A compressor bypass valve is a valve that releases intake pressure when the throttle is closed. (That is, when you let off on the gas.) The reason you want this pressure released is that the pressure can back up inside the intake pipe and into the compressor wheel, which will come to a halt against the pressurized air. (The phenomenon where the turbo blade stalls against this pressure is called "compressor surge," and it has a distinctive chuffing sound.) This can hurt the turbo bearings and slow spool-up when you get back on the gas.
So, most every turbo car (including the Lancer Evolution) is equipped with a vacuum-actuated bypass valve that releases the intake pressure when you let off the gas.
These valves are almost always vented back into the intake AFTER the air metering system, so that previously-metered air stays in the system, ensuring perfect air/fuel ratios at all times.
A blow-off valve is a version of a compressor bypass valve that vents to atmosphere. There is no performance reason for doing this, but it does make a really loud noise.
The only performance-related reasons to upgrade the compressor bypass valve is to fix a leaky factory valve. The OEM Mitsubishi valve on the Lancer seems to hold pressure pretty well, at least at or near factroy boost levels. At higher boost levels (like 25+ psi), this may not be the case.
Pretty much all the aftermarket blow-off valves work equally well, although the HKS one didn't perform that great according to a recent Sport Compact Car magazine test.
Most factory bypass valves are sprung softly, so they're very reponsive and will vent at part throttle. If you try to vent them to atmosphere, they'll leak air into the intake under vacuum, which can result in bad idling and poor part-throttle fueling.
Most aftermarket blow-off valves are sprung more stiffly, and/or are adjustable, so they will not leak at idle. This can result in increased compressor surge if the valve is too stiff.
On the Evo, the short intake tract means if you add an aftermarket intake, you get some pretty loud bypass valve noise just from the factory valve.
If you really want it loud, you can install and aftermarket BOV. We stock the TurboXS RFL, which is the loudest valve we have ever heard.
But as for performance, the BOV doesn't give any. It's strictly for show. And depending on the valve, setting it stiff enough so it will not leak air at idle (which can cause rough idle or stalling) may induce "compressor surge," which is a fluttering noise that resutls when the turbo blade stalls out against high pressure intake air.
Bypass valves are pretty easy to install. The valve is located right near the turbo on the front of the engine. It's stuffed down low, so it's easiest to access if you take of the turbo inlet pipe and airbox.
In conclusion: If you want a big loud BOV noise, go ahead and install one. But if you're looking for more horsepower, you probably don't need to change your stock valve.
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