HIGH PERFORMANCE ACADEMY
Blow-off valves are not fitted for the reasons most people think. BOVs are usually marketed at improving the boost response in between gear changes, but the reality is not the case. If you look at many forms of professional motorsport, BOVs often aren’t used at all. The main reason they’re used is to protect the turbocharger from the destructive forces resulting from surge which can occur during a gear shift — essentially the pressurized air has nowhere to go. You do need to size them for your application — a BOV suitable for a 200kW Evo, for example, isn’t going to be adequate for a 500kW Evo, as you need to be able to vent a much larger mass of air. If you have a low- to mildly-tuned set-up, going to a larger BOV is not essential unless the factory one can’t hold the boost pressure. A good example is the early Evo BOVs which leak above about 15psi. When it comes to BOV design, I prefer a diaphragmstyle BOV as you’re guaranteed the BOV chamber will seal, whereas the cheaper range of BOVs will still work, but you’re relying on a seal that’s dependent on the tolerance between the body of the BOV and the piston — which is fine when the unit is brand new, but over time the tolerances will increase. If there is a leak inside the BOV chamber, it’s possible to affect the boost pressure in the vacuum line to the BOV. This can be a problem if the vacuum line is also connected to a boost gauge, or even the fuel pressure regulator, as these components may see false pressure signals. Basically, you want to find a good quality BOV that isn’t going to leak — you get what you pay for.