Hi,

I was wondering if anyone could give me a detailed description of how an internal BOV works and if it is possible to convert to external. Thanks.

NATE

 

I have never heard of an internal BOV. It sounds like you mean wastegate, which is a totally different device with a different purpose.

BOVs are for reducing compressor surge when you slam the throttle shut for a shift. They are connected between the turbo and the throttle and include a small vacuum hose that goes to intake manifold, between the engine and the throttle. When you snap the throttle shut under boost, the boost between the turbo and the throttle and the vacuum between the throttle and the engine open the BOV, which vents air out of the turbo plumbing between the turbo and the throttle. For cars with metered air (a flapper valve or hot wire MAF sensor), it is best to route the vented air back into the pipe between the MAF sensor and the turbo to avoid a moment of rich mixture caused by the missing air. For a car that does not have an air flow meter, you can just vent the air to the atmosphere with no ill effects on the mixture.

Wastegates are for controlling the amount of boost you get when running with your foot on the gas. The wastegate allows exhaust to go around the turbo, or vent to the atmosphere before it gets to the turbine. It is operated by a hose connected between the turbo and the throttle on the intake tract. When the pressure gets high enough in the intake tract, it will open the wastegate which is normally held shut by a spring, reducing the amount of exhaust available to spin the turbo. It is a variable arrangement, so more boost will open the wastegate more, and less pressure will open it less or allow it to close. There is a point at which you reach some kind of dynamic equilibrium that holds boost to a certain level.

So back to the question... Some turbos have internal wastegates built into the turbine housing in the form of a flapper door that are operated by an actuator that connects to the intake tract (usually right at the compressor outlet). An external wastegate is a stand-alone valve that has both the exhaust valve and the actuator section together in a single unit. I have seen some setups where the turbine outlet flows through one pipe and a separate pipe handles the flow out of the internal wastegate. The flapper door is removed in this case (rendering the internal wastegate non-functional), and an external wastegate is installed on the tube that connects to the hole left where the flapper door was. I assume you could also block the outlet for the internal wastegate and add a tube to the exhaust manifold to vent exhaust from. I haven't seen these for RX-7s, and I think most people just keep the internal wastegate or switch to a turbo with no internal wastegate and a manifold and external wastegate that are designed to work together.

-Max

 

I think you mean a "bypass valve", which is a BOV that recirculates the air back to the compressor intake rather than to the atmosphere. You can find a lot of info on venting to the atmosphere if you search the 2Gen forum for "atmosphere". If you are still using the stock AFM and ECU, it is not a good idea to vent this to the atmosphere because of the below statement. However, some people don't mind slower acceleration times, worse gas mileage, and less response if they can make cool noises. :)