MR2 Catalytic Coverters – Explained

A post to clear up if your car should have a catalytic converter and what they do.

First, what do they do?Exhaust Tips

Catalytic converters where first introduced to treat and monitor the gases emitted from the engine and remove certain harmfull pollutants before they are let in to the atmosphere.

As an enigine is running a number if gases are produced as exhaust.

  • Carbon dioxide – Carbon deposits that have mixed with the oxygen in the combustion process
  • Water vapour – Hydrogen from the fuel that has bonded with oxygen
  • Nitrogen gas – An inert gas that makes up the mix of gases found in the air
  • Carbon monoxide – Formed when there is not enough oxygen, this is colourless, poisenous and odourless
  • Hydrocarbons – These are particles of fue that have not been fully burned, they can be broken down by sunlight later and then turn in to oxidants which contribute to the break down of the ozone layer
  • Oxides of nitrogen – This contributes towards acid rain and smog and can cause irritation to people breathing
  • Particels of fuel – These have not been burned in the combustion process and cause breathing problems

The first three in this list are the main outputs from the exhaust and are mostly harmless, the last 4 or aimed to be vastly reduced by the use of a catalytic converter.

How they work

Modern units use 2 different catalysts to lower the harmful emmisions: a reduction catalyst and a oxidation catalyst. Each type uses a” honeycomb” structure inside, coated with a metal catalyst – platinum, rhodium and/or palladium

Reduction – This part uses platinum and rhodium inside the unit. As the oxides of nitrogen in the exhaust gas pass through they will attach on to the elements mention previously, this will then cause a chemical reaction to occur and break up the oxides of nitrogen in to nitrogen gas and oxygen.

Oxydation – This second stage targets the unburnt hydrocarbons and the carbon monoxide, which flow over a membrane made up of platinum and palladium. As the gases flow over the membrane a chemical reaction occurs which converts the carbon monoxde and hydrocarbons in to carbon dixode and water.

Technically speaking there is a third part as well. This is an oxygen sensor(s), older cars used just 1 sensor fitted upstream of the catalyst, this monitored oxygen content in the exhaust. The readings from this are sent to the ECU which adjusts the air/fuel ratio to compensate for the lean/rich mixture. Ensureing that the engine is running at the ideal stochiometric point while recieving enough oxygen to burn off as much of the hydrocarbons and carbon monixide as pssoble. Occasionally newer models use a secondary sensor after the catalytic converter, this largely is used to monitor the performance of the cataytic converter itself.

This then means the harmfull pollutants are massively reduced. The material in the catalytic converter is not used up during the chemical reactions.

Should your MR2 have one?

“Back in the day” cars could get by emmisions laws easily without a catalytic converter, but with ever stricter laws coming in, they began to be fitted as standard to common production cars. After 1992/3 it became near impossible to pass the new regulations without the use of one (UK).

This means that a lot of revolution 1 mk2 mr2s did not have one fitted in the exhaust as standard, and as they where built before the updated regulations they do not need one still for passing MOT’s and staying legal.

In the revolution 2 models this did lower the power output to 156hp (N/A), though by the time the revolution 3 was introduced in 1993 the power was back up to around 173hp for most naturally aspirated 3s-ge models even with them all including the catalytic converter.

Aftermarket parts

Alot of aftermarket or performance manufacturers that either build complete exhaust systems or simply just the catalytic converter section do commonly use a “de-cat” system, This is basically just not using a catalytic part as there main goal is to build an exhaust system to give as much extra power and toque as possible, not emmisions. This design is offered by Japspeed while they do also offer an option to include a “cat-back” option, something that Mongoose offer as well.

A cat back is basically everything in the exhaust system after the catayltic converter. These are usually installed straight after the exhaust manifold as the extra heat in the gases will heat the membrane up quicker than if it were further down the exhaust. The extra heat is beneficial as it will begin the reaction quicker to reduce all of the harmful emissions.






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