S70 T5 CBV question

Vuym · Post by **Vuym** » 22 Jul 2025, 02:48

Hi everyone, I finally got around to pulling the stock CBV off my 16T and found the port was completely clogged with tough rubber gunk - explains why I was getting compressor surge despite a new vacuum line. Surprisingly though, the diaphragm wasn’t torn. Cleaned it up and freed the port, but now I’m wondering:

Should I just upgrade the CBV while I’m here? I'm currently running 13psi and AFR levels are between 11-12 WOT. I think the most I will take this car is stage 2 but will likely end up being a stage 1 project. What do you guys think? If my CBV is not going to hold and it will make a big enough difference, should I go for the Forge Recirculation Valve or would it make more sense long-term to blank off the CBV and go with an external BOV setup?

Thank you!

Post by **scot850** » 22 Jul 2025, 08:36

I have a 2000 V70R with a tune running a claimed 310bhp (tune from Rica in Holland). It is on the standard CBV and it has up to 20psi boost.

Neil.

Post by **Sveedy** » 22 Jul 2025, 10:43

I have the Forge unit on my '96. It's a work of art. Do you need it ? Probably not. But it is nice to have, and changing springs is easy if your needs change.
I'm not sure about a BOV. I've heard it is more for sticks than autos.

Vuym · Post by **Vuym** » 22 Jul 2025, 11:14

Thank you both for your advice, that's very helpful. It is reassuring to know that it isn't something I have to be worried about for now, at least. The Forge unit looks amazing, I will admit that part of my reasoning for considering it is because of its design.

If anyone here has fitted a BOV, could you please let me know how it affected performance? Do you have to make any changes to fueling or ECU settings to compensate for the air being vented to atmosphere?

Vuym · Post by **Vuym** » 23 Jul 2025, 11:57

Ended up running the Forge recirc valve in the end, I think it was the right call.

Increased to 15psi via MBC so untuned (AFR 10.8 so went down somehow) but it does drop off after a few seconds of WOT. Pipes arrive tomorrow for me to make custom RIP kit so I suspect I might have some worn stock intercooler couplers which are making it hard to sustain peak boost.

Post by **abscate** » 24 Jul 2025, 11:41

The stock CBV and a BOV are two different functions.

The stock valve pulls off boost when you lift, the intake manifold vacuum sucks the valve open and reduces boost in this case. You can buy some stiffer springs which will allow more boost but IPD warns you that the red spring without a tune will mean bent rods.

Vuym · Post by **Vuym** » 27 Jul 2025, 05:59

abscate wrote: ↑24 Jul 2025, 11:41 The stock CBV and a BOV are two different functions.

The stock valve pulls off boost when you lift, the intake manifold vacuum sucks the valve open and reduces boost in this case. You can buy some stiffer springs which will allow more boost but IPD warns you that the red spring without a tune will mean bent rods.

That's interesting - thank you for the explanation. I was under the impression that using stiffer springs would make it harder for the CBV to open which makes it more likely for the pressure to return to the turbo and cause compressor surge rather than being diverted back to the intake manifold.