dont understand the # of pounds boost???

Home  \  Asian Imports  \  dont understand the # of pounds boost???

what does it mean when people say they are running like 10 pounds of boost? how does it work out? how do you change the amount of boost?

posted by  silvia_star

boost. PSI. Bars. All refer to the ammount of pressure a turbo/supercharger is putting out. Boost can be changed via boost controllers, and other ways also.

To figure out how it works, visit www.howstuffworks.com

I'm sure they have a section on turbochargers.

posted by  Godlaus

what about like when people say they're running like 12 psi? what does that mean?

posted by  silvia_star

someone tell me plz!

posted by  silvia_star

No, no, no, no, NO! I have explained this previously on these forums. Turbochargers in and of themselves DO NOT create pressure. Turbochargers produce increased airflow. It is the fact that this increased airflow is then restricted by flowing into the intake manifold and into the combustion chambers that creates pressure or boost.

Here is a perfect example. Everyone has a blow dryer in their house. Go get it and plug it in. Turn it on. How much pressure is it putting out. None. Zero. Zilch. Nada. Now cup your hand and put it about a foot in front of the nozzle. Do you feel a little pressure? That's because there is a little airflow restriction. Now move your hand closer. More pressure? More restriction. Closer still. More pressure? More restriction. And so it goes until you reach the mechanical or dynamic limits of the machine.

The boost numbers that you read are based on the ratio of airflow verses restriction. Obviously a pop off valve, wastegate, boost controller, etc will reduce pressure at the appropriate time because... If you have enough turbine speed, therefore enough airflow AND at the same time enough airflow restriction the boost pressure will keep building until you reach the mechanical or dynamic limits of the turbo or something blows up. That something will be the engine because turbo housings can withstand in excess of 50 psi.

Please guys, let's try to keep this simple, correct and real. :banghead:

posted by  vwhobo

I learn something new everyday.

posted by  Godlaus

Not from me, I don't help anybody... Just ask DildoRida67.

posted by  vwhobo

Ok wvhobo, turbos use centrifugal force right, which increases the speed of the air going into the cylinders. So most of the PRESSURE BUILD UP IN THE CYLINDERS because thats when the air gets restricted by the intake valves right? When the compresser is spinning, more velocity of air if going into the engine so if the intake valve is closed, than pressure build up there and when the valves open, the air is already pressured to fill the combustion cambers. Am I right that when the velocity of the air meets a closed intake valve or top of a piston, the velocity is turned into pressure?

posted by  CarEXPERT

Based on this post I would say you have a pretty good grasp on the subject.

posted by  vwhobo

They are using slang. They are saying the pressure is 10 psig (pounds per square inch gauge), which simply means relative to atmospheric pressure is 10 psi. Saying "pounds" sounds pretty tough and knowledgable though.

When listening to your mates carry on, remember that some measure at the diffuser of the compressor, others before the throttle body and others at the plenum or intake runners. The plenum is where a map sensor is generally located and where you should measure for bragging rights.

posted by  Wally

carexpert, what you said is correct except the part about centrifugal force. the turbine spinning simply moves air, as a fan does, there is no centrifugal force involved. centrifugal force pertains to the force exerted on any part of the turbine by it's rotation. what we are concerned about is the air coming off the turbine, and entering the intake or exhaust

posted by  carls47807

Well actually compressor side is a centrifugal pump. It's all very basic fluid dynamics kind of stuff. The intake air is pulled into the center and flung (for lack of a better description) o-utward by the rotation of the blades. Centrifugal force in action.

Note: O-utward is spelled as such because the language filter won't let it through written properly. Try it.

posted by  vwhobo

What makes you think that the compressor on a turbocharger cannot raise the pressure of the air at the outlet with respect to the inlet?

I am assuming that you are referring to the outlet static pressure component being lower than the inlet total pressure which may in fact be true (for a given scenario). However, the outlet total pressure is going to be the sum of the outlet static and dynamic pressure components. The outlet total pressure must be higher than the inlet total pressure otherwise all the shaft work going into the compressor is for nothing.

As for the hair dryer it employs a low cost fan which is not design to generate high pressures to begin with. Nevertheless, the total pressure in the hair dryer is higher than atmospheric pressure otherwise the air would not flow out of the hair dryer (high pressure always moves towards a low pressure).

posted by  Regular.Stormy

The compressor does not have a turbine. it has an impellor that approximates a mixed flow fan (centifugal + axial). The contours of the impellor blades create a high pressure region, until they enter a choke region where stall flutter starts.

Even when there is no connection to the discharge snout, there will be a higher pressure in the diffuser than atmosphere, because of pressure drop.

posted by  Wally

You can only have pressure if you have restriction. The nozzle pressure is in fact slightly higher than the inlet pressure, but... Keep in mind the true inlet pressure is negative due to the the fact the impeller is spinning which explains the differential. Also keep in mind that the nozzle has high velocity air flow and as we all learned in 5th grade higher velocity equals lower pressure.

As for the hair dryer analogy, if you're honestly trying to equate the two this subject is above your head. Perhaps I should have said perfect example for those of us not fortunate enough to have a manometer and a turbo laying around the house. :roll:

posted by  vwhobo

Correct me if I'm wrong. Would'nt the measurment of pressure with the addition of a turbocharger or supercharger start after 14.7psi. Because if a car is at sea level then there would allready have to be 14.7psi of ambient air pressure within the intake manifold upon the intake stroke.

posted by  DSMer

The gauge already has 14.7 psi acting on the bellows, so you will be reading differential pressure wrt atmosphere, thus we use the acronym psig in the egineering game.

When working out your density ratios, pressure ratios and volumes you work in absolute, just as you would for ideal gas equations.

posted by  Wally

Makes sense, thats how a gauge can read below 0 bar.

Ahh my favorite Pv=nRT :hi: .

posted by  DSMer

Beware the bar measurement too. Some instruments are rounded to 100kPa for bar instead of the 101.32kPa, not that it matters too much on a car engine.

People can get a bit too cute when it comes to huffered or hairdryered engines.

posted by  Wally

I do not disagree.



As at the outlet side there must be a high pressure which can then expand as the restrictions allow. If the intake manifold is pressurized at 5 psig than the total pressure at the outlet of the compressor must be higher for more air to enter the manifold.



This is true for the static pressure but what about dynamic pressure?



Maybe I could mail you my pitot-static tube? :roll:

posted by  Regular.Stormy

You are alluding to Tp= Vp + Sp ?

posted by  Wally

Yea originally I thought I just might be miss-understanding what pressure vwhobo was referring too. But now I honestly do not know what direction the discussion is taking, off topic I guess.

posted by  Regular.Stormy

I think he is simplfying it so that people with only a bourdon gauge can appreciate that although the pressure reading (static pressure) may be high the flow may be correspondingly low due to pressure loss of the fittings.

Not many people would be able to pitot traverse the pipes to see total pressure and work out velocity pressure.

posted by  Wally

Thanks for the clarification.

posted by  Regular.Stormy

'sOK. Of course if the pressure is taken at the plenum a lot of that concern is nullified and becomes more or less a question of post TB VE.

posted by  Wally

This kid couldnt understand what people were talking about when they said:
"hey, im running 8 pounds on my integra"

Hes going to came back and check the post, read all this stuff you guys are talking about, and his head is going to explode...But then again I think this thread kinda veered away from the original question, so he probably wont care.

posted by  Zalight

He asked the price of an apple and came away with a cornucopia :wink2: I would say that was money well spent.

posted by  Wally

Yes its the differential pressure. When the throttle plates are closed the gauge says inches of mercury so the higher the inches, the lower the pressure. When you're not under boost, it's just a regular N/A motor, producing vacuum to suck in air and make power. When you give it some throttle, it's at 0, that means there is no longer just engine vacuum, it's at a steady flow of air (meaning the turbo is producing just as much air as the engine is sucking, then there is booost where the turbo is actually forcing high speed air into the combustion chambers like vwhobo said.
But what I'm confused about is if the psig measures the relative psi in the manifold, than doesnt the air get more compressed as it goes into the combustion chambers because of its velocitty? than the gauge is not that accurate and so does the wastegate. cuz the wastegate uses pressure in the manifold so it wouldnt be accurate right?

posted by  CarEXPERT

Your Message