit hole saw bits matching the calculated diameter and electric drill, bore out circles following the marked drill map. Let the tool operation pace itself without forcing contact, preventing binding or bit damage. Append rigidity bars as needed to stabilize the drill unit.

Carefully drill out calculated hole-saw patterns properly sized to not compromise converter body rigidity when exhausted.

Smooth Drilled Holes

File down any remaining burrs or closure rings around bored holes using a die grinder, Dremel, or file tools. All edges should smoothly transition, avoiding exhaust flow snags that can perpetuate turbulence, eroding walls over time through abrasion wear mechanisms.

Debris Cleanout

Post-drilling, residual metal shavings likely contaminate new holes. Flush the interior using cleaning solvents applied with spray bottles and funnels. Soak and wipe down with alcohol-dampened rags. Verify no particulate matter remains before reassembling the converter.

Reseal Housing

With internals hollowed out and smoothed inside the drilled catalytic converter unit, reseal external housing using high-temp RTV silicone. Clamp end caps while adhesive cures—fresh stainless hardware ideal, though original fasteners can be reinstalled if undamaged.

Now properly hollowed and sealed, the modified catalytic converter integrates optimized exhaust flow dynamics, reducing restrictions for amplified engine output potential on racing vehicles while maintaining adequate structural reinforcements at critical junctures to prevent cracking or integrity loss from prolonged use.

Proceed with testing with caution, looking for any signs of abnormal converter behavior.

Performance Impact of Hollow Catalytic Converters

Hollowing out stock catalytic converters delivers profound performance enhancements by reducing exhaust backpressure but also incurs caveats around emissions and risks.

Reduced Exhaust Backpressure

Eliminating the internal honeycomb unleashes exhaust flow volume and velocity through converter channels, minimizing hydraulic resistance—effects amplified by enlarged through-holes and smoothed components, further diminishing turbulence. Expect ~15-25% lower measured backpressure post-modification.

Horsepower & Torque Gains

Lower backpressure decreases pumping losses as piston exhaust strokes encounter less residual cylinder pressure. Reduced restrictions also improve scavenging, expelling more spent combustion gases during overlap.

Dyno tests typically show +8-15% horsepower and +5-10% torque output positive shifts, cutting over 100 lbs load off the exhaust cycle. Engines reach power peaks more easily.

Improved Throttle Response

Lessened exhaust restrictions allow intake pulses to rapidly draw fresh air charge into manifolds, syncing with fuel delivery. Minimized exhalation lag times tighten coupling between driver input via pedal actuation vs powertrain response.

Sound Enhancement

With the sound muffling substrate removed, acoustic internals amplify and tune exhaust audio profiles. Increased volume and pitch changes make engines vocalize a deeper growl or higher-pitched scream depending on engine specs and hole-drilling effects.

Legality Issues

Voiding catalytic emissions conversion properties generates up to 30 times more nitrous oxides (NOx) and carbon monoxide (CO). The road is illegal without meeting diesel or gasoline vehicle emissions standards. Further, hollowed converters trigger check engine lights as onboard computers detect missing critical components. Racing applications only.

While performance indexes improve, hollowed converters no longer filter toxic gases, making street operation illegal. Further risks exist.

Risks of Hollow Catalytic Converter Modifications

Hollowing out catalytic converters also introduces reliability threats from structural weakening, irregular exhaust flow, and removing contaminant catchers requiring awareness.

Structural Durability Decline

Excess drilling jeopardizes casing integrity, risking cracking from prolonged thermal plus mechanical fatigue as hot exhaust gases pass through enlarged yet thinned channels. Insufficient reinforcement sections similarly risk rupturing over time.

Clogging Risks Remain

Despite smoothing drilled seams, residual microscopic burrs may persist, catching carbon particulate flowing through. This collects downstream, clogging exit holes and reducing the efficacy of enlarged passages. Further debris from blown engines still enters the exhaust path with nowhere to entrap it.

Thermal Impacts

Removed catalytic media eliminates a heat sink effect previously absorbing some thermal energy. This subjects downstream components to increased temperatures – mufflers, oxygen sensors, etc – potentially shortening service life or triggering failure modes.

Rougher Exhaust Notes

Larger hollowed channels and drilled hole edges may introduce new harmonics, decreasing sound pleasantness and producing unpleasant resonating notes at specific rpm ranges and requiring additional acoustic tuning modifications.

Thus, while power gains result, longevity assurances change, requiring careful iteration and testing to achieve an optimal balance between restriction reduction and sustainable reliability parameters.

Alternatives to Hollow Catalytic Modifications

Before attempting to hollow factory converters, first consider these alternative options.

Aftermarket High-Flow Converters

Aftermarket manufacturers offer direct replacement catalytic converters removing bottlenecks via:

  • Enlarged substrate volume
  • Reduced cell density
  • Smoothed inlet/outlets
  • Oversized piping

Performance improves with emissions control retained and no altering of factory components.

Delete Pipes

Delete pipes, remove catalytic converters, and replace them with straight exhaust tubing to improve flow. However, deleting pipes completely discards emissions control equipment, which conflicts with environmental protection efforts.

Editor’s Note: We previously mentioned deleting pipes as an alternative, but further evaluation finds their environmental impact incompatible with eco-friendly practices. According to EPA emissions control research, deleting pipes increases harmful emissions like nitrogen oxides and carbon monoxide 30-fold. We retract recommending them. Better alternatives exist, like high-flow converters using advanced catalysts to remove bottlenecks while retaining purification properties. Companies also recondition factory converters via media blasting, restoring flow rates without drilling while maintaining compliance.

The key consideration is balancing performance interests with responsible emissions practices. Convert modification for racing vehicles focuses largely on power – hence hollowing procedures covered here in detail – but street legality requires retaining the chemistry processing of expelled compounds. Beyond legal compliance, protecting the atmosphere aligns with ethical imperatives for the greater societal good.

We encourage exploring options, not discarding converters entirely despite power benefits. Secondary air injection components also reasonably handle small power losses through electronic tuning adjustments. Choose solutions upholding both interests responsibly.

Factory Part Reconditioning

Some specialists remanufactured used catalytic converters to clean out contamination and remove flow obstructions by media blasting rather than drilling to retain structural stability and law compliance.

Consider alternatives balancing both performance and responsibility factors. Modifying emissions systems remains contentious, requiring prudence and mindfulness of repercussions. Evaluate options fully before proceeding.

FAQs Related to Hollowing Out Catalytic Converter

Why would someone consider hollowing out a catalytic converter?

Enthusiasts seek to reduce exhaust backpressure, enhancing engine performance, throttle response, and sound.

What are the legal implications of hollowing out a catalytic converter?

Hollowed converters are generally not street-legal, as they bypass emissions control, potentially violating environmental laws.

Are there risks involved in modifying catalytic converters?

Risks include structural weakening, irregular exhaust flow, and increased emissions, impacting reliability and environmental concerns.

Can hollowing out a catalytic converter improve engine performance?

Yes, it can lead to reduced backpressure, resulting in horsepower and torque gains, improved throttle response, and enhanced exhaust sound.

How can I choose the right catalytic converter for hollowing?

Consider factors like converter type (dual bed or three-way), location, intended application (racing or street), and the unit’s physical design.

What tools and safety precautions are necessary for the modification process?

Safety gear includes gloves, respirator mask, eye goggles, and ear protection. Tools range from diamond-grit hole saw bits to a plasma cutter, with measurements and cleaning agents essential.

Is hollowing out catalytic converters environmentally responsible?

No, hollowing out removes emissions treatment, increasing toxic emissions. Responsible alternatives, such as high-flow converters, are recommended for eco-conscious drivers.

Can I revert a hollowed catalytic converter to its original state?

Reverting is challenging. Consider alternatives like aftermarket high-flow converters or factory part reconditioning for a more eco-friendly approach.

What are the alternatives to hollowing catalytic converters?

Alternatives include aftermarket high-flow converters, delete pipes, and factory part reconditioning, each with varying impacts on performance and emissions.

Should I seek professional advice before attempting modifications?

Consulting with professionals is highly recommended to ensure the modification aligns with local regulations, emissions standards, and safety practices.

Can I Just Hollow Out My Catalytic Converter Instead of Replacing It?

Yes, hollowing out a catalytic converter is an option for performance enthusiasts seeking increased engine power. However, it comes with legal and environmental risks, and alternative options like high-flow converters are more suitable for street vehicles. Always consider the intended use and local regulations before making modifications.

Conclusion on Hollow Out A Catalytic Converter

Hollowing factory catalytic converters unlock increased horsepower and fuel economy in racing vehicles by reducing exhaust path restrictions. However, sacrificed emissions treatment directly conflicts with environmental protection efforts through increased atmospheric toxin outputs. Further, extensive drilling risks long-term converter integrity, introducing new failure possibilities.

Ensure catalytic modification legality given intended vehicle operating conditions in your local region before investing in alteration procedures. Where racing applications apply, mindful design optimization balancing backpressure relief against stability retention helps engines exploit maximum power while maintaining a degree of longevity confidence despite sacrificed catchers and structural thinning. Consider professional fabrication guidance adjusting injection timing alongside physical changes to align engine operating points with lower discharged backpressure effects.

Yet for street-driven vehicles, more eco-conscious high-flow converter or filter reconditioning alternatives are better suited to delivering moderate performance gains while upholding responsible emissions standards, proving critical to global pollution reduction efforts. Consider the greater good along with personal interests when modifying catalytic systems.

I am Nicolas, an automobile engineer with over 5 years of experience in exhaust systems and catalytic converters. I am passionate about learning and understanding how things work, and I am always looking for new ways to improve the performance and efficiency of automotive exhaust systems. Know more about me. As an Amazon Associate, we earn commission from qualifying purchases.

Sharing Is Caring:

Leave a Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.