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Catalytic converter removal on motorcycles (cat elimination)

Removing the catalytic converter (decatting) is a popular motorcycle modification, but there are a few things to consider.

The cat converter is a restriction in the exhaust flow, just like the standard exhaust end can. But as the two devices are placed at a different distance from the cylinder head, the back pressure wave they create will peak at different RPM’s. 

There is another difference in the nature of the restrictions in the end can and the catalytic converter: The large volume exhaust end can will not be acting as a restriction at low RPM, because the volume in the exhaust will let the smaller amount of exhaust gasses through without slowing them down to any significant degree. The cat has a much smaller volume and the restriction will be more linear in relation to engine RPM.

The back pressure wave in a resonance chamber exhaust is extremely important in a two stroke engine, but the back pressure have much less significance in a modern four stroke engine - so when removing the catalytic converter or changing the exhaust on a four stroke engine, the focus should be on how the increased air flow will make the air/fuel ratio leaner than stock.

No matter if your motorcycle have the stock end can or an aftermarket exhaust - and/or have removed the cat converter, the O2 (Lambda) sensor in the exhaust will be able to cope with the conditions in closed loop mode (High RPM), so removing the cat. converter will never damage your engine, unless you are foolish enough to remove, tweak, or disable the O2 sensor.

(Do not remove or disable the O2/Lambda sensor

But the freer air flow through the engine after a cat removal means that the engine is running a bit leaner in open loop mode too, and this is why you need the BoosterPlug.

Will the BoosterPlug solve the decat issues ?


The short answer is: Yes, the BoosterPlug and the O2 (Lambda) sensor will prevent the motorcycle engine from running too lean - with or without the catalytic converter in the exhaust system.

As mentioned above, the O2 sensor will adjust the air/fuel ratio in closed loop mode (High RPM), so the decat will not damage your engine.

But at idle, low RPM and during acceleration and engine braking, the engine will be running lean because of the increased air flow when you remove the cat. This will have a negative effect on the motorcycles rideability and is why the bike develops a terrible on/off like throttle action after a decat. You will probably also notice how the engine easily stalls at idle and take off.

Open loop is mostly a low RPM thing, and at low RPM, the difference in air flow through the engine is not huge whether you have the cat installed or not - So the BoosterPlug's operational span is big enough to keep your motorcycle engine covered both with and without the catalytic converter.

- The technical details on the cat removal is the same as for the previous page on aftermarket exhaust and air filters