Two Guys Rally

The Volvo Chronicles: CBV Diaphragm
[ August 2nd, 2008 ] By: Mark Ozimek Posted in » Ramblings

Last time I posted about my S70’s engine, I had just replaced some vacuum lines. Although it helped, there’s still something strange going on. I had ordered a new Compressor Bypass Valve (CBV) diaphragm to replace the worn out one that is allowing air to escape from the compressor outlet back to the inlet. Unfortunately it was out of stock everywhere I looked, so I had placed an order and waited for the part to arrive.

Now, a few weeks later, I get home from work to find a small package at the door. “What could it be!?”, I wonder. Opening up the bag reveals the impossibly rare diaphragm:

(Click for larger view)

What this diaphragm does is open and close to allow air out of the compressor outlet back into the inlet to prevent compressor surge when there is a sudden decrease in airflow through the engine at higher loads. The actuation method is pretty simple. There is a spring that pushes the valve assembly closed under most conditions. The pressure from the compressor pushes against this spring. There is a air hose that runs to the intake manifold that allows for a change in pressure behind the diaphragm. When near or at WOT, the pressure in the intake manifold is close to that of the compressor outlet, meaning the spring holds the valve shut. When the throttle is suddenly closed so that there is a lot more pressure in the compressor outlet than in the intake manifold, the spring can no longer hold the seal closed, allowing air to flow past it.

Over time, the heat, oil, cyclic action, vibration, dirt, etc., break down the diaphragm, it stiffens up, develops holes and tears, which generally decreases its performance. When there are holes in it, air can always escape through the CBV, significantly decreasing turbo performance and efficiency.

So I plan to change the diaphragm in my car soon. It isn’t a terribly complex job, there is just very little room to work, as it is wedged between the turbo housing and engine block, with a bunch of stuff in the way from both the top and bottom.

It’ll be a fun project to work on, and I’ll be sure to make a post when the old one gets replaced!