Does x psi = x psi regardless?

[Enigma_Man]

Redsand187, I apologize, I read your message backwards. You're right, and I agree with you Sorry man.

Mr Dre.... Eh, I dunno if you're confused, or if I'm using the wrong terminology, but if you're MAKING 600 CFM @ 15 psi, then yes, that's much better than 300 CFM @ 15 psi, but the engine is what determines the CFM.... Just because the turbo can possibly supply 600 CFM @ 15 psi doesn't mean it automatically will when you bolt that turbo onto your car. You can't force more flow into a closed volume, like the analogy with blowing into a straw with your finger in the end.

Mr. Dre, the pressure and density are actually different. If you have a bottle full of very very hot air at 15 psi, there's a lot less molecules than a bottle of cold ait at 15 psi, and therefore the hot air is less dense.


Heh, I didn't mean this to turn into a big argument-fest, but I guess clearing up the info is good.

15 psi = 15 psi, as long as the temperature is the same. That's all I was really trying to say, so that people don't go slapping on t3/4 turbos onto their stock cars, and expecting a difference.

I suggest to the moderators here that this either be locked now, cause we're all basically arguing the same point or that it be moved out of extreme tuner forum, cause basic laws of science are far from extreme, heh.

[Mod edit: The thread will stay in Extreme Tuners and stay open for now. This question has been asked and butchered in the general forums too many times for it not to be resolved. So far its a discussion. If it degenerates to name calling, it will be closed.

-Tev ]

-Jes

 

[GalantVR41062]

but the concept of how the gasses flow and getting people to understand what is going on in the engine, and how to read a compressor map might be "extreme" for some people.

 

[pneumo]

I hope I'm not beating this to death, but I think I found another way to show what Enigma Man is saying; changing from a small turbo to a big turbo, all else being equal, doesn't get more power, especially at 13 psi.

Our cars have a 2 liter engine. That means for every two rotations of the crank it inhales two liters of air. Two rotations are needed because its a four stroke engine. This assumes 100% volumetric efficiency. More on that later.

So, how much CFM will our engine displace at 7,000 rpm? Converting to cubic inches of displacement makes it easier to convert to Cubic Feet, so use 122 ci multiplied by 7,000 rpm divided by 3456 = 247 CFM at atmospheric pressure. The # 3456 is derived from converting cubic inches to cubic feet, 12 cubed, multiplied by two because it takes two crank turns for full CFM displacement.

Now how much CFM is needed at a given boost pressure? That's where the compressor maps come in handy. scroll back up to post # 14 for more info on compressor maps. Since standard atmospheric pressure is 14.7 psi, plus 14.7 psi of boost means the engine is getting twice as much air as atmospheric pressure. That's a pressure ratio of 2. That means at 15 psi (rounded off) our engines are proccesing as much air and fuel as a four liter engine. CFM doubles to 494 at 7,000 rpm.

Now before all the T25 owners start saying "crap! My turbo only flows 360 CFM! I'm screwed!" I still have to factor in Volumetric Efficiency. VE means that an engine won't displace all of the air it can displace due to flow limitations in the intake, exhaust, valve sizes, cam durations, etc. Typical engines operate around 80% VE. So 494 CFM X 0.80 = 395 CFM at 15 psi at 7,000 rpm. OK now the T25 owners can say," I'm screwed!" Airflow needs drop off at lower rpm. Do the math for 6,000 rpm and you get 338 cfm. 5,000 = 282. Doesn't boost tend to drop off at high rpm with a T25? That's why. Will a 600cfm turbo help at 5,000 rpm? Not yet! Spin that sucker up to 9k and see what happens. Improving VE will help an engine use more cfm, but that's another topic. Of course, turning up the boost will cram more cfm into the engine, but that's also off topic.

I made an assumtion regarding the VE of our engines. I could be off significantly, so don't take these #'s as gospel when choosing a turbo. It's best to plot the pressure and flow #'s on the compressor maps.
Does anyone out there have more accurate VE numbers for our engines? I'd also be interested in finding out how VE changes as boost pressure goes up. HTH. and if I'm the one confused just shoot me now!

 

[GalantVR41062]

http://www.auto-ware.com/combust_bytes/eng_sci.htm

http://eric.virginia.com/install_un...metric_efficiency/ve_computation_9.012000.htm

http://www.turbofast.com.au/tfcalc.html

a lot of good stuff on the net if you type in Turbo Volumetric Efficiency under a serch engine(Google) good reading.

 

[doug]

Originally posted by pneumo
Does anyone out there have more accurate VE numbers for our engines? I'd also be interested in finding out how VE changes as boost pressure goes up.

There were some similar conversations in a thread I started a few weeks ago. The original title was "Converting between CFM and lbs/min" but it evolved into a flow and turbo theory thread. Good reading, if you ask me (yeah, I know, no one asked me). I also did ask at the end of that thread if anybody knew what the VE numbers for our engine was, but haven't gotten a response yet.

 

[Enigma_Man]

pneumo, thank you for that good explanation

-Jesse

 

[GalantVR41062]

with turbo application with 4vavles per cylender etc it is possable to get more than 100% VE but I have been unable to find the VE for the dsm stock or modified I guess you just go bigger untill you dont get full boost on the juice then the trubo is to big(jk of course) But the big guys are running Supra(3 liter I-6) sized turbos and making mad power but I guessed we were talking about the smaller side(ie less than 60lbs/min.) if I were to guess on the VE for our motor I would think it would be about 70% at 7,500rpm and max around 100%(4500-6000rpm or so) thats my guess though, as it apears VE goes down as RPM goes up. Good stuff.

 

[HighPsi91]

True, VE on a stock motor decreases at higher rpm... but cams, intake manifolds, etc work wonders to help that. If you run the numbers on some of the high HP guys it seems like they would have to be over 100% VE. If you look at the torque curve on a dyno plot that is a very good indicator of VE. VE and torque are very closely related.

 

[GalantVR41062]

Makes sense but the VE will still drop off(less as you modify more I would think just like torqe drops), but the solid lifter dsms keep making more HP it does not drop off so I am thinking on buying them, dam ajustments and $500 price....... o well.

 

[greyforestgst]

ill make it easy for everyone.

15psi does = 15psi no matter what. what you want is more molecules of gasses in your engine. 300 degree intake air at 15psi has far less moles of gaseous mixture than 150 degree intake air at 15psi.

15psi of 300 degree gas causes as much knock as 15psi of 150 degree gas. take a wild guess at which makes more power.

 

[danthman3]

this got me thinking a lot (and it hurt)
so with the help of my brain plus the brains of all of my physics a nd chemistry teachers this is the concept we came up with:
consider a cylinder once all valves are closed
the pressure and temperature are the only variables (since the volume is set by the chamber)
lets assume the cylinder can equalize pressure with the set boost
15 psi at the same temp will be = to 15 psi on any turbo (assuming same temp) so at that point the minor temp difference probly won't have a big effect on the power (remember this equation is based on the kelvin temperature scale)
if you fill the cylinder with a fan or a gigantic monster, assuming its at the same temp its the same amount of o2
now the trick is maintaining 15 psi at all rpms (pneumo explained the cfm calculation quite well so i won't reiterate that)
the max cfm of a turbo determines when it will start to "fall off" in psi but it does not flow less air, there just is more required to reach x psi
as we all know the t 25 falls off at higher rpms
thats cuz it can't flow enough air to maintain x psi
at low rpms almost all turbos are created equal
at higher rpms efficiency, temperature, and cfm are the determining factors
sorri if i restated too much i did this for my own sanity too cuz this question bothered me for a bit so i had to lay it all out

btw: n is moles
and the bottle collapse to lower the volume

thanx

 

[DCJ98GST]

If you havent already, READ THIS THREAD!

http://www.dsmtuners.com/forums/showthread.php?s=&threadid=20184

This is the same one that doug mentioned above.

It basically explains all of this and then some!

There are two things that change when bolting on a bigger turbo and keeping the same 15 psi (and everything else the same).

1. A lot of the gain of bolting on a bigger turbo is the increased VE of the engine due to the less restrictive turbine housing and more efficient turbine and compressor wheels at the same psi. This lets the engine flow more at the same psi therefore more cfm and more power. This really has not been discussed in this thread and is a significant gain.

2. The charged air is heated less creating more air per volume as you know by now.

I think most of the gain is from #1 unless you get really out of the compressor wheel's efficiency range.

Darryl

 

[danthman3]

my point is that you cant flow more air into the same engine at the same rpm at the same boost level
neglecting the temp diff
at the same psi 2 turbo's will flow the same amount of air in the same engine at the same rpm
gas laws show theat you can't put more air in an engin without raising the pressure or lowering the temp (since u can't increase the volume)
correct me if im wrong

 

[BatmanGSX]

I think what the guy was originally asking is "does x psi of boost = x psi of boost regardless. The answer is "no", and I'm going to try and make this as simple as possible. Our boost gauges and boost levels as we know them are relative pressure. What this means it is a level of pressure over the ambient pressure, ambient pressures change. HOWEVER boost controllers work on abselute pressure which is the reason if you went from 14.7psi in CA to 12.8psi in CO your boost gauge would actually read HIGHER. With the boost controller set to open at 15psi, on top of an ambient pressure of 14.7psi, your gauge would read 15psi and you would have an abselute pressure of 29.7psi. Go to Colorado with an ambient pressure of 12.8 and your boost controler still isn't going to open until you hit 29.7psi of abselute pressure, so now your gauge is going to read 16.9psi.

I don't know if that's what the original poster was asking. If he was trying to get all scientific and stuff then yes, x psi does always = x psi. However, as others have said, in our application pressure isn't all that matters.

And yeah, you can have more volume FLOWING through the same space and have the same psi. Duh. Most people are under this crazy assumption that the turbo is "stacking" air into the intake system, causing it to compress. WRONG. The air is compressed in the compressor housing and then FLOWS in a compressed state to the combustion chamber, losing a little pressure as it goes. The reason we can fit more volume into the system and have the same pressure is that the demands of the engine should be going up as more volume goes in. If it's not then something is wrong and your compressor wheel might surge. Think about it, you're pumping 30lb/hr into the engine at 20psi and making 300hp, if you are pumping in 60lb/hr into the engine at 20psi and making 300hp then something is REALLY wrong. You should be making around 600hp. Double the air at the same pressure fits in the same space because, for lack of better termonology, it's being "sucked on harder".

You have to think about it all dynamically, remember, there is an engine on the other side of that intake system sucking it's heart out.

 

[BatmanGSX]

Originally posted by danthman3
my point is that you cant flow more air into the same engine at the same rpm at the same boost level
neglecting the temp diff
at the same psi 2 turbo's will flow the same amount of air in the same engine at the same rpm
gas laws show theat you can't put more air in an engin without raising the pressure or lowering the temp (since u can't increase the volume)
correct me if im wrong

I'm correcting you because you are wrong. Think about what you are saying, you are saying that at 20psi both a 20G and 16G will make the same ammount of power at 8000rpms.

<h1>WRONG</h1>

 

[Tevenor]

Originally posted by danthman3
my point is that you cant flow more air into the same engine at the same rpm at the same boost level
neglecting the temp diff

This is absolutely wrong. How can you neglect temp differences? Why in the hell do we use an intercooler then? For looks?

It's all about mass of air. To do that you compress a known volume and keep the air as dense as possible. To do that, you keep the temperature as low as possible. It is a huge key in the puzzle.

Originally posted by danthman3
at the same psi 2 turbo's will flow the same amount of air in the same engine at the same rpm
gas laws show theat you can't put more air in an engin without raising the pressure or lowering the temp (since u can't increase the volume)
correct me if im wrong

Your first statement is wrong. Your second is right. You can't discard air temps as part of the equation. You can't just drop the speed of light from the equation E=MC^2. Same as you can drop the temp from PV=nRT.

 

[RS/377]

I would like to add one thing here.

Yes, the turbo is a compresor, and yes in its fins the air is compressor.

However, the air is stacking up in the engine.

It maybe compressed in the fins of the compressor wheel to x psi, but when it leaves the housing, the pressure changes depending on the requirements of the engine and the pressure (or vacuum) in the intake system.

Best example I can think of here is comparing my truck to my eagle. the truck has a 300hp 396 in it, if I were to put a 14b on it, the thing would never make any signifigant boost. However, like you said, and I agree, the air is getting compressed in the compressor housing, however, once it leaves the blades of the compressor, the engine is going to accept all of it, without it ever "stacking up" to register boost.

Now take the DSM, with the same turbo, and the mini-me motor. The turbo flows more air than the engine can ingest, so the air begins to stack up.

I know these examples show extremes, but, I find things easier to understand when put in extremes.

Another example, you have two air compressors with the same compressors, but one has a tank twice that of the other. For each revolution, the larger one is going to have half the pressure, regardless of the fact that the compresors are exactly the same.

 

[BatmanGSX]

Originally posted by RS/377
Best example I can think of here is comparing my truck to my eagle. the truck has a 300hp 396 in it, if I were to put a 14b on it, the thing would never make any signifigant boost. However, like you said, and I agree, the air is getting compressed in the compressor housing, however, once it leaves the blades of the compressor, the engine is going to accept all of it, without it ever "stacking up" to register boost.

<h1>WRONG</h1>

It will pressurize your 396.

 

[Enigma_Man]

Okay, I'm going to summarize everything:

In a cylinder, once all the valves are closed, the only variables are temperature and pressure, so as long as you can maintain the same temperature, x psi = x psi. (please bear with me)

More flow does equal more power, yes. But you cannot flow more air through an engine than the engine can physically take. The engine is an air pump, and the only way to make more air molecules go through it without changing the physical characteristics of the engine, is to increase the pressure and/or decrease the temperature of the air flowing into it.

In order to get more flow through an engine at the same PSI and temperature, things need to be made more efficient, or big, with cams, porting, stroking, etc. A bigger turbo cannot just *make* more flow through the engine.

What will happen if an engine can outflow a turbo, is the PSI will decrease, because the engine is eating more than the turbo is producing (as seen on T25s). This is the only case where the flow of the turbo matters, when the turbo cannot keep up with the engine, and maintain the specified PSI.

A bigger turbo _will_ give you more gains, but not _necessarily_ because it can flow more. On a stock 4g63, the only benefit to a large turbo will be decreased air temperature, due to the better efficiency of the larger compressor housing. So, a bigger turbo will give you gains, but I originally started all of this to deter people with stock engines from expecting large gains with a bigger turbo.

So yes, bigger turbos are better. But the gains you are going to see will be dramatically less on a stock engine, and you will also have to deal with more lag on a stock engine.

I would like to thank everybody who thought about this before just blabbing out something, and tried to be constructive.

-Jesse

 

[greyforestgst]

Originally posted by BatmanGSX
I think what the guy was originally asking is "does x psi of boost = x psi of boost regardless. The answer is "no", and I'm going to try and make this as simple as possible. Our boost gauges and boost levels as we know them are relative pressure. What this means it is a level of pressure over the ambient pressure, ambient pressures change. HOWEVER boost controllers work on abselute pressure which is the reason if you went from 14.7psi in CA to 12.8psi in CO your boost gauge would actually read HIGHER. With the boost controller set to open at 15psi, on top of an ambient pressure of 14.7psi, your gauge would read 15psi and you would have an abselute pressure of 29.7psi. Go to Colorado with an ambient pressure of 12.8 and your boost controler still isn't going to open until you hit 29.7psi of abselute pressure, so now your gauge is going to read 16.9psi.

reviving a dead topic, but i came across this and had to correct it.

if you are in CA or CO and your mbc is set to 15psi, you will hit 15psi. it will take a shorter time in CA. in CA, your boost gauge will read right at 0psi with the engine shut off. in CO, your boost gauge will read slightly vacuum. your mbc will take the same amount of psi to push the spring open and then the wg solenoid no matter where you are. in CO, though, you have to spool your turbo up a little faster to do so, since it has to pressurize less dense molecules.

 

[RS/377]

You do realize that to move enough air to bring my 396 up to 12psi, you need something in the neighborhood of a T61 right? Also, this is with calling 5500 the redline, even that low the 5500 rpm point is haging past the max airflow on the compressor map.

So, with that being said, you still think a 14b could produce some sort of boost on a 396 that would be worth while?

 

[BatmanGSX]

Originally posted by greyforestgst
in CO, your boost gauge will read slightly vacuum.

No it won't. They read 0 same as everywhere else.

 

[BatmanGSX]

Originally posted by RS/377
You do realize that to move enough air to bring my 396 up to 12psi, you need something in the neighborhood of a T61 right? Also, this is with calling 5500 the redline, even that low the 5500 rpm point is haging past the max airflow on the compressor map.

So, with that being said, you still think a 14b could produce some sort of boost on a 396 that would be worth while?

You do realize that a 396 making 300hp flows about the same ammount of air through it as a 300hp 4G63 right? 14B is a bad example because it can't support 300hp but a 16G would work just fine. Engine size has nothing to do with anything, it's all about the power it makes. A 16G can pressurize a 300hp 396. It probably can't pressurize one that makes 500hp though, well it would drop off. Don't assume that the airflow of your engine is related to it's size, you have everything coming in through 1 measly little valve that is going to pose a major restriction and prevent the engine from sucking in what it is physically capable off.

 

[danthman3]

Originally posted by Tevenor


This is absolutely wrong. How can you neglect temp differences? Why in the hell do we use an intercooler then? For looks?

It's all about mass of air. To do that you compress a known volume and keep the air as dense as possible. To do that, you keep the temperature as low as possible. It is a huge key in the puzzle.



Your first statement is wrong. Your second is right. You can't discard air temps as part of the equation. You can't just drop the speed of light from the equation E=MC^2. Same as you can drop the temp from PV=nRT.

actually i can neglect the temp change of two different compressors
as i went on to explain the only diff between one turbo at 15psi and another turbo at 15 psi would be the efficiency (how much the air is heated)
in the PV=nRT equation you can "drop" any variable that is a constant for the purpose of a ratio (ie PV/nRT=PV/nRT with P and V constant becomes 1/nT=1/nT or nT=nT) meaning as i said two compressors are equal at x psi except for the amount they heat the incoming air
all of this is assuming that the given turbo can maintain the given pressure until redline
obviously temp plays a role in the n (moles) of oxygen ingested by the engine, but in comparing you can use ratios
to use ur weak metaphor E=MC^2, E is proportional to M E/M=E/M so if your M is the same so is your E
if you wanna challenge my car knowledge go ahead
but don't even try to argue with my physics or chemistry (and that of 3 of my teachers)

basically what im saying is enigma man is right
u want more o2 in the same engine? either put more in (raise boost, lower temp, use an oxidizer) or make the effective volume bigger (cams, stroker, etc)

 

[Tevenor]

Originally posted by danthman3

actually i can neglect the temp change of two different compressors
as i went on to explain the only diff between one turbo at 15psi and another turbo at 15 psi would be the efficiency (how much the air is heated)

So how are 'how much the air is heated' and the 'temperature changes' different? This is exactly what I said. You cannot ignore the temperature changes in the air flow when figuring the mass of air per unit of time. I am not sure why you can't figure this out.

Originally posted by danthman3
in the PV=nRT equation you can "drop" any variable that is a constant for the purpose of a ratio (ie PV/nRT=PV/nRT with P and V constant becomes 1/nT=1/nT or nT=nT) meaning as i said two compressors are equal at x psi except for the amount they heat the incoming air

Since you want to make this into a 'look who's smarter discussion' please inform the class how exactly you get PV/nRT = PV/nRT from the base equation of PV=nRT. It's basic algebra so you and your 3 chemistry/physics teachers should have no problem showing your work.

Originally posted by danthman3
all of this is assuming that the given turbo can maintain the given pressure until redline
obviously temp plays a role in the n (moles) of oxygen ingested by the engine, but in comparing you can use ratios
to use ur weak metaphor E=MC^2, E is proportional to M E/M=E/M so if your M is the same so is your E
if you wanna challenge my car knowledge go ahead
but don't even try to argue with my physics or chemistry (and that of 3 of my teachers)

What are you arguing? You seem to be confirming my point for me. (oh and tell your high school teachers that Mech Engineer w/ a concentration in thermodynamics thanks them).