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Discussion Starter · #1 ·
I did a search but found nothing ? I am sorry if this is a stupid question but why do I get almost 1 psi more when it is low 60's outside opposed to normal boost from high 60's to 105 ? I even have a greddy profec b and for some reason when it is cold at night it will creep right past my preset point of 16.5 psi and it will hit boost cu if I am not carefull ? Any info would be great . For now I have the low boost set for 65 deg and below 16.5 psi and for 65 and up I have the high boost set for 16.5 psi . This seems to work fine but I was just curious on why more boost when cooloer ? Thanx .
 

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Colder air is more dense. The Intercooler works more efficiently with cooler air being run through it. So cooler air going into the turbo and the intercooler working more efficently makes for more boost.

Just taking a stab at it.
 

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Richie03 said:
Colder air is more dense. The Intercooler works more efficiently with cooler air being run through it. So cooler air going into the turbo and the intercooler working more efficently makes for more boost.

Just taking a stab at it.
that's right. colder air is more dense, so the turbo is still working the same, but the air it's compressing is already denser. the end result is a charge denser (higher boost) by the value of the colder intake air.

dR
 

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I don't think the answer is that simple.

The guy's question was why does colder air = more pressure, based upon his gauge.

The fact that there's more O2 molecules in colder air than warmer air should not affect measured pressure.

i.e. 15 psi of cold air = 15 psi of hot air, right?

I don;t know the answer either, but I do think the boost controller may not be consistent managing the boost when there is a significant temp change.

Hg.
 

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You guys are going down a slippery slope into the world of gas physics. :)

15PSI of cold air = 15PSI of warm air, but there are more molecules in the cold air.

Effect is the same: More power. But it's not a PSI increase. I agree with QuickSilver. The gauge isn't 100% accurate across all temps.

And you thought you'd never see this again after school! Remember the ideal gas law. PV=NRT. N=Number of moles of gas. Drop the temp, and you can put more gas into the same volume at the same pressure.
 

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The gauge is plenty accurate. I have yet to pilot my STi through a North Dakota winter, but my WRX ran significantly higher boost when on the MBC in cold weather. I had to turn it down a couple times to keep from overboosting. The reason that I think the gauge is fine is because I have tested the stock Lampco gauge against a large, high-quality, properly calibrated mechanical gauge and it was spot on. Also, the gauge itself is nice and toasty warm inside the cabin of the car, even when the outside temps are 20 below zero (F!). Plus, it is common knowledge among tuners and owners (even of non-suby brands) that this occurs, and it happens with non-subi cars with quality aftermarket gauges as well. There must be more to it than gauge quality.
 

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QuickSilver said:
I don't think the answer is that simple.

The guy's question was why does colder air = more pressure, based upon his gauge.

The fact that there's more O2 molecules in colder air than warmer air should not affect measured pressure.

i.e. 15 psi of cold air = 15 psi of hot air, right?

I don;t know the answer either, but I do think the boost controller may not be consistent managing the boost when there is a significant temp change.

Hg.
yes, but your turbo isn't thinking in PSI, all it's doing is working as hard as you tell it. you make the adjustments to it's duty cycle to the end result of your desired boost.

let's say your turbo is working "x" hard. regardless of what the intake air temp is the turbo still works "x" it's not compressing to 15psi, it's just working on a duty cycle (based on YOUR input.)

so when it gets colder, the turbo has no idea the intake air is denser, it's still going to compress "x" the outcome is the turbo made more boost because of the higher multiplier (colder intake air).

in short, the higher count of 02 molecules when subjected to a static compression situation, will result in an increasing relationship of pressure. (there's more to compress in that fixed volume)

dR
 

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Oh we're not bashing the gauge quality. It's probably the colder temperatures effecting how the boost controller reads and/or adjusts boost.

I'm just saying that cooling the air will not, itself, cause PSI to go up. There's something in the mechanics that's allowing it to go up. Did that make any sense? :confused:

I do have a theory, but not knowing the internals of the BC, I could just be pulling it out of my backside... The pressure sensor for the BC is sealed. Ambient temperature drops and cools the sensor. Thus, the sensor believes it is seeing less boost than it actually is, and lets the turbo build up more pressure, which is registered in your boost gauge.

I told you this was a slippery slope! :D
 

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But that does not explain the same phenomenon occurring with cars running MBC’s. The MBC is completely mechanical, and should not be affected by temps. Of course, maybe the metal in the spring changes properties....... OK, Maybe the 3/16 mod on an open tube folks can check it out. They have no mechanical nor electronics in their boost circuits.
 

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Discussion Starter · #10 ·
The boost gauge is an Omori and the controller is always dead on ! The only time that I get more boost is in cold weather . I dont believe that it has anything to do with the gauge . Possible it has something to do with the boost controller BUT my friend has a supercharged dakota that is also making more boost when it is cold ! My only question is why ? Is it my boost controller or a sientific reason for boost presure to rise in colder weather even though a preset value of boost is set at a warmer temperature ?
 

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I'm going to go in a different direction than what everyone else is on this. Even though the air coming in is more dense, the PSI reading is a pressure reading. On the other hand, a denser air charge will cause a leaner mixture, which increases Exhaust Gas Temperatures (EGT's), which can cause an increase in boost.

You might want to see what your AF ratios look like with the increased boost during the cold temperatures you're having.
 

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I just reread your post and noticed that you said boost creeped up. If it's actually creeping past your set limit and not just boosting to a level that is slightly higher, the stock wastegate may not be keeping up with the boost pressure.
 

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The answer is - compressors are constant volume machines (at constant speed), while turbines are dependent mostly on enthalpy. So as the air temp decreases the density of the air increases, and more mass flow is compressed for an equal volume, when driven at the same speed. The turbine sees a higher temperature at the inlet when the intake charge pressure increases, which is equivalent to a higher inlet enthalpy.

So at colder temperatures the compressor is capable of putting more mass of air in the engine, which increases the backpressure on the compressor and increases boost. The turbine, at the same engine rpm, spins faster because it is receiving higher temperature gas from the engine exhaust valves, so the compressor spins faster. The result is that at colder temperatures you will see both more boost and possibly an earlier boost peak.
 

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VVVV, would this only be true for a mechanical boost controller? I'm just thinking that if an automatic controller is set to 16PSI, it ought to dump from the wastegate at 16PSI, regardless of air temp. From your comments, it's obvious that you'd get quicker boost, though.

I love geeky discussions. :p
 

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Discussion Starter · #15 ·
VVVV,
Thank you very much for respoding . That was the kind of answer I was looking for . That makes complete sense . I have set my low boost to 16.5 psi when the air temp is 58 degrees and below and my hi boost is set at 16.5 psi for 59 degees and above . This seems to be working very well for me . I have been testing it for three nights now .
 

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vtjim said:
VVVV, would this only be true for a mechanical boost controller? I'm just thinking that if an automatic controller is set to 16PSI, it ought to dump from the wastegate at 16PSI, regardless of air temp. From your comments, it's obvious that you'd get quicker boost, though.

I love geeky discussions. :p
not sure what you mean by automatic. (ebc?) but it doesn't matter. all boost controllers work on the same principle. i don't know of a BC that you can set a "PSI" and it will hold it. all of them (manual or electronic) are set by changing the gain from "wastegate only" boost.

so back to my point. the turbo doesn't know PSI. it's going to work the same no matter what temp it is. so if it's colder out, the turbo will work as hard as it did when it was warm out, thus allowing for more air in the same volume of space.

dR
 

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I get it. I'm learning about wastegates and boost controllers here, hence my questions. :) I thought a boost controller let you set a boost level in PSI and the wastegate would hold it. But that's not how they work.
 

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A wastegate is just a relief valve that prevents the boost from exceeding the set point pressure. If the wastegate doesn't open, or it is too small to relieve all of the flow from the compressor, the boost will increase to a higher peak value on a cold day when compared to a hot day.

Usually the biggest difference between a MBC and a EBC is the reaction time of the feedback from the pressure sensors. An EBC will react quicker, shutting the wastegate sooner, and maintaining higher boost pressure during transient (re: throttle lift) conditions.

Wastegates typically only have open and close control, meaning that they pop fully open when they open, and have no partial open positions. So whether you spike at 16.5 psi or 20 psi, it opens the same amount, and closes when it reads the set pressure of 16 psi.

So the reaction time to close it is the only wastegate variable that can be improved to help engine response, while changing the set pressure only affects when the controller signals to open and close it.

I think what dark_rex is getting at (correct me if I'm wrong) is that the turbo (compressor and turbine assembly) is free spinning. There is nothing besides the turbine, compressor, and bearing that contact the rotating shaft. Nothing slows it down or speeds it up besides the air and exhaust that come in contact with the blades.

Technically a turbocharged engine is two engines working in a symbiotic relationship. The turbo is a gas turbine engine that only "sees" the piston engine as a combustor. As you adjust the throttle, you are directly controlling the piston engine, and indirectly controlling the turbine engine, which is why turbos have lag.
 

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A little off topic but still relevant I think:

From an undergrad M.E. course -

For every 10 degree F raise or drop in temperature: horsepower in an internal combustion engine will decrease or increase by 1%.
(as I recall turbocharging was not factored into this equasion).

So, a modded Rex with 300 HP on a standard 68 degree day should have 306 HP on a 48 degree day.

Having posted the above, I disagree with myself....my Rex absolutely screams on a 48 degree day (2% HP increase my ass).

One of you MIT or Worcester Poly undergrad Thermo geeks with knowlege of the dark and arcane equasions for turbocharging corrections need to refute and expand upon the above antiquated equasion.
 

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Hang onto your slide rules boys, (I'm ALMOST old enough to have used one.), but cold air is the key to making more power with an intercooled turbo engine. I could reference thermodynamic text books, but those are hard to understand for me. Instead I refer you to Hugh MacInnes's "Turbochargers". If you crunch the numbers you can see the big increase in mass flow at cooler temps.

I don't have a good answer as to why the boost goes up a cooler temps. A guess is restrictions in the system....
 
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