SolidSpruce

When im sitting in my car, and look at my boost gauge, it read 20psi of VAC. Have you ever noticed that a throttle body will suck Think about this, when your turbo reads 10psi on a boost gauge, its not simply 10psi into a manifold with zero pressure but 10psi ON-TOP of the 40+ psi of vacuum the motor is already sucking.

so technically isnt a turbo making like 50psi when it reads 10psi on the + side?

i dont know if this makes much sence, or i just thought into that too much. but it only seems logical.

to fight all the vacuum a motor makes and push it into a positive value, its more than just simply the positive value given, it would be the vacuum + positive value = overall pressure the turbo puts out.

let me know what you all think.

while ur at it, check out this funny arab music video - http://flightline.highline.edu/ic/tunak.html

keebler65

keep in mind that your boost guage probably doesnt read in psi for vacuum, rather inches or mm of hg....although your question is still valid.

VanHonda

Won't an engine, even NA run much less vac when yu are driving it or there is a load on the engine. Before i was turbo, i noticed that my vafc would read almost 0 after i would drive it hard. Just a thought

MaxSterling

boost gauge is reading "gauge pressure" (Pgauge = Pturbo - Patmoshpere)

Ricey McRicerton

Yup. An NA motor will run closer to no vaccum at all the more you hammer it.

Mase

Im not sure what you're really talking about though, but actually, most b series motors, at idle, have around 10 psi of vac.

SolidSpruce

see, thats just the thing, u are making a decision based off of what the gauge reads - the gauge just says 0 or absolute manifold pressure simply because there is no resistence of air it has to struggle for, so no vacuum is registered - but put ur hand or throw a fist full of dust infront of the throttle at wide open throttle, and tell me if its at 0 pressure or at about 40psi of vacuum. its pulling harder than a freakin vacuum cleaner, the gauge simply cant tell because it has no resistence to register.....think about that.

Ricey McRicerton

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Mase &raquo;</TD></TR><TR><TD CLASS="quote">

Im not sure what you're really talking about though, but actually, most b series motors, at idle, have around 10 psi of vac.</TD></TR></TABLE>

Isnt vaccum measured in in/hg?? I know for a fact my D series motor sits at 20 in/hg at idle.

bretx0r

the pistons going up and down create a low pressure and the high pressure outside goes towards the low pressure. Basically when you boost a motor you are raising the compression, becuase more compression + more displacement + power stroke = more power That is why most all motor motors are high compression because they have to utilize the amount of air that the low pressure is taking in as much as possible but instead of having something pushing it in they have to have something physically subbing in for that absence of pressure...

The guage reads 0 because all that pressure is being pulled into the cylinders so fast that the guage cant read it because the pressure in the manifold is absent... someone corerct me if im wrong Im sure if you somehow hooked up a boost guage to one of the cylinders it would read higher at WOT. And nobody say that the air is 'sucked' in because nothing sucks.

thats kindof it in a nutshell...

Seraph0

Well boost gauges always measure relative pressure.

There is relative pressure and absolute pressure.

grndcont

good, now we all have a basic understanding of how a speed density system works. you see, at a higher inch of vacum the ecu can assume the load and demand on the system is less, because something is restricting flow ( throttle plate). now when u floor it, the air flow will increce but the vacum will drop (n/a). and when the ecu sees less vacum and is still geting rpm inputs it can asume a higher load and demand is now needed. boost is a hole other animal. we're relly ading cfm's of air to the clinders, but we mesure this in psi. now back to the o.g. question, no... or atleast i dont think so...when i can think of exactlly why i'll wright back. it's hard u see, becouse the vacum is stil there, but u can't read it at wot, yet when u boost you can read that! so meby your reading of say 5psi, is 5psi above 0 becouse u have negleated the vacum, or filled it up as u could say. p.s. if you know for a fact that my obove statment is flamingly false, just say so and dont be a dick...i'm just an m.e. in traning.

SJP0tato

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by SolidSpruce &raquo;</TD></TR><TR><TD CLASS="quote">
to fight all the vacuum a motor makes and push it into a positive value, its more than just simply the positive value given, it would be the vacuum + positive value = overall pressure the turbo puts out.

let me know what you all think.
</TD></TR></TABLE>

There's actually 2 frames of reference you can think of when thinking about this. One is without the earth's atmosphere, and one is with. In the earth's atmosphere, the engine's vaccum is actually just removing the atmospheric pressure that exists everywhere on earth (most pressure at sealevel or below, less pressure the higher you go), so 10psi from the turbo in a way "replaces" that displaced atmospheric pressure, and then pressurizes it above atmospheric to 10psi. So in reference to absolute vaccum you're making 10psi + atmospheric. In the earth's atmosphere you're making 10psi.

What's really neat to think about, each time your lungs exhale, you're putting out a little bit extra pressure over atmospheric pressure, which is a LOT more than absolute vaccum... so your lungs are kinda like a little mini-turbo creating pressure & vaccum of their own.

Probably clear as mud,

Ryan P.

SolidSpruce

well at WOT simply pressurizing 1psi of air only and supplying it to a N/A motor will not push it 1psi into positive pressure....you would need to first combat all the vacuum it is currently drawing in through the throttle, once that vacuum is canceled, it can build up boost above absolute manifold pressure. a boost gauge has nothing to do with what we are talking about here, because a boost gauge cannot register the extreme amount of air being drawn into the motor at wide open throttle, because it has no resistence to measure it against.

its a difficult concept to grasp because i dont think anyone has analyzed it or really thought about, but there is a serious amout of vacuum a turbo has to cancel out before it builds positive boost pressure, meaning that a turbo is truely putting out more than what meets the eye, we simply dont relate to it in that way, simply its final output above absolute manifold pressure.

Ricey McRicerton

Wow, I was trying to read and comprehend all of this, and just realized that there is no possible way for me to contribute anymore to this thread. to you guys who can help and understand this stuff!!

96-EJ6 B18C

Vac is measured in mm of mercury or inches of mercury while boost is measured in psi or bar. When your boost gauge is reading "20" and it is below 0 that means that there is enough of a vac in your manifold that the ambient air pressure would force mercury up a tube 20 inches. Hence you read the gauge "20 inches of vac". When the gauge is at 0 all it is doing is reading atmospheric pressure. In an n/a car at WOT it will read very close to 0. Your manifold doesn't see any pressure until the turbo compresses the air (readings above 0 on the boost gauge). So when your boost gauge is reading "10 psi" that is exactly what pressure is in your manifold. The standard atmospheric pressure is 14.7 PSI (29.92 mmHg) (1.0 BAR) at sea level on a standard day (59 deg. F) which does not count as boost (unless you have a boost gauge that is calibrated to read 0 in an absolute vac), and can be read on a boost gauge as 0. So...over an ABSOLUTE vac you are running 24.7 PSI...but we don't live in an absolute vac fortunately, so the standard atmospheric pressure DOES NOT COUNT AS BOOST! Therefore when your boost gauge reads "10PSI" that is exactly how much boost your motor is seeing! Good God I'm out of breath!


Modified by 96-EJ6 B18C at 6:40 PM 10/27/2003


Modified by 96-EJ6 B18C at 6:49 PM 10/27/2003

96-EJ6 B18C

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Ricey McRicerton &raquo;</TD></TR><TR><TD CLASS="quote">Wow, I was trying to read and comprehend all of this, and just realized that there is no possible way for me to contribute anymore to this thread. to you guys who can help and understand this stuff!!</TD></TR></TABLE>

Just think logically about it for a few seconds and you will grasp the basic ideas. The **** is really simple once you "get it." And your absolutely right vac is measured in mmHg or in.Hg (see above post) HOW THE **** CAN YOU HAVE A "PSI OF VAC"???

bretx0r

Just think... Would engines be able to run in outer space? Dont think so.

Thats so cool how it relies on out atmospheres naturally high pressure to work.

Ricey McRicerton

Yeah I understand what you are saying 96-EJ6 B18C and agree with you, just some of the other concepts are a little out of my undestanding.

lazerus

standard atmospheric pressure (at sea level) is around 14.7 PSI/in^2 when you are running 10 psi at sea level, that means your Total or absolute pressure inside the intake manifold is 24.7 PSI above zero pressure, or 10 psi above atmospheric pressure...

96-EJ6 B18C

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by bretx0r &raquo;</TD></TR><TR><TD CLASS="quote">Just think... Would engines be able to run in outer space? Dont think so.

Thats so cool how it relies on out atmospheres naturally high pressure to work.</TD></TR></TABLE>

Lol, yeah a damn rotary would probably find some way to LOL

Ricy McRicerton (love the sn) basically all a boost gauge measures is: total pressure (atmospheric+any pressure the turbo is making)-atmospheric pressure=boost hopefully that helps a bit.

MachAF

People seem to forget that on a standard day at sea level there is 14.7 psi.

this is a good topic and has great answers!!

SiRkid

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by lazerus &raquo;</TD></TR><TR><TD CLASS="quote">standard atmospheric pressure (at sea level) is around 14.7 PSI/in^2 when you are running 10 psi at sea level, that means your Total or absolute pressure inside the intake manifold is 24.7 PSI above zero pressure, or 10 psi above atmospheric pressure...</TD></TR></TABLE>

SolidSpruce

ok, everyone forget ur boost gauge, who even cares about what it has to say at this point....im not talking about 20 inches of vac or psi or whatever method the 'boost gauge' wants to read....im talking about physically how much vacuum an engine can suck at WOT, if u hold ANYTHING in front of the throttle or try to suffocate it in any way, it rips/sucks/implodes/destroys anything trying to obstruct it, unless its can resit enough to cough the motor out of breath and suffocate it. THAT is the vacuum im am talking about, that is the vacuum that the turbo is overcoming, the vacuum a boost gauge at WOT cannot register.

lets take this to a simpler level, if u have a plumbing pipe in your house that has 40psi of water pressure flowing in one direction, it does not simply take 10psi coming from the opposite direction to cause it to change paths of flow...it would simply lower the resitance to 30psi, it would require 50psi of pressure to get the 40psi flowing in the opposite direction at 10psi.

nobody mention a thing more about a boost gauge or mercury or atmospheric 14.7, its not the issue here.


Modified by SolidSpruce at 5:42 AM 10/28/2003

grndcont

like i said, flow is the issue here, not vac - or +.

Bottoz

You're reading the guage wrong. At idle, the boost/vac guage sits at about 20in/hg. Notice how the guage goes from 30-0-30? Its reading Negative 20in/hg. -20in/hg = -10psi of boost....which is vacume! Its like going outside on a sunny day and saying, "Wow it sure is Negative Dark outside."

http://www.stealth316.com/2-converters.htm