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Compressor Bypass Valve Information

Starion Bypass Valve Hookup.
Starion Racing Bypass Valve with Tee pipe.
Starion Over the Valve Cover Pipe with Racing Bypass Valve.
Starion Over the Valve Cover Pipe with Large and Small TurboXS Valve.

Starion Over the Valve Cover Pipe with TurboXS Type H Valve. $289.00 Order


The Compressor Bypass (or Blow-Off)Valve seems to be the most misunderstood part in a Turbo system. Everyone wants one. Almost everyone buys one. And almost no one understands what it does or how it works.

We sell an awful lot of Bypass valves every month to Starion and other car owners and about 75% of those people call not understanding how to make it work or claiming it does not work. We always get them fixed up but it can be frustrating.
Here is a breif lesson in Bypass valves. Once you understand how simple it is and what it does you will never have a problem again.

The object of a Bypass valve is to relieve the excessive boost pressure that builds up between the Turbo and Throttle body when you let off the gas to shift or decellerate. It really was not designed as a noisemaker although many people think that is all it is for.We do agree that it sounds pretty cool when it blows off, but that is a by-product of its function, not the design.
If you are running 15psi boost and slam the throttle shut the boost pressure spikes up and tries to drive the compressor wheel backwards into the Turbo. This does 3 things...

First...it stalls the Turbo so that when you open the throttle back up it has to spool up allover again causing lag and hesitation.

Second...It puts a terrible load on the thrust bearing of the Turbo which can cause premature wear and failure.

Third...It weakens all your soft connections in the intercooler plumbing from excessive expansion and contraction.

It is a very simple device. it has a Diaphragm and a Valve (Much like an engine valve) When the Valve is shut the boost stays in the system and when it is open you get the Whoosh sound and the boost blows off. This is all done with a Single Vacuum and Boost connection. When the engine is under boost the valve is pressed shut by its own spring and the force of the boost pressure on it. This insures no boost leaks. When the throttle is shut and the engine goes from pressure to vacuum the valve is LIFTED off its seat and the excess pressure vents to the Atmosphere (or possibly back into the intake tract)depending on your particular installation. It is critical that you have a good connection to the bypass valve. it has strong vacuum demands to operate. You must be hooked up to a source that is below the throttle plate when it is closed. This will insure PRESSURE under boost conditions and VACUUM under closed or part throttle conditions. To insure you have a good source start the engine and put your finger over the connection you want to use. Make sure that it sucks good against your finger at idle then stops sucking when you open the throttle.( or hook up a vacuum gauge to it and make sure it makes at least 18-25 inches of vacuum when you open the throttle and snap it back shut again.)

On an Airflow Meter or Mass Air Sensor equipped car it is critical that the bypass valve be closed at idle. If it hangs open under idle conditions it will cause a few problems like stalling when you come back to idle. poor idle quality, and poor part throttle driveability.
The HKS Racing bypass valve that we use has an adjustment to correct this type of problem. Some of the non-adjustable valves or those with very light springs will (if they stay open at idle )have to have the discharge routed back into the plumbing betwen the Turbo and the Airflow Meter or Mass Air sensor.
If you are uncertain if the valve is staying open at idle just block off the discharge with your hand and see if the idle changes. if it has no effect you are probably safe.



Starion Bypass Hookup: To answer the question we get asked 10 times a day. You hook up the Bypass valve to the 3rd port back on the throttle body.If you are standing at the drivers side fender looking at the throttle body. there are 3 small vacuum ports in a row. You want to use the one closest to the firewall.To insure you have a good source start the car and put your finger over this hose. It should have good strong vacuum going to it. If you do not feel good suction on the hose at idle the bypass valve will never work properly.
This is also the same port you use for an aftermarket boost gauge and Rising rate fuel regulator.

When hooking up the HKS racing bypass valve you only use the vacuum port that is next to the valve discharge. The one coming off the base at an angle is not used.Leave it open. DO NOT CAP IT OFF.



85-89 Starion Conquest Over the Valve Cover pipe with HKS Racing Bypass valve.$319.00


Starion OVC with Large Turbo XS valve shown below $289.00
Also available with small Turbo XS valve $265.00




85-89 Starion Bypass Valve with Tee Pipe. $229.00
Note the location of the vacuum hose. This is the connection that is used.