turbo pressure vs. volume

[AMink]

I'm a little confused about turbos moving more air at a lesser pressure. If the pressure gauge pickup is in the manifold, a given volume of air would be a given pressure, correct? Same volume of air going through the same size hole. I am wondering how a larger turbo can move more air (lower egt's, burn more fuel efficiently) at a lower pressure. Or am I way out in left field on this? My main reason for asking is I am hoping (still!) to get a new turbo and want to understand what I am doing. TIA

 

[mberry]

The quick answer is: pressure does not equal volume and vice-versa. More pressure does not mean "good" air. For a given turbo, once it goes beyond it's performance map, all it does is make hotter air at a higher pressure which is not what we want. That sometimes does not mean more air as there may be a restriction in the system somewhere.



Think of it this way, turn your garden hose on less than full flow. Put your finger over the end, make a small hole for the water to get out and what happens? Pressure goes up, volume goes down. Change the size of the obstruction to a larger hole, pressure goes down but volume goes up. It works the same with air. The bigger turbo usually has a larger outlet (more volume at a given pressure, or same volume at a lower pressure), or a more effeciently designed housing.



Hope that makes sense. I also hope I explaied that correctly.



Bottom line, get the bigger turbo. I am too someday.

 

[AMink]

I understand the pressure map and how that works, but I guess what I don't get is, that once the air is out of turbo and through the intercooler and into the manifold, the manifold is the "finger on the hose". With the boost gauge, we are measuring manifold pressure, I think. If a larger turbo moves more volume, less pressure, when it gets to the end of the line, doesn't the pressure have to go up to move the larger volume of air? OH, MY BRAIN!

 

[cojhl2]

Hope this is a good place to hijack this thread for a question regarding pressure in the system.



When we install a turbo boost pickup (I plan to in the near future) does it not make a difference where the pickup is in terms of the air stream? If the probe is at the outer edge of the stream is not the pressure less than if it were a pitot tube for instance. (Dynamic vs Static vs Stall).



So if I put the sender in the plenum where air flow is fast vs where it is essentially stationary, isn't the recording of turbo boost gonnna be different?



This quandry has bothered me for ages but I have not had the guts to ask!!

 

[NIsaacs]

When you are talking pressure v/s volume on turbo's, they mean the exhaust flow, not intake flow. The waste gate will help overcome this issue and bleed off some of the restriction of the exhaust flow.





"NICK"

 

[PC12Driver]

Air density is based on pressure and temperature. Pressure alone is only part of the equation.



Hot air molecules take up more space than cold molecules. If the pressures are equal, a cubic foot of hot air will have less oxygen molecules in it than a cubic foot of cold air, simply because you can't fit as many of the larger (hotter) molecules into the same space.



So if you have 30 psi in your manifold, a hot 30 psi charge (small turbo) would have fewer O2 molecules than a cool 30 psi charge (bigger turbo). If you add more fuel molecules, you need more O2 molecules to balance things out. Pressure, in itself, isn't what keeps our egts down. It's the amount of oxygen molecules.



As a small turbo overspeeds, the friction between the air and the blades, the quick ramp up in pressure, and probably even the blade tips approaching sonic speeds will superheat the air charge. The larger turbos can keep the speed down to move the same mass of air, which in turn will keep the discharge temps down.



Using the water hose analogy is a little confusing since liquid is incompressible and doesn't really follow the same rules as a gas.

 

[MCummings]

How about this,,



Lets assume this. . You can only fit 1" of air into the cylinder at a given RPM/Pressure...



Here is air at 100*F.



... ... ... . (ten "dots" of air in one inch)



Now, here is air at 300*F



@@@@ (four "dots" of air in one inch. )



The more "dots" you can get into you engine, the leaner it will run.



To get the same amount of air at 300* we have to raise the pressure tremendously, but with turbo chargers, we have a problem with the fact that, as we raise pressure, we also raise temperature.



That is why An HX-35 at 35PSI will perform very similar even though boosted to 50PSI.





Did I make any sense? I tried to do a volume vs. temperature illustration for dummies,, or maybe I just confused myself. LOL



Merrick

 

[AMink]

BINGO! Now I get it. Thank's for the replies. ... @@@... :-laf

 

[DMcPherson]

hmmm, I diddnt see alot about volume aposed to pressure just temp.

even though they are related. with dealing with turbos for a few years now I have come to realize a few things. like said volume does not equal pressure.

so if a turbo can only supply 10cfm and is required to make 20 psi of boost to make power, a turbo that can flow 20 cfm may only need 10 psi to make the same.

so basically a turbo that will move more cfm will fill the cylinder more requiring less pressure to do so.

eficiency is also important like said, because you can only fit so much air at a certain temperature into one place. and at the same time the less boost pressure means less drive pressure (exhaust pressure) and reduced air temps as well as exhaust temps. also better cylinder scavengine, but thats another story.

anyeay thats my . 02 cents

 

[cojhl2]

<<10cfm and is required to make 20 psi of boost to make power, a turbo that can flow 20 cfm may only need 10 psi to make the same. >>



I dont get this,, The pressure the turbo produces is a reaction to the downstream restriction. If it takes 20psig to load the cyl how can 10psig do the same.



This of course ignores temp differences. .

 

[chrleb1]

<<10cfm and is required to make 20 psi of boost to make power, a turbo that can flow 20 cfm may only need 10 psi to make the same. >>



I dont get this,, The pressure the turbo produces is a reaction to the downstream restriction. If it takes 20psig to load the cyl how can 10psig do the same.



This of course ignores temp differences. .

You can't ingore temp diferences. That is the whole principle of getting the same volume of air moved at a lower pressure. Air density is the key. Intercooling is a major player in this. If you have a turbo that will "fit" a 11 liter motor and you throw it on a 5. 9, what is going to happen. You won't be able to get it lit without tons of fuel. Now if you have a very efficient intercooler, that turbo all of sudden thinks it is back on the 11 liter motor. As the air cools, more air has to fill that void. All of this action drops pressure while supplying more air. It only took me a semester of Thermodynamics to get all of that..... :-laf

 

[Forrest Nearing]

and just FYI, the cylinder head and camshaft profile are the nozzle, not the manifold...

 

[cojhl2]

EGaads,, my brain is getting full. . Thank you so much for the explanation.



I guess (although looking back it was mentioned) that the turbine itself by inefficient action would raise the temp. I thought it was only a PV/T thing.



From my standpoint guys I do really apprecaite the time it takes to explain these things.



My first job out of college I worked with this elderly self taught engineer. My dad asked him once if I was learning anthing, to which he answered " Well I don't know if he is learing anything but he is getting a heck of a lot of information".



Things haven't changed!!!

 

[Alan Reagan]

The volume is a constant. In other words, a 5. 9 liter engine will take in 5. 9 liters of air every two revolutions. Whether it is compressed or not, it is still 5. 9 liters in volume.



Once you put in the temperature and pressure, then you are talking Pounds of Air, which is the true measure you are looking for when dealing with the turbo calculations.



To get pounds of air:

n(lbs/min)= P(psia) x V(cu. ft. /min) x 29/(10. 73 x T(deg R))







Also, air is heated during when compressed because of the friction between molecules as they are forced together during compression. Larger turbos typically deliver cooler air because they are not spinning as fast or working as hard to compress the air. The larger turbos just don't spin up as fast so the downside is their lag time.