Question about BOV and Wastegate.
12-05-2002, 03:06 AM
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Question about BOV and Wastegate.
What are the pro's and con's of setting your wastegate and BOV to two different pressure's. For example if I had the wastegate set to 12-14 psi and the BOV set to 8 psi for low-boost until I cuold upgrade the internals etc. Would this cause any harm? I can see several pro's in this.
1.) Ability to use a larger turbo in low boost apps meaning better thermal effeciency.
2.) Ability to "crank up the boost" by only adjusting the BOv and not sync'ing the whole system.
3.) Less boost creep because wastegate is set for 12 psi so it wuold start to crack at about 8-10 which is what you want anyway.
I just wasn't sure if the constant venting would damage the BOv or if would be able to handle that. Anyway just thinking aloud here please respond with any thoughts or questions.
1.) Ability to use a larger turbo in low boost apps meaning better thermal effeciency.
2.) Ability to "crank up the boost" by only adjusting the BOv and not sync'ing the whole system.
3.) Less boost creep because wastegate is set for 12 psi so it wuold start to crack at about 8-10 which is what you want anyway.
I just wasn't sure if the constant venting would damage the BOv or if would be able to handle that. Anyway just thinking aloud here please respond with any thoughts or questions.
12-05-2002, 04:44 AM
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Blowoff valves shouldn't be used to control your boost pressure. I think you have the wrong concept of how a blowoff valve actually works...
On a blowoff valve, you of course have the main portion of it that vents from your intake pipe to the atmosphere. You also then have a second part that connects a line from your intake manifold (after the throttlebody) to the BOV.
What happens is, when you are at full throttle, the boost on the intake pipe is the same as the boost inside your manifold. Essentially, the pressure on the intake pipe versus the manifold cancel eachother out inside the BOV.
But when you shift and close the throttleplate, suddenly you have all this boost in the intake tract but you have vacuum on the manifold.... BOV plate is suddenly very biased and between the boost on one side and the vacuum on the other, the plate opens and vents pressure until the two are near equilibrium.
However, there is a spring in the wastegate to give the manifold side of things a bit more "oomph", because you know as well as I do that 12psi against a small blockplate is a lot more force than 12psi from a tiny vacuum line off the manifold. Pressre x surface area = force -- lots more surface area on the intake pipe side of the BOV.
That spring often has to be adjusted to work correctly at your boost pressure so that it doesn't crack prematurely but still opens fully when needed.
You always want to set pressure at the wastegate, and just make sure that your BOV is set so that it pops off correctly.
Also, using a bigger turbo for lower boost to gain better thermal efficiency is an illogical statement. Bigger turbos are more thermally efficient at higher boost levels; smaller turbos for lower boost levels. Trying to make a big turbo run lower boost will simply increase lag time and really will gain you very little.
However, using a small turbo to create big boost will generally make more heat than air pressure, so sometimes you end up using a big turbo to make "weenie boost" for the daily driver racket.
On a blowoff valve, you of course have the main portion of it that vents from your intake pipe to the atmosphere. You also then have a second part that connects a line from your intake manifold (after the throttlebody) to the BOV.
What happens is, when you are at full throttle, the boost on the intake pipe is the same as the boost inside your manifold. Essentially, the pressure on the intake pipe versus the manifold cancel eachother out inside the BOV.
But when you shift and close the throttleplate, suddenly you have all this boost in the intake tract but you have vacuum on the manifold.... BOV plate is suddenly very biased and between the boost on one side and the vacuum on the other, the plate opens and vents pressure until the two are near equilibrium.
However, there is a spring in the wastegate to give the manifold side of things a bit more "oomph", because you know as well as I do that 12psi against a small blockplate is a lot more force than 12psi from a tiny vacuum line off the manifold. Pressre x surface area = force -- lots more surface area on the intake pipe side of the BOV.
That spring often has to be adjusted to work correctly at your boost pressure so that it doesn't crack prematurely but still opens fully when needed.
You always want to set pressure at the wastegate, and just make sure that your BOV is set so that it pops off correctly.
Also, using a bigger turbo for lower boost to gain better thermal efficiency is an illogical statement. Bigger turbos are more thermally efficient at higher boost levels; smaller turbos for lower boost levels. Trying to make a big turbo run lower boost will simply increase lag time and really will gain you very little.
However, using a small turbo to create big boost will generally make more heat than air pressure, so sometimes you end up using a big turbo to make "weenie boost" for the daily driver racket.
12-05-2002, 05:07 AM
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Ah! I get it. Duh! rolleyes.gif I for some reason was thinking of the BOV reading the boost in the intake plumbing pre-throttle and venting anything above it's set pressure ratio into the atmosphere or before the turbo. But as far as a bigger turbo being more effecient at higher boost than lower boost, that still makes sense to me. Let m esee if I can dig up some images to show what I'm trying to say. I'm so bad about getting my thoughts across typing. smile.gif
Ok, I'm going to try and send some photos to Random to post up here so maybe they will explain a little better my question on the thermodynamic side.
T04e w/ 62 AR
T04e w/ 50 AR
T04e w/ 57 AR
The thick dark lines should be read as 14 psi.
The thin grey ones are 8 psi.
Again assuming you could create/find a BOV that would vent anything above X psi regardless of manifold pressure why wouldn't this be more beneficial. To me it looks like in these cases making the turbo put out more boost would increase the thermal effeciency significantly. This would especially be good to those of us that want to start out small then gradually work up where 300 BHP lives wiithout replacing a turbo every time you turn the boost up a couple of notches.
Ok, crap the 50 AR one didn't show up with the thin grey lines but I'm sure you can still read what I'm getting at Red.
[ December 05, 2002, 01:21 PM: Message edited by: Y2KTIBLTD ]
Ok, I'm going to try and send some photos to Random to post up here so maybe they will explain a little better my question on the thermodynamic side.
T04e w/ 62 AR
T04e w/ 50 AR
T04e w/ 57 AR
The thick dark lines should be read as 14 psi.
The thin grey ones are 8 psi.
Again assuming you could create/find a BOV that would vent anything above X psi regardless of manifold pressure why wouldn't this be more beneficial. To me it looks like in these cases making the turbo put out more boost would increase the thermal effeciency significantly. This would especially be good to those of us that want to start out small then gradually work up where 300 BHP lives wiithout replacing a turbo every time you turn the boost up a couple of notches.
Ok, crap the 50 AR one didn't show up with the thin grey lines but I'm sure you can still read what I'm getting at Red.
[ December 05, 2002, 01:21 PM: Message edited by: Y2KTIBLTD ]
12-06-2002, 05:30 PM
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You still seem to be confused. There are three definitions that you need to know.
1. Wastegate - determines how much exhaust flow your turbo recieves.. and vents exhaust gases away and allows your turbo to make X psi.
2. Blow Off Valve - Used between the turbo and the throttle body. Opens when the throttle body is closed to release (blow off) the high pressure air going backwards from the throttle body to the turbo. Only to decrease lag between shifts and to increase turbo life.
3. Pop off Valve - Used between the turbo and the throttle body. Releases air (pops off) when boost pressure go above X psi.
I think you are thinking about a pop off valve. Most people use it as protection against boost creep and wastegate failures. The pop off pressure is set for a couple of psi above the wastegate setting so that if something goes wrong... you only boost a couple of psi too high and not to engine wrecking levels.
You will not want to use the pop off valve as your primary control for boost. Such as setting the pop off... at lower than your wastegate. What happens is once you reach the pop off level... it bleeds the boost out.. like a intake leak... but your turbo / wastegate only knows that it hasn't reached its set psi... so it keeps spinning and the wastegate remains closed.
You basically end of overdriving the turbo more than necesssary.
Control boost levels with the wastegate... use the pop off valve for protection only... and use a BOV to help the car spool between shifts.
I hope this is clear.
Cheuk
[ December 07, 2002, 12:31 AM: Message edited by: Cheuk in Seoul ]
1. Wastegate - determines how much exhaust flow your turbo recieves.. and vents exhaust gases away and allows your turbo to make X psi.
2. Blow Off Valve - Used between the turbo and the throttle body. Opens when the throttle body is closed to release (blow off) the high pressure air going backwards from the throttle body to the turbo. Only to decrease lag between shifts and to increase turbo life.
3. Pop off Valve - Used between the turbo and the throttle body. Releases air (pops off) when boost pressure go above X psi.
I think you are thinking about a pop off valve. Most people use it as protection against boost creep and wastegate failures. The pop off pressure is set for a couple of psi above the wastegate setting so that if something goes wrong... you only boost a couple of psi too high and not to engine wrecking levels.
You will not want to use the pop off valve as your primary control for boost. Such as setting the pop off... at lower than your wastegate. What happens is once you reach the pop off level... it bleeds the boost out.. like a intake leak... but your turbo / wastegate only knows that it hasn't reached its set psi... so it keeps spinning and the wastegate remains closed.
You basically end of overdriving the turbo more than necesssary.
Control boost levels with the wastegate... use the pop off valve for protection only... and use a BOV to help the car spool between shifts.
I hope this is clear.
Cheuk
[ December 07, 2002, 12:31 AM: Message edited by: Cheuk in Seoul ]
12-08-2002, 10:54 PM
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That's what I was afraid of. It would end up prematurely wearing out the turbocharger. Thanks for clearing up my confusion on the pop-off valve vs. BOV. That makes more sense now.