Vortech Race Valve. Swapped the stock plastic Bosch unit for this one... much better.

 

Quick question ... do these bypass valves stay open while in vacuum?

 

Yes, but the opening/closing point is adjustable, via the preload spring. Stiffer spring = valve that closes earlier and under lower vacuum.

Jim

 

Incorrect. A blowoff valve opens from overpressure hence the psi ratings used by manufactuers of those valves. A bypass valve is vacuum operated. It bypasses the compressor's airflow during low demand operation. Vacuum pulls it open and boost pressure applies to the diaphragm holding it shut.
Just want to clear the air on that. : )

All of ATI's valves are considered bypass valves.

 

Oh really? That's news to me.

 

OK, that's a new one for me. HKS talks about bypass and blowoff valves here:

HKS compressor blowoff and bypass valves play a vital role in any turbo system because they not only improve the performance of your vehicle but they also increase the longevity of the turbocharger as well. The primary function of any blow-off or by-pass valve is to prevent compressor surge by relieving the compressed airflow that reverses direction as the throttle is closed. HKS blowoff and bypass valves reroute the pressurized air, that is normally forced backwards into the turbo compressor, into the intake stream or by venting it to the atmosphere. Subsequently, engine performance is then increased from quicker turbo spool up, as the compressor speed is not reduced by the backward airflow. The longevity of the turbo is also increased, as its bearings will not suffer from the impact that is also produced by compressor surge. Though many vehicles incorporate factory by-pass valves, they are often undersized and quickly are beyond their capable operating range as airflow levels are increased. HKS offers four types of compressor blowoff and bypass valves: Standard, Racing, Super Sequential Blow-Off Valve, and the Racing Blow-Off Valve Type II.
From HKSUSA.COM, in their Blowoff/Bypass Valve Section.

It sounds like a blowoff is the same as a bypass, and only works in throttle-off conditions.

Here's Wikipedia: A blowoff valve (BOV, also known as a bypass valve, compressor relief valve, or sometimes hooter valve) or Charge Air Bypass Valve which is the correct term for this device according to SAE J1930 is a vacuum or in some cases a solenoid operated valve that is located in the intake tract on an internal combustion engine between a turbocharger and the throttle body. It's used is to prevent compressor surge by venting the charge air when the throttle is closed, such as during a gearshift. When the throttle is closed after the engine has been used with boost there will be an increase in pressure after the compressor while the flow though the compressor decrease to a very low level. This will cause a condition commonly called compressor surge, the flow in the compressor stalls and the compressor will no longer be able to uphold the pressure increase over it. This will eventually cause the flow to go backwards through the compressor until a pressure increase that the compressor can uphold will be reached. This may slow down or even stop the rotating assembly in the turbocharger, thus causing turbo lag when the throttle is pressed again. It can also cause vibrations which can damage the turbocharger itself.

Some bypass valves are equipped with a second port so that severe overboost can cause them to open and act as a safety valve.

It's the wastegate that starts to dump over a certain pressure.

I do see your point about my comment - I was wrong. That's a vented or divorced bypass.

Jim

 

IIRC, the difference between a BPV and BOV is not when they vent, but where they vent. BOV's vent to atmosphere, BPV's vent back into the intake tract to prevent the loss of metered air.

 

Keep in mind that a bypass valve is not positive pressure activated, though positive pressure does hold it closed. Manifold vacuum operates the bypass valve as its normal state is to be closed. With a bypass valve the idea is that there is no appreciable pressure increase in the intake tubing when the throttle is closed. The closing throttle creates manifold vacuum that pulls the valve open dumping excess compressor airflow before it can cause damage. The engine's airflow demand has been replaced by the bypass valve's area. If the valve is sized correctly the pressure in the intake tubing will drop instead of spike upwards where it can cause damage.

The blowoff valve works off of the pressure spike that occurs when the engine's airflow consumption is removed. When the throttle is closed airflow will immediately build up in the intake tubing as it has nowhere to go. This pressure spike builds up in the tubing until it unseats the valve's pressure rating which releases the pressure in the system to the set point, usually tuned to produce a specific sound for those that like that kind of thing.

Supercharger manufactuers generally don't like the blowoff valve as the compressor is solidly driven by the crank so its airflow is fixed to the engine rpm. Removing airflow consumption by closing the throttle does not change the airflow delivered. What may have produced 8 psi on a 346 cubic inch engine will now spike up to whatever the compressor is able to deliver before backflow occurs if the air is not given somewhere to go. As you can imagine this causes problems. You could easily blow off a tubing connection, rupture a hose, damage an intercooler core or tank, force open the throttle on a drive by wire car with a centered throttle plate and force air back against the impeller on its way back through the inlet. The bypass size need to be matched to the airflow of the supercharger and not necessarily the boost level. There is a big difference in airflow say from 8 psi on a 346 vs 8 psi on a 540.

Turbo enthusiasts for one aren't generally used to using a bypass valve. Many people hear the squeak or whistle of a blowoff valve and immediately associate that with a turbocharger. I've seen videos here of turbo cars mildly going through the gears with a high pitch squeak between them. The turbo is driven by exhaust gas mass. When the throttle is closed the driving mechanism is greatly reduced and only inertia keeps the compressor at speed. If there were no valve at all the backup of airflow would quickly slow the turbo down to a point where no positive pressure would continue to build up as the energy used to create it has been removed. The turbo rpm is independant of engine rpm. While the same damage can occur on a turbo car, the pressure buildup is of lower duration due to the above reasons, but pressure must build to load the turbo into surge. If the tubing pressure is lowered with a bypass valve between lift throttle shifts the load is reduced on the compressor and it is able to maintain rpm. The wastegate will no doubt close some with the reduced pressure and the car will recover quicker due to the higher airflow from the compressor after the shift completes. Even with its performance benifits the bypass valve is largely ignored on many street turbo systems with people opting for the noisy blowoff valve.

Companies such as Precision Turbo sell the ATI race bypass valve for their drag racing customers as it able to dump the airflow of large frame turbos at throttle lift. Sometimes the valves are used in pairs.

With all of that said the two terms are often used interchangeably by enthusiasts due to just not caring what its called. They want a certain effect. Its up to the company to determine what the customer is after. This is demonstrated by the above Wiki quoted.

Sorry for the long post. I hope it helped somewhat.

 

I think the mini Vortech will work or the HKS type R. I just got a Hyperboost which is similar in size to Bosch but a little larger but has a aluminium body and no rubber diahgpram( i know i didn't spell that right). My bosch vents to the atmosphere with a little filter over the outlet. If the valve is before the MAF meter then it's no problem to vent to the atmoshpere-I think? Also this Hyperboost is pretty small and supposed to be able to handle 20psi all day long-even though i'm amking only 9psi do you think that's correct? Here's a link to the Hyperboost-I got the HX compact:

http://www.stratmosphere.com/hyperboost.htm

The thing that still worries me is the small inlet size-I think it might be fine for a P1SC making 10-12psi but I think I may need to upgrade to either 2 valves or a larger valve when I upgrade to a D1SC?

 

I'm certainly no expert, but I did research on this a year or so ago when a buddy of mine bought a BOV for his supercharger. To my knowledge and what my research told me was the difference between a bypass valve and a blow off valve is that bypass valves are normally open and only close when the manifold sees boost. A blow off valve is a little more complex in the way it operates. It's normally closed so as not to let in unfiltered air at idle. But it uses spring pressure and manifold vacuum to know when to open and close. So as boost is seen at the BOV and in the manifold the BOV stays closed. As soon as the throttle blade closes and the monifold sees vacuum, the BOV still sees pressure and opens.

As far as I know the difference between BOV and BPV had nothing to do with their location relative to the MAF nor whether they recirculated air. But again I'm no professional.

Mike

 

I'm not saying you're wrong, but HKS and the blowoff valve diagrams they have at their site do not show them operating as you describe. They show them working the same way as a bypass, only vented to the air. The piston movement is controlled by vacuum generated behind a closed throttle.

What you're describing sounds like a spring-loaded safety valve, or pop-off valve, where the pressure overrides the spring and opens the valve.

Jim

 

Shortly, I'll be running the the Vortech Race Valve as well: