Myth's busted! How our RPA Mechanical BOV system works (and why you need it)

Myth's busted! How our RPA Mechanical BOV system works (and why you need it)

There is a heap of misinformation on how this system works, on the internet and even supposed "expert" mechanics.
We wont go into all this on this post, or we would be here all day. We will bust some myths in the future though.
We assume you have some basic mechanical knowledge here, but all the same here are some basic principles.
1) an engine is essentially a big air pump. More air in and out = more power.
2) a throttle is a valve that is controlled by your right foot! It controls the air / fuel into the engine by opening or closing the butterfly valve.
3) A turbo has 2 "fans" connected by an axle, one is spun by the waste gases (exhaust) exiting the engine, this turns the other fan, that draws and compresses more air into the engine.

The diagrams are based on M177 platform but the concept is the same on all petrol cars.
The primary purpose of a BOV (blow off valve) is to relieve the pressurized when the throttle is closed.

The way this valve is actuated is either electronic solenoid by the ecu (factory valve) or mechanically using manifold pressure/vacuum.
The upgrade systems we offer are mechanically operated.
The purpose of the spring inside the valve isn't to hold the valve closed under boost, it is to stop the valve opening under idle, where vacuum maybe present in the inlet manifold. As soon as the throttle starts to open (you depress the pedal) the engine is fed more air, it will increase in revs, producing more exhaust gas, this spins the turbine on the turbo, which is obviously attached to the compressor wheel on the turbo, and this sucks in air and produces boost.
That pressurized air enters the engine and the cycle continues as you accelerate.

The vacuum hose from the manifold to the top of the BOV is also pressurized. This is what keeps the BOV closed as the pressure is equal both sides of the valve.

When you take your foot off the accelerator pedal the throttle is closed. The air is blocked from entering the manifold past the throttle.
A BOV that is too  small or equipped with a spring that is too stiff, may not adequately  vent the pressurized air, forcing this air back up through the compressor wheel. Causing the distinct "dose"(Aussie slang) sound as the air fights to escape between the compressor wheel blades(Sounds like a "chew chew chew" or bird sound). This is VERY BAD for the turbo, causing premature bearing failure (a common issue in M133 and M157 AMG engines that are not equipped with a bypass / blow off valve.

A BOV DOES NOT slow the turbo speed, producing "lag". quite the opposite! It allows the turbo to continue spinning.


For the W205 C63 owners out there who have been blowing the rubber couplings off the throttle to the manifold, you can see it isn't because of your BOV setup. A BOV is also not going to help you exploding the plastic inlet manifolds either. Both these issues are a result of the flex and eventual fatigue of the factory plastic inlet manifold.

A BOV upgrade MAY help intercooler life as this is the only component before the throttle that can experience over boosting if the BOV is not large enough for the amount of air you are pushing into the engine.

The main advantage of our BOV conversion is elimination of the ECU controlled system. The ECU can open the factory BOV whenever it is programmed to, for example for traction control, or between gears to make the changing smoother, n
ot generally what you want when you are after more performance.

The factory valves are known to melt when subjected to the increased heat of tuned engines. Furthermore the factory valves are small, electronic actuators, and prone to leaking.

Factory AMG Recirculation valves are prone to melting

The RPA Turbosmart BOV is controlled by the boost pressure, the more boost pressure, the tighter the valve closes.
We hope you find this post useful.
In the future we will bust some other common myths such as:

  • BOV spacers are the same as a BOV conversion
  • Venting to atmosphere produces more lag, this is why the factory system recirculates to the air intake (this one is a favorites and SO WRONG🤣🤣)
  • Difference between a BOV and a wastegate

The ultimate AMG Blow Off Valve upgrade


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