Prime Power Posted February 4, 2007 Share Posted February 4, 2007 I installed my boost gauge and see most of the time it is a vaccum. So the turbo is in a vaccum relative to what? Is it not pushing any air into the system. Basically help me get my head around it better. Thanks Link to comment Share on other sites More sharing options...
BDII Posted February 4, 2007 Share Posted February 4, 2007 your gauge should read positive and negative, anywhere over 2500 RPM you should read some sort of boost. As long as the turbine (hotside) is spinning you will have some sort boost being created. When the air from the cold side of the turbo exceeds the CFM of the Intake pressure you are technically "in boost". Need forum help? Private Message legGTLT Link to comment Share on other sites More sharing options...
Prime Power Posted February 4, 2007 Author Share Posted February 4, 2007 I am understanding when I am in boost, but when it is negative, what is going on. Thanks. Link to comment Share on other sites More sharing options...
bosco Posted February 4, 2007 Share Posted February 4, 2007 I am understanding when I am in boost, but when it is negative, what is going on. Thanks. the turbo is just free wheeling waiting for your command to boost ie. increase air/fuel. bosco Stay Stock Stay Happy Link to comment Share on other sites More sharing options...
BDII Posted February 4, 2007 Share Posted February 4, 2007 the CFM of the intake charge exceeds that of the turbo. Need forum help? Private Message legGTLT Link to comment Share on other sites More sharing options...
Prime Power Posted February 4, 2007 Author Share Posted February 4, 2007 Got it, thanks so much, this forum is great. Link to comment Share on other sites More sharing options...
unstablementaly Posted February 4, 2007 Share Posted February 4, 2007 wow, could you guys explain it simpler? your negative readings are vacuum. all cars run in a vacuum. intake sucking in air. sucking thru the turbo untill it spools. once the turbo is spooled up and FORCING air thru the rest of your intake then you are pressurized aka boost my vBGarage Link to comment Share on other sites More sharing options...
ZinFreak Posted February 4, 2007 Share Posted February 4, 2007 Well, I can take a stab at explaining what you are seeing. One of the most common areas to connect the pressure line for boost is on the Air Bypass Valve (ABV). This line frm the ABV connects directly to the intake manifold. If you can get a of the Legacy repair manual for 05/06 look at "102 - Basic Description Turbocharger System". If you can see the picture it would make sense. OK. So when the turbo charger spins up from the engine management, the ABV will open allowing air pressure at roughly the same pressure the turbo is boosting to, to "bypass" through the valve and into the intake system. That is how the gauge reads the positive "boost pressure". When the turbo is not in play, the ABV is closed. However, the intake system pulls a negative pressure across that now closed line. With a boost gauge connected at this point, you are effectively seeing the negative pressure that the intake system is pulling. Another way to look at it. The line from the ABV to the intake manifold has positive pressure when the turbo is active, and negative pressure when inactive (due to the vacuum created by the intake). Does that help? -Zin 06 LGT LTD GRP 5MT 07 FXT LTD OBP 4EAT Link to comment Share on other sites More sharing options...
BDII Posted February 4, 2007 Share Posted February 4, 2007 wow, could you guys explain it simpler? your negative readings are vacuum. all cars run in a vacuum. intake sucking in air. sucking thru the turbo untill it spools. once the turbo is spooled up and FORCING air thru the rest of your intake then you are pressurized aka boost we didnt want to confuse the guy:lol: Need forum help? Private Message legGTLT Link to comment Share on other sites More sharing options...
bosco Posted February 4, 2007 Share Posted February 4, 2007 now you got me confused. bosco Stay Stock Stay Happy Link to comment Share on other sites More sharing options...
unstablementaly Posted February 4, 2007 Share Posted February 4, 2007 lol, sorry!!! i just deal with pressure and vacuum all day long. the long explaination always seems to be the most accurate but i guess for those that don't get it can get lost in translation. my vBGarage Link to comment Share on other sites More sharing options...
ZinFreak Posted February 4, 2007 Share Posted February 4, 2007 Did you skip physics in high school Unless your car was in space, it runs in atmospheric pressure (+14.7 psi @ S.L.), which varies with altitude. wow, could you guys explain it simpler? your negative readings are vacuum. all cars run in a vacuum. intake sucking in air. sucking thru the turbo untill it spools. once the turbo is spooled up and FORCING air thru the rest of your intake then you are pressurized aka boost -Zin 06 LGT LTD GRP 5MT 07 FXT LTD OBP 4EAT Link to comment Share on other sites More sharing options...
unstablementaly Posted February 4, 2007 Share Posted February 4, 2007 Well, I can take a stab at explaining what you are seeing. One of the most common areas to connect the pressure line for boost is on the Air Bypass Valve (ABV). This line frm the ABV connects directly to the intake manifold. If you can get a of the Legacy repair manual for 05/06 look at "102 - Basic Description Turbocharger System". If you can see the picture it would make sense. OK. So when the turbo charger spins up from the engine management, the ABV will open allowing air pressure at roughly the same pressure the turbo is boosting to, to bypass the valve and into the intake system. That is how the gauge reads the positive "boost pressure". When the turbo is not in play, the ABV is closed. However, the intake system pulls a negative pressure across that now closed line. With a boost gauge connected at this point, you are effectively seeing the negative pressure that the intake system is pulling. Another way to look at it. The line from the ABV to the intake manifold has positive pressure when the turbo is active, and negative pressure when inactive (due to the vacuum created by the intake). Does that help? and i'm confusing??!! my vBGarage Link to comment Share on other sites More sharing options...
unstablementaly Posted February 4, 2007 Share Posted February 4, 2007 umm, the intake sucks in air, creating a vacuum. hense the negative numbers on your gauge. did you skip physics? my vBGarage Link to comment Share on other sites More sharing options...
ZinFreak Posted February 4, 2007 Share Posted February 4, 2007 umm, the intake sucks in air, creating a vacuum. hense the negative numbers on your gauge. did you skip physics? I taught university level mechanical engineering. I've had more physics, thermodynamics and fluid dynamics classes than I care to recall. Your "simplistic" explanation, well, it doesn't explain much at all. -Zin 06 LGT LTD GRP 5MT 07 FXT LTD OBP 4EAT Link to comment Share on other sites More sharing options...
unstablementaly Posted February 4, 2007 Share Posted February 4, 2007 well, i think it did. he just wanted to know why he saw negative readings on his gauge. i don't play well with mechanical engineers, especially those in the thermodynamics field. hvac mechanics only know what works in the field!!! my vBGarage Link to comment Share on other sites More sharing options...
Prime Power Posted February 4, 2007 Author Share Posted February 4, 2007 I am a prime power engineer and have had thermodynamics and physics in college, however that was a few years ago and everything I do is based on large generator power plants (2.25 megawatts). Therefore we use 16 cylinder diesel engines with turbo's, but at a steady 1800 rpm, there is no variance. We work on more than that but is the mainstay of what we use. Also I have a specialty of instrumentaion. This being my first turbo car that I have modified and really got into to, I wanted to understand it better. I have been on recruiting duty for the last year and have forgot some of my old job. Can't wait to go back. I appreciate all your help guys. Link to comment Share on other sites More sharing options...
Jon in CT Posted February 5, 2007 Share Posted February 5, 2007 the turbo is just free wheeling waiting for your command to boost ie. increase air/fuel. boscoSubaru says the turbo is spinning at about 20,000 RPM at idle (see http://endwrench.com/images/pdfs/WRX.pdf#page=3). I wouldn't call that "free wheeling." The turbo IS compressing the air at idle above ambient pressure. That extra pressure is being relieved by the BPV, which redirects the excess air back into the turbo compressor inlet tube. the CFM of the intake charge exceeds that of the turbo.At idle, the CFM of the turbo greatly exceeds that of the intake charge. That's one reason why there's a bypass valve, diverting excess turbo pressure back into the compressor inlet. Why on earth do you people feel free to post crap here when you're clueless? You're not helping anybody. Link to comment Share on other sites More sharing options...
Prime Power Posted February 5, 2007 Author Share Posted February 5, 2007 ;986558'] Why on earth do you people feel free to post crap here when you're clueless? You're not helping anybody. Thanks, good info. I see that so much on forums. I am a smart guy, but had some questions as too this, and am not to proud to ask. Thanks. Link to comment Share on other sites More sharing options...
Jon in CT Posted February 5, 2007 Share Posted February 5, 2007 Therefore we use 16 cylinder diesel engines with turbo's, but at a steady 1800 rpm, there is no variance. We work on more than that but is the mainstay of what we use.One of the major differences between compression ignition engines (your diesel engines at work) and spark ignition engines (your subaru engine on the road) is the use of a throttle plate on the Subaru to control torque. Diesel engines don't employ a throttle plate (i.e. they're always "wide open throttle") and show little, if any, negative pressure in the intake manifold. They control torque via fueling. Spark ignition engines control torque by modulating the amount of air into the cylinders with a throttle plate. When the throttle plate is mostly closed, a partial vacuum develops downstream from the throttle plate. Link to comment Share on other sites More sharing options...
Faster_nu Posted February 5, 2007 Share Posted February 5, 2007 Ouch, when a car is naturally aspirated you usually have vacuum. When you place a turbo in the path of the intake and the turbo fins spin fast enough the air becomes pressurized by the turbine in the turbo. Your measurement point is after the turbo so when the turbo is free wheeling it reads vacuum and when the turbo is being turned at a fast enough rate by exhaust gas it pressurizes the air after the turbo. Link to comment Share on other sites More sharing options...
Faster_nu Posted February 5, 2007 Share Posted February 5, 2007 Jon, is that true for diesel cars as well? Link to comment Share on other sites More sharing options...
Evilleg Posted February 6, 2007 Share Posted February 6, 2007 does the car stay on full boost like 13 until u shift? Link to comment Share on other sites More sharing options...
Prime Power Posted February 6, 2007 Author Share Posted February 6, 2007 Good info. Like I said, my knowledge comes from the training and courses I have recieved to do my job. My auto knowledge is basically a carryover and from research. I really appreciate this guys. Link to comment Share on other sites More sharing options...
Faster_nu Posted February 6, 2007 Share Posted February 6, 2007 Evil - Boost usually tapers off the closer you get to redline, this is one of the mods that is usually done with products that reprogram your ECU. The mods keep boost up longer and usually increase the amount of boost as well. At every additonal psi such as 13.5 to 14.5 you increase hp aprox 30hp as I remember it. I think the stage one cobb hits 14.5 hold this value longer and tapers off to almost stock at redline. A stock unit hits 13.4 takes longer to do this and tapers off faster to redline. All this is effected by Gas, Air temp, intake temp, throttle position, and load. Poor gas will limit boost targets. Link to comment Share on other sites More sharing options...
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