• Hello Guest, welcome to the initial stages of our new platform!
    You can find some additional information about where we are in the process of migrating the board and setting up our new software here

    Thank you for being a part of our community!

PSI vs. CFM and fueling upgrade

NotSoFresh said:
You choose the hotside based on the exhaust flow of the engine at the desired operating parameters. If it is too small, a restriction happens, which effects the gas flow through the engine. If it is designed efficiently and matched well, it turns the shaft, which turns a compressor. This also has to be matched to the amount of air the engine uses. If this is too big or too small, adverse things happen here as well.
I also understand that the intake air turns into the exhaust. More intake=more exhaust. More exhaust=more faster turblow=moreintake=more exhaust.......They are related. In the end, a well designed system has both sides sized for the engines desired operating range, and matched to each others abilities.
It is a hell of a confusing process, as this thread illustrates.
I am trying to break the concept down into chunks that are simpler to understand.
If the bigger turbo places less exhaust restriction, it will flow more at 18psi than one that creates more restriction. This still works with what I said previously. A cylinder with a given intake will flow 200cfm at 18psi, with a given exhaust restriction. To increase cfm through the engine you can either increase intake pressure or decrease backpressure or both to increase flow. Its a pressure differential in which either side can be manipulated. They are independent, but related.
Psi is a direct measure of the backpressure before the intake. Put on a custom head and manifold, drop the intake backpressure and the same turbo will flow the same cfm at a lower psi. At the same psi, the cfm will go up, within the limits of the exhaust backpressure and turbines ability to transfer power to coldside compression.
It is an interrelated system no doubt but you can look it as separate pieces without losing sight of the whole.
Click to expand...

Wrong. That is what you are failing to see. They are not at all independent and can't be treated as such. You don't get higher boost pressures without restricting the exhaust. There is "work" going on. To say otherwise is to believe in perpetual motion, or some such analogy. The turbine is a restriction no matter what size or AR turbine you run.
 
If you hooked the hotside up to a *different* motor just to power it, and the engine got the 18 psi of air crammed into the intake, and a free flowing exhaust with little restriction, it would matter very little that the turbo pushing the air was tiny and working HARD or big and just happily in the middle of its efficiency map. Maybe a slight loss due to hotter compressed air from the smaller turbo. And the motor would be making more power than it would with either turbo on it.

OTOH, the poor other motor revving its guts out to spool the turbo and not getting any boost out of the deal would not be making much power at all. Far less power than it would be making without the turbo corking up the exhaust flow.

The main difference between big and little turbo really is in the pressure required in the exhaust. That very directly reduces power, almost to the degree that pressure in the intake manifold increases it.

You really can throttle a motor down to 0HP just by restricting the exhaust, in the same way you can throttle it down to 0 HP by restricting the intake (throttle valve).
 
NotSoFresh said:
Moles per minute. Pounds are weight and weight is dependant on gravity. Moles will give you number of molecules, regardless of gravitational pull.
Click to expand...


Pound of fuel, pounds of air are the typical engineering units used to describe power and efficiency in the English/US system. SI would be Kilograms. Nobody uses moles to evaluate pumps, which is what an engine is. It is also what a turbocharger is.

https://en.wikipedia.org/wiki/Pound_(mass)
 
vwbusman66 said:
So, in research, I have discovered the concept (and reality) that a bigger turbo producing the same PSI (let's say 1 bar/14.5 PSI for argument's sake) is actually flowing a larger volume of air.

Does this mean that if I'm running a b230ft with a 13c at 1 bar and I replace said 13c with a 15g at the same 1bar, I will need more fuel to maintain a stable/safe AFR?
Click to expand...

linuxman51 said:
yes.
Click to expand...

Serious question.
At the the same RPM, Same PSI, if the 15g car needs more fuel to have same AFR, there is more air mass in cylinder, agreed?
Only way to do that is lower temp air.

I'm trying to break down simply as I can for me, not pissing match like gseller says, but just learning.:)

(PV)/T=(PV)/T

share the wisdom, Please

edit:
not asking about power, just OP's Q re: fuel
 
Volume is not constant. otherwise we wouldn't need bigger turbos.

Seriously, this is 101 level turbo ****, if you don't understand it, don't opine with bull****
 
JohnMc said:
FWIW a pound is a perfectly fine measure of mass. It can be used either way. It just assumes 1G when used as a measurement of mass.




I think?
Click to expand...

Yes. It is a mass unit when standard temperature and pressure are the operating parameters. It is a funky English unit, which we here in the states have to suffer using. Just like inches, feet, yards, miles. Conversion factors required to make the connection between them. Please start teaching/adopting the SI system here in this country.
 
NotSoFresh said:
Moles per minute. Pounds are weight and weight is dependant on gravity. Moles will give you number of molecules, regardless of gravitational pull.
Click to expand...

The pound (in this context) is a unit of mass, which is not depending on a gravitational constant. 1 pound on earth is the same mass as one pound on the moon.

If you are going to be a smartass at least try to be correct. :)
 
apachechef said:
Serious question.
At the the same RPM, Same PSI, if the 15g car needs more fuel to have same AFR, there is more air mass in cylinder, agreed?
Only way to do that is lower temp air.

I'm trying to break down simply as I can for me, not pissing match like gseller says, but just learning.:)

(PV)/T=(PV)/T

share the wisdom, Please

edit:
not asking about power, just OP's Q re: fuel
Click to expand...

Looks to me like you "get it".
 
linuxman51 said:
Volume is not constant. otherwise we wouldn't need bigger turbos.

Seriously, this is 101 level turbo ****, if you don't understand it, don't opine with bull****
Click to expand...
hope that wasn't pointed at me, I'm just looking at Op's Q.
The instant the intake valve closes, the vol in the cylinder is equal in the two engines, right?
(dif mass)

at same V and P and AFR, T is only var left if more fuel is used, agreed?

F turbo101, if the above is wrong, charles, boyle, and some french guy need an ass whooping
 
Another thing to consider when thinking about exhaust backpressure is how much exhaust remains in the cylinder after the exhaust stroke. If you have a very free flowing exhaust there might not be any at all - all the exhaust leaves, plus perhaps even a little of the intake charge, pulled out by the existing exhaust on the overlap.

But with considerable pressure in the exhaust, and the piston having to force the exhaust out, more exhaust remains. Even at TDC there's still some volume left (40 - 50 - 60 ccs, depending). It makes a difference if that is still full of exhaust at whatever pressure the exhaust has in it, or it got much more completely evacuated because there was no backpressure, maybe even a little scavenging.

I really have no idea what exhaust PSI ranges you typically get in a WOT fully spooled up turbo engine, other than it varies depending on the turbo. But I do know that totally independent of the intake side, valve opening, and throttle setting and intake manifold pressure that exhaust back pressure can, in extreme cases, reduce the power output all the way down to 0. The intake manifold can have 1 bar of pressure in it, WOT, no restrictions in the intake, the cylinders get to breathe in as much of that 1 bar air when the intake valve opens, and the motor is still moving no air, and not producing any power.
 
Last edited:
apachechef said:
hope that wasn't pointed at me, I'm just looking at Op's Q.
The instant the intake valve closes, the vol in the cylinder is equal in the two engines, right?
(dif mass)

at same V and P and AFR, T is only var left if more fuel is used, agreed?

F turbo101, if the above is wrong, charles, boyle, and some french guy need an ass whooping
Click to expand...

I may have taken the previous post in a way it wasn't intended, for that I apologize.


However, If this is your view on things... and T is the *only* factor that changes (in your opinion), we can change T to almost whatever value we want via various methods (very large intercooler to sink down to almost ambient, a2w to sink to whatever T we want, etc).

So if that is the case, then why do we have various turbo sizes... thousands... tens of thousands.... when we could cover the entirety of need with a few dozen perhaps, and some fancy intercoolers?
 
2manyturbos said:
Looks to me like you "get it".
Click to expand...

no, I think instead you've missed it.

If volume is the sole dictator of performance, all 2.3L turbo engines would make the exact same power at the exact same boost levels.

But this is not the case, is it. Ford 2.3 turbo guys struggle to get "big numbers". 2.0 mitsu engines throw down big numbers.
1.6L hondas, big numbers.

Ergo, volume cannot be considered static.


consider also the parable of boost cut on lh 2.4 cars.

We *know* that it is based on mass air flow
We also know that you hit that value at a lower boost level with a 15g than you do with a 13c.
And lastly, we know that it is measured pre-compressor (so it cares not about the T in pervert).

In fact, lh doesn't see "boost" at all, it simply sees air mass. More mass, more fuel, more power so the story goes.
 
2manyturbos said:
Wrong. That is what you are failing to see. They are not at all independent and can't be treated as such. You don't get higher boost pressures without restricting the exhaust. There is "work" going on. To say otherwise is to believe in perpetual motion, or some such analogy. The turbine is a restriction no matter what size or AR turbine you run.
Click to expand...

apachechef said:
Serious question.
At the the same RPM, Same PSI, if the 15g car needs more fuel to have same AFR, there is more air mass in cylinder, agreed?
Only way to do that is lower temp air.

I'm trying to break down simply as I can for me, not pissing match like gseller says, but just learning.:)

(PV)/T=(PV)/T

share the wisdom, Please

edit:
not asking about power, just OP's Q re: fuel
Click to expand...

linuxman51 said:
Volume is not constant. otherwise we wouldn't need bigger turbos.

Seriously, this is 101 level turbo ****, if you don't understand it, don't opine with bull****
Click to expand...

apachechef said:
hope that wasn't pointed at me, I'm just looking at Op's Q.
The instant the intake valve closes, the vol in the cylinder is equal in the two engines, right?
(dif mass)

at same V and P and AFR, T is only var left if more fuel is used, agreed?

F turbo101, if the above is wrong, charles, boyle, and some french guy need an ass whooping
Click to expand...
Exactly where i was going with splitting it up.
Science. You can't experiment and have usable data changing more than one variable at a time. In order to look at it one variable at a time, you have to keep the others constant. What effect does changing turbo have on the system if nothing else changes? If I change compressors, but not turbine, what effect on the system will it have at a given psi and rpm? More total cfm through? More heat? What if I change cam overlap and not the turbo? What does this do to the power curve and the intake temps? What would the cam overlap do to the total cfm throughput of the engine?

That is the scientific method.

And the moles comment was a joke. I was being pedantic about a pedantic comment. pounds work fine in this discussion.
 
linuxman51 said:
I may have taken the previous post in a way it wasn't intended, for that I apologize.


However, If this is your view on things... and T is the *only* factor that changes (in your opinion), we can change T to almost whatever value we want via various methods (very large intercooler to sink down to almost ambient, a2w to sink to whatever T we want, etc).

So if that is the case, then why do we have various turbo sizes... thousands... tens of thousands.... when we could cover the entirety of need with a few dozen perhaps, and some fancy intercoolers?
Click to expand...

If this is your view on things... and T is the *only* factor that changes (in your opinion),
Click to expand...
Course not, bud. Just trying to understand the basics and build from there.
control as many var as possible to figure out what happening in bite size chunks.
In this case, OP q was only thing asked, so I asked "how does the cyl know what turbo is upstream if it is taking equal vol bites at equal PSI?"
seems like T is answer in the most reduced mind experiment I can handle.

I don't think turbos are labeled delta T @15psi, nor do you.
As to your Q re: fancy intercoolers,:) funny, but zip up, its not a pissing match:-P

The posts saying, "same psi, but more flow" strike me as overall correct, but problematic when narrowed down to a very static, very limited q.(what My brain can handle)

perhaps all here( not pointing fimgers) share their understanding more, and share less of what they heard or sayings or truisms.
If I'm not confident, I try to ask instead of preach.
Thanks Ken, good info is appreciated, I mean it.:)
 
apachechef said:
Course not, bud. Just trying to understand the basics and build from there.
control as many var as possible to figure out what happening in bite size chunks.
In this case, OP q was only thing asked, so I asked "how does the cyl know what turbo is upstream if it is taking equal vol bites at equal PSI?"
seems like T is answer in the most reduced mind experiment I can handle.

I don't think turbos are labeled delta T @15psi, nor do you.
As to your Q re: fancy intercoolers,:) funny, but zip up, its not a pissing match:-P

The posts saying, "same psi, but more flow" strike me as overall correct, but problematic when narrowed down to a very static, very limited q.(what My brain can handle)

perhaps all here( not pointing fimgers) share their understanding more, and share less of what they heard or sayings or truisms.
If I'm not confident, I try to ask instead of preach.
Thanks Ken, good info is appreciated, I mean it.:)
Click to expand...


there has been no unzipping or stretching for measurement over here :lol:
 
linuxman51 said:
no, I think instead you've missed it.

If volume is the sole dictator of performance, all 2.3L turbo engines would make the exact same power at the exact same boost levels.

But this is not the case, is it. Ford 2.3 turbo guys struggle to get "big numbers". 2.0 mitsu engines throw down big numbers.
1.6L hondas, big numbers.

Ergo, volume cannot be considered static.


consider also the parable of boost cut on lh 2.4 cars.

We *know* that it is based on mass air flow
We also know that you hit that value at a lower boost level with a 15g than you do with a 13c.
And lastly, we know that it is measured pre-compressor (so it cares not about the T in pervert).

In fact, lh doesn't see "boost" at all, it simply sees air mass. More mass, more fuel, more power so the story goes.
Click to expand...

I didn't miss it at all. What apachechef said is exactly correct. The only difference in his scenario is the temperature. Otherwise, the two turbos would be moving the exact same amount of air. Ideal gas law, and all that. You guys are getting lost in the forest. The difference is in density of the charge. How can the same volume of air be of different density? Temperature. Forget the intercooler. It isn't magic. Think of it in terms of mass of "gas" moved. Air being a gas. Volume of air is a bad measure because the discharge temps are not going to be the same 13C vs. 15G at much higher boost levels. They maps may cross over where they are the same at a certain flow.

I reread your post above and it appears to me you are on the right track. Mass flow is what I've been saying throughout the thread. Including in the paragraph above this one. Go back to what apachechef said and point out where his statement is wrong. Exactly what he said, word for word. There is that instant where the valve closes and you have a certain swept volume. Now, what could be different in those two comparative volumes that would require one to require more fuel than the other?
 
Last edited:
linuxman51 said:
no, I think instead you've missed it.

If volume is the sole dictator of performance, all 2.3L turbo engines would make the exact same power at the exact same boost levels.

Ergo, volume cannot be considered static.


consider also the parable of boost cut on lh 2.4 cars.

We *know* that it is based on mass air flow


In fact, lh doesn't see "boost" at all, it simply sees air mass. More mass, more fuel, more power so the story goes.
Click to expand...

Ergo, volume cannot be considered static.
Click to expand...
:-(
. More mass, more fuel, more power
Click to expand...
:)

Agreed, it's all about the mass of air squeezed into the 2.3L per 2x revolution, the vol will always be 2.3L, at diff P and T.
 
Well, my question got answered on page one.
Liking physics and chemistry has done me well.

Wasn't trying to start an argument/pissing match/ whatever over something I already understood.

I just wanted to make sure I was applying my knowledge to tuning needs correctly.
 
You must log in or register to reply here.
Back
Top