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BLOW OFF VALVE vs WASTEGATE


In a turbo setup, the exhaust gas spins the turbine which in turn spins the compressor, therefore the compressor wheel is constantly in motion.

When you are accelerating, the compressor is spinning at X rpms over idle. Therefore it takes Y amount of time for the compressor to slow down to idle RPMS (no boost).

Now, when you are accelerating, all of the air that the turbo is pushing is injested by the motor and expelled out the exhaust however, when you let off the gas (such as during a shift) the throttle plate is instantly closed off, thereby stopping the flow of air that the turbo is pushing.

Because the turbo requires that Y amount of time to slow down to an RPM where it is no longer providing boost, during that instant when the throttle plate is closed, the turbo is STILL pushing pressurized air into the intake pipe.

Because of this, during that moment, the air is backed up and has no where to go except back through the compressor side of the turbo. That is not an ideal situation because of two main points:

1. its creates "compressor surge" which puts unnecessary stress on the bushings/bearings

2. slows down the compressor, so you need to spool it back up again after the shift. (slower ET's)

Because of this phenomenon, it would be benificial if one could stop this charged air from flowing backwards through the turbo when you let off the gas and the throttle plate closes.


Hence the use of a BOV. It sits on the intake side of the turbo. When it senses vacuum the internal piston/valve within the BOV is instantly opened the moment you let off the gas, allowing the charged air an ALTERNATE route to take instead of flowing backwards through the turbo. It can now vent to the atmosphere, which eliminates compressor surge and maintains spool up of the turbo during the shift.

You may be wondering why they don't equip production cars with BOV's? Well, MANY cars have factory BOV's. The 1G-2G Eclipses ALL have a stock BOV. You have to remember, the average Joe, that drives an Eclipse may not like the loud BOV sound, so they are designed to be as quiet as possible, therefore you may not have noticed it was there.

Some people have asked if whether or not venting the BOV to the atmosphere is legal. It is perfectly legal! Why? Because its just regular air... the air that surrounds us. Its just the air from outside that the turbo has sucked in.

Venting the WASTEGATE to the atmosphere on the other hand IS very illegal, because it is venting EXHAUST GASES to the atmosphere which to be legal, MUST go through the catalytic converter.

Therefore, all factory wastegates plumb the exhaust back into the downpipe so that it is routed through the stock exhaust system.

Why do people vent them to the atmosphere then? Because plumbing the wastegate back into the downpipe generally LOSES anywhere from 10-20hp.

So what exactly is the wastegate? The wastegates ONLY function is the regulate the maximum boost pressure that the turbo will provide.

Similar to a BOV, the wastegate also has a piston/valve that is opened, however NOT by vacuum like the BOV, it is opened from the exact OPPOSITE - boost pressure. That's what makes them different from each other.

The wastegate has a spring inside of it (internal and external wastegate) that hold the piston/valve shut. The vacuum line attached to the side of a wastegate allows the boosted air to enter the lower chamber of the wastegate and when the boost pressure inside the wastegate OVERCOMES the spring holding the pison/valve closed, it cause it to open which allows the exhaust gases that normally flow through the turbo, to exit via the wastegate.

Because the exhaust gases now have an alternate route to take, the turbo can no longer continue to spool up and create higher boost pressures. Therefore it will maintain the same boost pressure that the spring is set at.. That is why if you have a 8psi spring in a wastegate, the wastegate will open at ~8psi and hold boost levels at 8psi and will not (should not) allow the turbo to build up higher boost, otherwise you'll end picking up pieces of your motor.

Some people may argue that "the turbo must be too big if you have to stop it from spooling too high, so why not get a smaller turbo?" That statement is INCORRECT.

ALL TURBO'S will continue to spool faster and faster until the exhaust gases cannot spool it up any further.

The problem you run into is the efficiency range of the turbo. Any given turbo will only operate effciently within a givin PSI range. The size of the turbo will dictate how fast boost will come on and how much maximum potential HP it can attain.

So if you want an ALL OUT high HP drag car, then you'd want a LARGE(R) turbo that can provide 20-30psi and still be within its efficiency range whereas if you wanted a daily driven streetable car or for autox etc, you'd want to get a smaller turbo so that it can spool up quicker and give you boost almost instantaneously however will NOT be able to provide the peak HP you're looking for (lower max potential HP).

Both turbo's regardless of what motor they are on, will still require a wastegate otherwise you can't set how much boost you want to run.

Thats where a boost controller comes into play, but that is another topic.

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