wastegate question

[chrispot]

I will be fitting a single turbo to my car soon and I am just curious about external wastegates, as i understand it there can be as much as twice the pressure in the exhaust manifold as there is boost pressure, so if i fit a external wastegate with a 7 psi spring my base boost pressure will be 7 psi, and i can up the boost with a boost controller,

 

my question is why does'nt the wastegate valve open with a 7 psi spring if the pressure in the exhaust manifold is higher than this, or have i got it wrong? i think that i am correct in saying that the ex manifold pressure can vary with different A/R used.

 

thanks chris.

[Wez]

Because the wastegate works with a reference port from the positive side of the turbo, when 7psi is exceeded the wastegate is pushed open

 

This can be from the compressor cover or anywhere on the engine inlet side.

[jevansio]

As Wez says, the wastegate is opened due to a pressure line coming from your boosted pipework operating on the diaphram inside the wastegate, rather than the pressure exerted on the valve face from the manifold.

 

The spring rate in the wastegate denotes the minimum amount of boost you can ever run (as boost controllers can only raise and not drop boost)

[chrispot]

thanks for the replies, what I was trying to say is if you have a wastegate with a say 10 PSI spring and no boost connection lines, so when making 10 PSI of boost there could be 20 PSI in the exhaust manifold acting on the wastegate valve why doesn't this get pushed open when there is only a 10 PSI spring keeping it shut?

 

regards chris.

[Wez]

Here is short video I did a long time ago showing a wastegate working with compressor

 

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[heckler]

thanks for the replies, what I was trying to say is if you have a wastegate with a say 10 PSI spring and no boost connection lines, so when making 10 PSI of boost there could be 20 PSI in the exhaust manifold acting on the wastegate valve why doesn't this get pushed open when there is only a 10 PSI spring keeping it shut?

 

regards chris.

 

Probably because the exhaust gasses will take the easiest route out - through the exhaust. Wastegate is only there to control the boost produced by the turbo.