Thread: Turbo whistle and blow off valve

The reason for the blow of valve is to relieve the pressure in the inlet manifold, at full noise the turbo is supplying the engine with a considerable amount of air when you change gear and the engine revs drop the engine no longer requires/can take that amount of air so the pressure in the inlet manifold rises rapidly if aloud to build up to much the pulse created will relieve through to the compressor suction side causing an audible vibration this is known as barking and if this is aloud to occur frequently it can cause damage to the turbocharger the wategate does help as it diverts the exhaust gas around the turbine in turn slowing the turbo down but if the rapid engine deceleration is to quick the turbo wont have enough time to slow down thats why they have fitted a blow off valve in the inlet manifold.

Originally Posted by Sea_Dog

The reason for the blow of valve is to relieve the pressure in the inlet manifold, at full noise the turbo is supplying the engine with a considerable amount of air when you change gear and the engine revs drop the engine no longer requires/can take that amount of air so the pressure in the inlet manifold rises rapidly if aloud to build up to much the pulse created will relieve through to the compressor suction side causing an audible vibration this is known as barking and if this is aloud to occur frequently it can cause damage to the turbocharger the wategate does help as it diverts the exhaust gas around the turbine in turn slowing the turbo down but if the rapid engine deceleration is to quick the turbo wont have enough time to slow down thats why they have fitted a blow off valve in the inlet manifold.

This valve can't stop that. It's simply a spring loaded 15psi valve. Normal operation in the trucks never sees 15psi.

For us, it's better off gone.

Compressor bark is normally only an issue with big boost on single turbos and big intake piping volumes (to intercooler and back again etc). Stock these ran about 13psi max. The later freefloating turbos only did about 11psi.

My 4BD1T engine never had one.

I recall commenting on another forum, about removing something that looked much like a blow-off valve, and it turned out to be something else entirely.

Thats my point exactly the normal boost pressure never reaches 15 psi so why install one, only because when the engine rapidly decelerates it is effectively trying to brake the compressor which inturn results in an increase in pressure if there is enough rotating mass in the turbo the pressure may increase above the 15psi, I work on large marine diesel engines where this is quite common especially when the bridge jockeys change the engine load rapidly I have seen it blow the air filter off the turbo I have had it happen with choked charge air coolers aswell where the restriction in the inlet manifold is too high pushing the boost pressure above what it should be.

Originally Posted by Dougal

This valve can't stop that. It's simply a spring loaded 15psi valve. Normal operation in the trucks never sees 15psi.

For us, it's better off gone.

Compressor bark is normally only an issue with big boost on single turbos and big intake piping volumes (to intercooler and back again etc). Stock these ran about 13psi max. The later freefloating turbos only did about 11psi.

Originally Posted by Sea_Dog

Thats my point exactly the normal boost pressure never reaches 15 psi so why install one, only because when the engine rapidly decelerates it is effectively trying to brake the compressor which inturn results in an increase in pressure if there is enough rotating mass in the turbo the pressure may increase above the 15psi, I work on large marine diesel engines where this is quite common especially when the bridge jockeys change the engine load rapidly I have seen it blow the air filter off the turbo I have had it happen with choked charge air coolers aswell where the restriction in the inlet manifold is too high pushing the boost pressure above what it should be.

I don't accept your assumption that "rotating mass in the turbo the pressure may increase above the 15psi"

You have to remember that the turbo compressor is a dynamic type and the pressure is a function of the tip speed.

Even if I'm wrong, so what if there is a transient pressure 1.5 to 2 times greater.

I haven't seen one of the valves as found on some Isuzu's, let alone had one apart, but unless it is extremely fast acting, it won't be effective against the pressure pulse wave that travels at the speed of sound.

Originally Posted by Bush65

My 4BD1T engine never had one.

I recall commenting on another forum, about removing something that looked much like a blow-off valve, and it turned out to be something else entirely.

I think the 4BD2T has a load resistor for the glow-plugs in the intake.

Originally Posted by Sea_Dog

Thats my point exactly the normal boost pressure never reaches 15 psi so why install one, only because when the engine rapidly decelerates it is effectively trying to brake the compressor which inturn results in an increase in pressure if there is enough rotating mass in the turbo the pressure may increase above the 15psi, I work on large marine diesel engines where this is quite common especially when the bridge jockeys change the engine load rapidly I have seen it blow the air filter off the turbo I have had it happen with choked charge air coolers aswell where the restriction in the inlet manifold is too high pushing the boost pressure above what it should be.

Your marine diesels though will have far bigger turbos and lots more air volume in the intake piping/coolers than we have.

The pressure must be increasing as you can see in the video from the original post the valve is relieving when he changes gear if the pressure was still the same then that would mean the valve was relieving constantly, I do agree with you that the pressure from the turbo has not increased only the factt that the velocity of air flow which was coming from the turbo at full speed has now been braked by the engine this energy must then convert to pressure energy i also agree that the valve would need to be extremely fast acting to stop the pressure wave but maybe it is enough to reduce the amount of bark as dougal has said the volume of the air charge space may come into it but i can only see that this would only effect the amount of bark and on a small volume system it would still be there but maybe not as pronounced I am not an expert on turbo's and am only trying to understand the anomoly better myself.

On large 2 stroke crosshead cathedral marine diesels this occurs when the revs rapidly decrease as we have a fixed pitch propeller so engine speed is changed to increase or decrease the ships speed, On other vessels where we use medium speed trunk diesels and a variable pitch propeller is used the revs stay constant this allows us to use shaft generators driven off the gearbox constant speed is required to maintain the generator output frequency so when more or less pitch is required to vary the ships thrust the only thing which changes is the fuel rack position but also if there is a rapid reduction in load the compressor will bark my understanding of this is that the exhaust gas flow to the turbine has been significantly reduced and because the charge air pressure in the manifold has remained the same but the turbine revs have dropped the pressure being developed at the the compressor tips has dropped therefore it barks.

I was taught that the inducer in the compressor creates kinetic energy which is then partially converted to pressure energy via the diffuser then the rest is done by the volute casing this would confirm that tip speed is indeed a factor of pressure whereby incresing tip speed increases the gas velocity into the diffuser and volute casing.

I find the topic of turbines extremely interesting

Originally Posted by Bush65

I don't accept your assumption that "rotating mass in the turbo the pressure may increase above the 15psi"

You have to remember that the turbo compressor is a dynamic type and the pressure is a function of the tip speed.

Even if I'm wrong, so what if there is a transient pressure 1.5 to 2 times greater.

I haven't seen one of the valves as found on some Isuzu's, let alone had one apart, but unless it is extremely fast acting, it won't be effective against the pressure pulse wave that travels at the speed of sound.