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Removal of the secondary air injection system

There is a lot of discussions about disabling or removing the secondary air injection (SAI) system from the modern motorcycles - and a fair amount of home spun assumptions and completely wrong information on this topic floating around on the internet.

So even though this is a bit beyond the scope of a BoosterPlug related FAQ, we want to offer a proper explanation on the basics of the secondary air injection, and the concequenses of removal or disabling of the SAI.

What you need to know about the secondary air injection system - and about disabling it.

- The Basics.

The primary air induction is the air that is passing the throttle valve, getting mixed with fuel, and is sucked into the combustion chamber. We never really refer to this at the "primary air induction", but it explains why we are talking about a "secondary" system.
The secondary air injection system will pump fresh air from the air filter box to the exhaust header pipes - right after the exhaust valves.
So the air in the secondary air injection system is not being mixed with fuel, and will not be a part of the combustion process inside the engine.

- Why ?

The idea about pumping fresh air into exhaust system is twofold:

  • Adding fresh air to the exhaust gasses creates an after burn process in the header pipes that will burn up the remaing fuel parts that have escaped into the exhaust. (Burned fuel is better for the environment than unburned fuel)

  • The additional air molecules and the after burn process will increase the temperature in the front part ofthe exhaust and help the catalytic converter reach it's operating temperature faster, and will help to keep the cat. converters temperature up at idle and low power conditions.

- How ?

In a car engine you will mostly see an electrical air pump that is activated for the first 30 seconds after you start the engine, and then shut off. This burst of air in the exhaust during the engines warm up phase will help the cat converter to reach it's operating temperature a lot faster - to reduce pollution during the warm up phase.

On a motorcycle, there is usually no room for the rather large air pump, so the secondary air injection system is mostly operating on the pulse pump principle, where the vaccuum in the intake is used to pump fresh air into the exhaust.
Here is how it works:

  • The vaccuum pulse in the intake is used to move a diaphragm up and down. (The diaphragm can be placed in the cylinder head or in an external box)

  • On the other side of the diaphragm, there are two one-way valves, and as the diaphragm moves up and down, fresh air will be sucked from the air filter box, and pushed into the exhaust header. (So the diaphragm will separate the air/fuel mixture that goes in to the combustion chamber from the fresh air that is pumped into the exhaust)

  • The pulse pump will only be pumping air when the vaccuum in the intake is high, so it is only operational at idle or very low throttle openings, and during deceleration when you close the throttle ar higher RPM's 

- Will the secondary air injection system increase or decrease the power output ?

As mentioned in the text above, the secondary air stream is not getting into the combustion chamber, so it will not interfere with the combustion process in any way.

The diaphragm in the pulse pump setup will only move when the vacuum is at it's highest level - at closed throttle or very low throttle openings. This throttle position is typical open loop mode where the reading from the Lambda (O2) sensor is ignored anyway, so the additional air that is injected in the exhaust will not affect the Lambda sensor readings.

So the secondary air injection system will not change engine power in any way, and that is why are no performance gains by installing the common blanking off plate kit to remove the SAI system.

If you install a high flow aftermarket exhaust, the secondary air injection may cause a bit of backfire in the exhaust on deceleration, but it is a (sound) cosmetic issue, and will not damage your engine in any way.

Will it influence the BoosterPlug if I disable or remove the secondary air injection system (SAS, SAIS, SIS, etc) on my motorcycle ?

The short and direct answer is: The BoosterPlug is not depending on the secondary air injection in any way, so for the BoosterPlug is makes no difference if you remove or disable the secondary air injection system.