Is 15psi the same as 15psi?

[only1db]

a guy at work (one of my bosses) was arguing that 15psi with ANY turbo will flow the same amount of air...i told him he was wrong...

isnt it based on CFM's?? more CFM's at the same psi will yeild more air and more power...i will be referencing maxium boost...when i get a minute to myself...

btw...what are the CFM measurements for the 13g and 15g?? :confused:

thanks in advance! :D

[hyunelan2]

I am not a turbo expert, but from common sense you could understand that 15psi on a bicycle tire is a lot "less air" than 15psi in a monster truck tire. I would expect the same would be applied in a turbo situation, since the "more" air you have the more fuel you can add to it?

[mtlelantra]

^^ exactly

[SWortham]

I know you're right. It has more to do with CFM than psi. I never did understand this very well. But you piqued my interest. I always wanted to know how to read these compressor maps.

I just came across this explanation...
I felt there needed to be a tech article on this subject to help people understand turbo compressor maps and figure out what type of turbo they will need for there perfomance needs. At first glance a compressor map is quite confusing, but they offer alot of information and hopefully after reading this you should be able to get a little bit better understanding of them.
Here we have map for a T04E-50 garret turbo, this is just for learning sake and not the turbo I am recommending.


The numbers you see on the left, listed vertically from 1.00 to 3.60 is the pressure ratio. Let's say you are boosting at 7psi. This is roughly half of the sea level atmospheric pressure (14.7psi), so your pressure ratio is 1.5. if you're boosting 15psi(rounded up for simplicitys sake), your pressure ratio would be 2. The formula for figuring pressure ratio is: pressure ratio=(14.7+boost pressure)/14.7

The numbers you see on the bottom that start at 0 and end at 50 is the flow rate of the turbo measured in lbs/min(some are in cfm and some are in cms). This is the amount of airflow that you are wanting to push through your turbo.
Now that that is done look at the map again, the next thing to look at are the lines that make it look like a thumb print. Those line are effciency lines. The outer one being 65% effciency, and the inner one being 78% effcient(the effciency % differ from map to map, but should be marked on the lines). The area inside the 78% ring is the most effcient area or "island" of the turbos range. Getting your turbo to spend most of it's time in there will give you the most power at the lowest charge temperture.

Let's say you were boosting at a pressure ratio of 2.20 and were moving 30lbs/min worth of air. Follow those two lines together and you will find yourself almost right in the middle of the most effcient island. This is the ideal area to run your turbo. But let's say you want to boost at a pressure ratio of 2.6 and were moving the same 30 lbs/min worth of air, that would put you right between 78% and 76% effciency which is still good, but once you drop below 70% you might want to reconsider your turbo selection. You set your boost at 2.0 pressure ratio, but want to move 45lbs/min of air, this puts your point outside the turbos effciency range, this means that your turbo is to small, you could boost more to get the effciency up but notice how much more you would have to boost to increase the effciency.

This brings us to another bit of infomation; turbine speed. The horizontal lines that slope down towards the right of the map. That is the turbine speed measured in x1000rpm. Notice how at higher pressure ratio the faster the turbine spins and the lower the effciency and how suddenly the effciency drops compared to lower pressure ratios, going outside the top line on the turbine speed is were you decide to go with a bigger turbo(to much speed and you will just be making the turbo work harder and not as effcient). Now to the far left is the surge line, this is the point were the turbo starts to make boost at a given pressure ratio and turbine speed, you do not want a point on the left side of this line doing so will cause your turbo to surge at WOT, and when I mean surge I mean the turbo starts to spool but doesn't have enough turbine speed to build more boost and the car will start bucking madly and your boost gauge will start bouncing, this is murder on a turbo. It is imperative to size the turbo correctly so that this does not happen. In closing you have to figure out how much power you will be making at what boost levels and plot those points on the chosen compressor map to see where they fall in the effciency range of the turbo selected. I hope this is of some help, PM me with any questions that I haven't answered here or that you might have. Happy boosting.
http://www.dsmtuners.com/forums/showthread.php?t=128139

[jalmir]

You're right, 15 PSI on a 16g will give more air then 15PSI on a 13g ...

Depending on the 16g (there are a few out there) it'll be around 500 CFM and the 13g is aroung 360 CFM

[KeWLKaT]

Yes its all about CFM.

Here's a simple explanation:

PSI = pounds * square inch.

Bigger turbo has more area (square inches). That means that to have the same PSI, you need less pounds for a bigger area (square inch).

Kind of like:

Small turbo ---> 12 psi = 6 lbs * 2 sq. inch
Big turbo ---> 12 psi = 4 lbs * 3 sq. inch

And CFM stand for cubic feet per minute. So, obviously, a bigger turbo will move much more cubic feet per minute at a given RPM than a smaller turbo.

[only1db]

thats what i thought...but these damn engineers will want some math... i guess its time to go look up the equation in maxium boost...besides i would like to see how effecient the 13g really is at upper boost levels.

another argument he was attempting to make was that our engines would not hold the sheer volume of air that a larger turbo would put out...this too i thought was incorrect...as long as the compression levels are down you could squeeze just about anything down in there...or is my logic flawed? :confused:

keyan posted up the compressor maps...but the 13g is not there anymore...does anybody have it??

link to compressor maps (http://www.elantraxd.com/forums/showthread.php?t=16189&highlight=compressor+maps+13g)

[KeWLKaT]

thats what i thought...but these damn engineers will want some math...

I just gave you math in my previous post. And this also comes from me, a major in mechanical engineering :abovelol: ;)

Also SWortham posted the 13g map in this very thread!

And you can squeeze ANY pressure in ANY volume, given that the materials (rods, block) can take the said pressure. Also that's when compression ration comes into play, but that's really to avoid detonation (pinging). THEORETICALLY it would be possible to squeeze in let's say, a tuned 20 psi of boost in 10:1 stock CR if you had 156 Octane Gas (if that even exist / fictitious numbers), if you had forged rods and pistons.

[only1db]

^ wow i didnt even see that thing...sheesh...i'm going blind.

[SWortham]

By the way, here's a huge list of compressor maps (includes both Garrett & Mitsubishi turbos):
http://not2fast.wryday.com/turbo/maps/all.html

[only1db]

thanks

[jalmir]

And I did some math about CFM and the Beta engine ... some big turbos ARE too big for our engine ... the engine falls in the surge part of the map ...

Here are the math I did :

Beta CID : 121
N/A cfm (@6500 rpm) = 229.456
N/A cfm (@6250 rpm) = 220.631
N/A cfm (@6000 rpm) = 211.81


Boost : 22psi (Presure ratio = 2.496)
N(lb/min) (VE@80%) = 29.673 / 429.429 CFM / 0.203 m3/s


Boost : 18psi (Presure ratio = 2.224)
N(lb/min) (VE@80%) = 27.497 / 397.931 CFM / 0.188 m3/s


Boost : 15psi (Presure ratio = 2.016)
N(lb/min) (VE@85%) = 23.054 / 333.633 CFM / 0.157 m3/s


boost : 12psi (Presure ratio = 1.81)
N(lb/min) (VE@85%) = 20.725 / 299.93 CFM / 0.141 m3/s


Don't know if the numbers are ok or not, from various source they look good, but I'm no pro in turbo ...

[Mahonroy]

See I used to think that CFM's were directly proportional to psi, but I was wrong lol. Just look at Tim's 96 turbo wagon, at 15psi he got like 350whp using some rediculiously large turbo, and at 22psi he got like 430whp or something. I meen u compare that to like my TD04 15T, at 15psi I was probably around like 240whp at most, compared to the 350whp.

One thing that was wierd though that I didn't really understand, was that I was argueing (possative conversation) with John at alpine about what we are talking about here. He said that its all based off the pressure ratio, and that a certain psi will yield a certain power level, and thats all. He didn't believe me that Tim at RDtiburon dynoed around 350whp at 15psi and he told me that thats impossible lol.

Also its kind of wierd also, because in the book Maximum Boost, sure they refer to CFM's, and compressor maps, but reguarding all of there equations, it seems that they are refering to a certain boost pressure equals a certain power level as well. And noone even refers to the A/R ration (on the exhaust side), which makes a huge difference between flows....

[OH_Beta_Accent]

15psi is 15psi no matter what turbo you are running. The confusion is that 15psi will be the same amount of hp gain regardless and that's not true.

It's real simple. The whole reason there is any pressure in the intake is that you are cramming a percentage more air than the motor can naturally take in. That's the pressure ratio. So say our beta's take in 240 cfm at torque peak and we feed it 6 psi, then that turbo is trying to push ~340cfm into the motor.

The only reason the same amount of boost on the same motor with a different turbo will make more power is because the turbo is in a better part of it's compressor map and therefore working at a better compressor efficiency. So it's able to flow and compress the same amount of air without increasing it's temperature as much.

[BlueRush]

Think of it this way, PSI is the pressure built up in a given volume of air, while CFM is the volume of air (at whatever pressure) that a unit (Turbo) can move.

[ricerrx7]

As far as I've always thought (although that doesn't mean that it's true), all other things being equal (same exact intake piping, intake manifold, throttle body etc.), and the only thing that is changed is the turbo, is that 15psi from one turbo is the same as 15 psi from another turbo. Here's my thought process on this;
It's going to take the same amount of air to presurize a given space to a given pressure. (not counting temperature differences). Take a bicycle tire. To inflate a bike tire to 20 psi, it's always going to take the same volume of air. Whether you're using a hand pump, or a 120v air pump, you're going to end up with the same volume of air in the tire to get the desired 20 psi.
Now, no matter what turbo you are using, it takes a certain volume of air in your intake tract to achieve 15 psi. Try to "flow" more air into that space, and it will open the wastegate.
Now, with a bigger turbo that allows you to therefor run larger intake runners, throttle body, intake tubing etc, you will get more power from that same psi than the smaller turbo with the smaller runners etc.
Thoughts?

[Cypher]

ok i'm going to take an attempt at this. i understand everything i just usually have trouble explaining things.

if i understand what you're saying then thats wrong. a 15t on ur motor at 15psi will make more power than the 13g at 15psi even if everything is the same. in ur example adam think of it like this. as the 13g spools and creates boost only so much air is allowed to get into the cylinder on the intake stroke. this is because the intake valve is only open so long and only so much of that compressed air can enter. now imagine the 15t. it flows more air then the 13g so on that same intake stroke MORE air is able to enter the cylinder just because it flows more.

i hope i explained it decently. i know what i'm trying to say but often i can't get it out right lol. also felix, i do believe that ur could do 20lbs on 10.1:1 compression ratio. run some 106 octane (which is available around here) and run a bit rich and i'm sure it can be done. you don't need insanely high octane. hell i'd run 12-13lbs on stock compression with just a lil backed off timing and a 11:1 AFR!!!

[ricerrx7]

^What you said was perfectly clear. Gives me some thinking to do...
Thanks.

[Cypher]

compressor maps are tricky when you don't deal with them all the time. i'm not to the point where i can just look at them and know. i still have to look up some forumlas hah.

[Rystibby04]

ish says pumpkins are rwd...

[babot]

Yes its all about CFM.

Here's a simple explanation:

PSI = pounds * square inch.

Small turbo ---> 12 psi = 6 lbs * 2 sq. inch
Big turbo ---> 12 psi = 4 lbs * 3 sq. inch



Isn't PSI = pound(pound-force) / square inch? PSI is for pressure and pressure is force divided by surface.

So..
small turbo --> 12psi with 2sq inch -> puts 24 lbf
large turbo --> 12psi with 3sq inch -> puts 36 lbf (to the "air surface")


Wikipedia - PSI (http://en.wikipedia.org/wiki/Pound-force_per_square_inch)

[jalmir]

^^^

Re-read what Felix said, his formula is self explanatory ... you mixed it up!

[KeWLKaT]

Isn't PSI = pound(pound-force) / square inch?
Wikipedia - PSI (http://en.wikipedia.org/wiki/Pound-force_per_square_inch)

It is that aswell. It's equivalent to what I said. You mixed yourself up in your explanations

[babot]

Uh.. I'm really confused....
Are you saying that lbf / inch^2 is equivalent with lbf * inch^2 ??
(you are using * for multiply.. right?)

PSI is pressure, so it is a value calculated by dividing force(lbf, N, kgf, ...) by surface (inch^2, m^2, ...). not by multiplying surface with force. You need to multiply surface and pressure to get force, which is totally different from pressure. When you push 10lbf on a 1sqin coin, it gets more pressure than a 10sqin mat pushed by 20lbf. (the coin gets 10PSI, the mat gets 2PSI) larger surface => more distributed => less pressure

So.. if the air is pushed by (or turbine blades are pushing) 6lbf on 2sqinch (turbine/air surface), the pressure there is 3PSI. (6lbf / 2sqinch) and 4/3 PSI for 4lbf on 3sqinch (4lbf / 3sqinch).

What I was going to tell with

small turbo --> 12psi with 2sq inch -> puts 24 lbf
large turbo --> 12psi with 3sq inch -> puts 36 lbf

was that more force(24<36) is required to push air if you have more surface(2<3) with same pressure, which requires larger turbo. And, larger surface is needed to get more air flow when the pressure is constant.



Therefore...

Bigger turbo has more area (square inches). That means that to have the same PSI, you need less pounds for a bigger area (square inch).

No, you need more pounds for a bigger area with same pressure. With constant pressure, bigger area means more force. Hydraulics works with such laws. You put smaller piston for smaller force (longer movement), larger piston for larger force (shorter movement) based on the law that the pressure is constant in the system.

Requiring more force for the same pressure for bigger turbo may seem to be a paradox that bigger turbo is 'bad'.. but no.. because it has larger surface, which means that it pushes MORE air. Amount of air flown is directly proportional to Surface * Pressure. However, the same engine with same condition would require the same amount of air. So, bigger than required (and.. when no more PSI is wanted) is no good. And bigger turbo will need more energy to rev because it would have more rotational inertia.

[KeWLKaT]

Ah! I was under the impression that PSI for some reason was lbs*sq inch. Yeah, MULTILPLIED :rolleyes: Sorry dude, I work with the international system all day everyday, not too keen on the imperial system that the only country to still use it is the USA :D

[babot]

i hate the imperial system.... :( Because i was grown up in Korea, i've always needed to convert all those numbers 'to feel it'.. aargghhh... my brain is still compatible with the international system only.... except MPH, which made me confused when i returned and drove cars in Korea last summer.. :D

[only1db]

ish says pumpkins are rwd...


and you say that a 400lb heavier tib at 7-8 psi (autotragic) will spank ish...


back on topic....

i'm going to be doing all the math when i get a chance...i hear what everybody is saying...but with a bigger turbo with same diameter piping...you would make more power...

i dont think anybody took into account of air velocity in the charge piping..if the pipe is filled with more air the air would be moving faster at the same psi and would be able to shove more into the cylinder...hence more power...charge temps would also be down with a larger turbo...and the air would be more dense...which also creates more power.

[Cypher]

and you say that a 400lb heavier tib at 7-8 psi (autotragic) will spank ish...

pwned!

i dont think anybody took into account of air velocity in the charge piping..if the pipe is filled with more air the air would be moving faster at the same psi and would be able to shove more into the cylinder...hence more power...charge temps would also be down with a larger turbo...and the air would be more dense...which also creates more power.

and i believe i said this.

[babot]

I don't know where the pressure value is measured here. (at the turbine? or at the throttle body/engine cylinder)

But, if it is measured at throttle body's pipe/engine cylinder, which is constant regardless of which turbo is used, the amount of air flow is same for same engine condition (rpm) and same pressure (PSI value). However, smaller turbo with 15PSI at the turbine blades will result in less PSI value than a bigger turbo with 15PSI at turbine blades. (This is the air velocity effect from varying surface..)

If you let air(compressible fluid) flow faster in the same pipe(at least the engine cylinders are same..), it makes higher pressure. So the PSI value wouldn't be constant there. If you could make 15PSI in cylinder with smaller turbo/lower air flow, you will get more than 15PSI there with larger turbo/faster air flow.

If try to send faster with same pressure, they will just become more pressurized leaving only two choices: send slower to match the pressure value.. or keep the pressure higher to keep the faster speed. Both speed/pressure measured at constant pipe: engine cylinder or throttle.

So, if the PSI value is measured right after the (compressor) turbine blade, with same PSI value, we get different air flow speed at the throttle or engine cylinders although the air flow speed will be same at turbo side. (bernoulli's principle and PV/T = constant)

Anyways, I think that it's just different 'descriptions' for the same thing.. :) (measure pressure at turbo or at engine...)


ah.. I forgot to take temperature into thinking.. but.. I'm not going to go further.. already tired of digging old general physics textbooks that I've never expected to open again.... :)

[only1db]

hahah keep digging....i want ot know more!

[KeWLKaT]

I don't know where the pressure value is measured here. (at the turbine? or at the throttle body/engine cylinder)

at the turbine

[babot]

at the turbine

Yeah.. then..

the amount of air is the PSI value (at the turbine) * surface (of the turbine) assuming that the temperature values are same.

so "15psi small turbo is not same with 15psi large turbo." still holds. :)

(psi value at throttle will be different for those two 15psi turbo)

[Cypher]

the actual boost a gauge can be different depending on the spot you locate it at. ie: right after the turbo, right before the intake manifold. etc

[KeWLKaT]

^^^ yeah but im referring to the boost source the wastegate would see which is what goes into the engine really

[Cypher]

i know. just throwing it out there for... well i don't really have a point but just wanted to throw it out!