Stock catalytic converter vs high flow

A catalytic converter is a device contained inside a car engine designed to reduce the number of certain pollutants from the exhaust. It does so by causing a chemical reaction, altering the structure of gases to create less harmful substances.

There are two types of converters: stock and high flow. A stock converter does not allow for replacement materials, whereas a high flow converter is designed to be retrofitted with aftermarket materials.

What is a stock catalytic converter?

The stock catalytic converter helps to reduce water and carbon dioxide emissions from gasoline and diesel engines. The part is made of a honeycomb substrate coated with the precious metals platinum, rhodium, and palladium.

This coating helps convert toxic gases into less harmful substances that are released into the air.

Stock catalytic converters come as OEM (Original Equipment Manufacturer) and aftermarket. The OEM type replaces the stock converter while the other type is installed in addition to the stock converter (to regulate emissions).

However, it’s important to note that installing an aftermarket catalytic converter causes a car’s computer system to go into a different mode, and this could trigger dash lights and eventually lead to engine problems.

In the United States, catalytic converters are required by federal law to be installed on all 1996 and newer cars to help reduce pollutants from gasoline engines.

For a car that was manufactured without a catalytic converter to get one, it has to meet certain requirements such as being certified by California Air Resources Board (CARB), having a six-digit identification number, and having the manufacturer’s name stamped on it.

What is a high flow catalytic converter?

High flow converters allow for greater airflow into the engine by opening up restrictions in an engine’s exhaust system. This can mean that less energy is wasted when getting rid of unwanted gases out of the car’s combustion chamber, allowing more power to be created from a smaller engine size.

In this way, high-performance cars with low displacement engines can have a high-performance exhaust system.

High flow converters are also installed in older cars that have been converted into burnouts or racing cars.

In this case, the builder knows that they want to get rid of as many exhaust gases from the engine as possible and will use a high flow converter to do so. High flow converters can also be used to make a car sound louder.

Differences between the stock catalytic converter and a high flow converter

1. Size and position

A stock catalytic converter is large and has a circular frame, while a high flow converter is smaller and square. The stock converter also sits low in the exhaust system at the place where exhaust gas enters into the engine compartment; it then rises and attaches to the exhaust manifold.

The high flow converter does not attach to the exhaust manifold but places greater stress on other components, especially valves; this is why these converters are sometimes called hot side converters.

2. Material

Stock catalytic converters contain palladium and rhodium-based alloy substrates. Rhodium is a heavier version of platinum, and palladium contains traces of nickel.

The substrate functions as the catalyst for conversion, meaning it acts as the chemical agent to change harmful gases into those that do little harm. High flow converters contain only one type of material: stainless steel.

3. Manufacturing process

Stock catalytic converters are built in a complex process that includes separate setups to weld, shape the converter, and heat treats it.

High flow converters can be formed with a simple stamping process for stainless steel, creating a more cost-effective manufacturing system.

4. Physical appearance

Stock catalytic converters have a honeycomb design on the outside. The interior contains material for filtering out toxic chemicals. High flow converters contain a straight-through design, with the stainless-steel construction allowing gases to pass through quickly and easily.

5. Pre-installation

A stock converter can be installed in any car before it is built; however, the adjacent systems must be changed to fit the converter. A high-flow converter is installed at the dealership or by a professional mechanic.

6. Cost

Due to their construction process and material cost, stock catalytic converters are much more expensive to produce than high flow converters. This makes stock converters generally two to three times more expensive than high flow ones.

7. Aftermarket usage

Due to the complex design of stock catalytic converters, they are very difficult to modify, even for professionals. High flow converters can be altered with ease, allowing mechanics and amateur car enthusiasts alike to install aftermarket materials, such as stainless-steel construction or aluminized ceramic substrates.

8. Lifetime

Catalytic converters are designed to last around 100,000 miles before they are needed to be replaced. High flow catalytic converters can be used for up to twice as long but might need replacement depending on how much the car is driven and what kind of materials are installed.

9. Effectiveness

High flow converters work just as well as stock catalytic converters, but not all mechanics agree that they are a good choice. One argument is that high flow converters put excessive stress on other engine components and could potentially cause malfunctions or mechanical errors in the future.

Another argument against their usage is that non-stock materials can cause irregular heat distribution, potentially leading to an explosion.

A stock catalytic converter is designed for cars that are driven regularly and relatively short distances. A high flow converter might not provide enough protection for a sports car, which requires greater fuel efficiency but also needs higher speeds to maintain optimum performance.

Similarities of stock catalytic converters and high flow converters

There are five common similarities between stock catalytic converters and high flow converters.

  • The goal is to make the most power possible while remaining street legal with a warranty
  • Both are tested in the same location under the same conditions
  • Both are tested on an engine dyno by trained employees of reputable companies
  • Both are advertised in similar ways
  • Consumer reviews of both are usually positive, even at optimal conditions.

The most important similarity is that the purpose for both high flow converters and stock catalytic converters is to increase horsepower without getting too loud or losing emissions.

Many consumers enjoy the increased responsiveness and throttle response that an aftermarket catalytic converter provides as long as it still meets emissions requirements.

How much horsepower does a high flow catalytic converter add?

A high flow catalytic converter generally adds between 10-20 horsepower. The quality of the fabrication is important to note when looking at how much horsepower a high flow catalytic converter adds.

Several types of engines can benefit from this modification, but it is most commonly installed in forced induction engines. High flow converters are not intended to be catalytic converters that last the life of the vehicle. These converters are intended for racing or engine tuning purposes that require frequent replacement.

Do high-flow cats make exhausts louder?

Sometimes, the answer is that if your exhaust system has high-flow cats and you’re still hearing a raspy noise, the problem could be the catalytic converter under the car. The high-flow cats may work too well and not allow enough of the bad gases to escape.

Are all aftermarket catalytic converters high flow?

No. There are three typical types of catalytic converters, “high-flow converters,” “mainstream converters”, and “stock converters.” The first two are also sometimes referred to as “performance converters.”

The three categories differ in their ability to increase exhaust flow compared to the OEM catalytic converter. High flow converters can increase flow by up to 70 percent; mainstream converters typically increase flow by around 25 percent, and stock converters do not usually provide any increased exhaust flow compared to the OEM catalytic converter.

Do high-flow cats require a tune?

High-flow cat converters are becoming more popular in performance vehicles, but do they require additional tuning to the engine management system?

The answer is not necessarily. High flow cats vary greatly in design and construction, so you should get specifics about your particular model before determining whether or not it requires tuning.

If high-flow cats are made of stainless steel, they can often be treated as if they were OE equipment. If high-flow cats are made of ceramics or other high-temperature resistant materials, additional tuning will likely be required to avoid driveability problems or catalytic converter overheating.

Will high-flow cats pass emissions?

Over the past few decades, there has been an increased effort by automakers to produce vehicles that are more fuel-efficient and emit reduced tailpipe pollutants.

To satisfy these requirements, many carmakers installed secondary air injection systems (SAI) on their vehicles to reduce smog during low-speed driving conditions.

However, many of these catalytic converters were not large enough to work properly, and they would often fail federal inspections.

Do high-flow cats cause the check engine light to come on?

Since high-flow cats replace stock catalytic converters, mechanics often worry that these high-flow converters will cause generic or specialized diagnostic trouble codes (DTCs) to be triggered when they are installed.

As far as common engine diagnostics are concerned, high-flow cats will not cause the check engine light to come on.

Bottom line

A high-flow catalytic converter is a great option for those who are looking to increase the performance of their vehicle. It offers many benefits over the stock catalytic converter, including increased airflow and better fuel economy. If you are looking to improve the performance of your car, be sure to consider a high-flow catalytic converter.

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