PSI vs. CFM and fueling upgrade

vwbusman66 said:

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.

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You started a discussion. One where everyone learns from each other, hopefully.

From apache.

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.

It's all that simple. How you cram all that through the 2.3 is where it gets difficult.

apachechef said:

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.

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POnder the word 'into' there.

HOld that in your head while you imagine an air compressor, hooked to an air tank. It pumps up more and more pressure, until, at some (fairly specific) point, it stops pumping.

The inlet is unrestricted, the valves open and close, the cylinder still displaces the same volume of air each stroke. But it'snot moving any air. Why?

The compressor has a certain compression ratio - it pulls in volume X, compresses it to volume Y, and forces air out into the tank. But that compression ratio only produces a certain amount of PSI. Any higher and when the outlet valve closes there is still considerable pressure in the cylinder, and as the piston moves downward and the volume expands and the psi drops, until it gets to 1 bar right as the intake valve opens. At which point no new air enters the cylinder. The *same* air just ends up getting compressed to the same psi as the outlet, doesn't move, re-expands, no new air comes in. Stuck in a cycle with no air moving anymore.

That's a situation where a cylinder is actively displacing air, compressing it, but not moving it. Obviously, a running engine is much more complicated, with the volume of the gasses in the cylinder dramatically increasing as a small amount of fuel turns into a large volume of gas. But you can still see how pressure on the outlet can reduce the air moving into the cylinder on the intake stroke. The example above of the 'topped out' compressor is extreme, but we're talking difference in HP produced and differences in airflow, it's just a less extreme situation.

But 2.3L of displacement with a given PSI in the intake is not a static situation.

JohnMc said:

POnder the word 'into' there.

HOld that in your head while you imagine an air compressor, hooked to an air tank. It pumps up more and more pressure, until, at some (fairly specific) point, it stops pumping.

The inlet is unrestricted, the valves open and close, the cylinder still displaces the same volume of air each stroke. But it'snot moving any air. Why?

The compressor has a certain compression ratio - it pulls in volume X, compresses it to volume Y, and forces air out into the tank. But that compression ratio only produces a certain amount of PSI. Any higher and when the outlet valve closes there is still considerable pressure in the cylinder, and as the piston moves downward and the volume expands and the psi drops, until it gets to 1 bar right as the intake valve opens. At which point no new air enters the cylinder. The *same* air just ends up getting compressed to the same psi as the outlet, doesn't move, re-expands, no new air comes in. Stuck in a cycle with no air moving anymore.

That's a situation where a cylinder is actively displacing air, compressing it, but not moving it. Obviously, a running engine is much more complicated, with the volume of the gasses in the cylinder dramatically increasing as a small amount of fuel turns into a large volume of gas. But you can still see how pressure on the outlet can reduce the air moving into the cylinder on the intake stroke. The example above of the 'topped out' compressor is extreme, but we're talking difference in HP produced and differences in airflow, it's just a less extreme situation.

But 2.3L of displacement with a given PSI in the intake is not a static situation.

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you know exactly what you speak of, and understand it fully.
from OP,

but we're talking difference in HP produced

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wasn't asked.
I think you are several steps ahead

your analogy of the stalled air compressor:
if i'm filling a rigid 1L syringe from 2 equal temp and psi air tanks, same mass will be in each syringe, regardless what spec compressor stalled out getting to that psi.
electrical bill will surely differ, but unless the Psi drops enough while filling the syringe, you can't tell which tank is which by the weight of the syringe.

factoring hp, you are correct, but I don't think you disagree with the basics of what i said, other than a tiny turbo would result in a higher cyl P when instant intake opens...
kinda beyond Op q

I, for one, have enjoyed this thread and learned a lot. Good discussion.

apachechef said:

you know exactly what you speak of, and understand it fully.
from OP, wasn't asked.
I think you are several steps ahead

your analogy of the stalled air compressor:
if i'm filling a rigid 1L syringe from 2 equal temp and psi air tanks, same mass will be in each syringe, regardless what spec compressor stalled out getting to that psi.
electrical bill will surely differ, but unless the Psi drops enough while filling the syringe, you can't tell which tank is which by the weight of the syringe.

factoring hp, you are correct, but I don't think you disagree with the basics of what i said, other than a tiny turbo would result in a higher cyl P when instant intake opens...
kinda beyond Op q

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Your two syringes wouldn't start off the same though. They both start out with the plunger up near the top. Only one has ambient pressure in it then, the other with a not insignificant amount of pressure. As the plunger drops, one starts pulling fresh air in immediately, actually moving air. The other has to let that pressurized air already in it from the exhaust expand for a while, until it drops below the pressure of the inlet, and only then does it pull in 'new' air.

Same temperature in the inlet, same PSI, same mass per volume, only one syringe pulls in less air because it had residual pressure still in it that displaced the air that would have otherwise entered. Residual pressure because the outlet/exhaust had pressure in it.

An engine has a compression ratio - it has one volume at TDC, and another at BDC. The exhaust back pressure leads both to lost power when the exhaust stroke has to work harder to push the exhaust out into a pressurized exhaust, and it loses effective displacement when residual pressure of the exhaust left over in the chamber expands on the intake stroke and displaces incoming air.

The cylinder may have the same volume and mass of air, but in one case, it's less 'new' air, thus the engine doesn't move as much air, more of it stays stuck in the cylinder.

hp requires fuel, so a discussion involving that aspect is not beyond the pale. more air, more fuel needed, more power. thats how all of this works on a simple level.

JohnMc said:

Your two syringes wouldn't start off the same though. They both start out with the plunger up near the top. Only one has ambient pressure in it then, the other with a not insignificant amount of pressure. As the plunger drops, one starts pulling fresh air in immediately, actually moving air. The other has to let that pressurized air already in it from the exhaust expand for a while, until it drops below the pressure of the inlet, and only then does it pull in 'new' air.

Same temperature in the inlet, same PSI, same mass per volume, only one syringe pulls in less air because it had residual pressure still in it that displaced the air that would have otherwise entered. Residual pressure because the outlet/exhaust had pressure in it.

An engine has a compression ratio - it has one volume at TDC, and another at BDC. The exhaust back pressure leads both to lost power when the exhaust stroke has to work harder to push the exhaust out into a pressurized exhaust, and it loses effective displacement when residual pressure of the exhaust left over in the chamber expands on the intake stroke and displaces incoming air.

The cylinder may have the same volume and mass of air, but in one case, it's less 'new' air, thus the engine doesn't move as much air, more of it stays stuck in the cylinder.

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should i repeat my last sentence again?

but I don't think you disagree with the basics of what i said, other than a tiny turbo would result in a higher cyl P when instant intake opens...

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WE AGREE

Pretty much.

Just to complicate things even further, the gasses that remain in the cylinder due to backpressure are also deoxygenated. So it displaces some of the fuel air in the intake that would otherwise flow in, and even though the same volume of gas is in the cylinder when it begins the compression stroke, it needs less air because there's less oxygen in there - it's not lean because of less fuel.

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

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Bernoulli called, said he wants his 15g back.

I’m no turbo engineer, but I am a process chemist and deal with ish like this every single day, pumps, reactors, scrubbers, systems capacity, enthalpy, latency, taming runaway large scale reactions, the lot:

You’re all, at points, and some more than others, getting the physics right. However, stop saying “simple”. It’s not. Dynamic is the word. Many of you want to compare a single snapshot of the system, which leads to threads like this. The system is dynamic and has variables inheritant to components that many of your are taking as constant. With the pedantry out of the way, here’s the simple truth:

There’s a practical reason higher flowing fuel injectors are a supporting upgrade for a larger turbo. There’s a reason one chooses a turbo based on the compressor map and VE. You can’t bench race fluid dynamics because the system isn’t ideal. Compressor maps are experimental data. VE is an assumption. And larger injectors are a necessary practicality. As has been pointed out, if you are moving more air, more fuel is required to react. Fuel injection measures starting state (MAF) and final state (O2 sensor) and connects the dots to adjust the fuel ratio. If it runs out of ceiling...


TLDR: I’m no expert, but I did stay at a Holiday Inn Express last night.

....you MS eet!

SlowRide said:

Bernoulli called, said he wants his 15g back.

I’m no turbo engineer, but I am a process chemist and deal with ish like this every single day, pumps, reactors, scrubbers, systems capacity, enthalpy, latency, taming runaway large scale reactions, the lot:

You’re all, at points, and some more than others, getting the physics right. However, stop saying “simple”. It’s not. Dynamic is the word. Many of you want to compare a single snapshot of the system, which leads to threads like this. The system is dynamic and has variables inheritant to components that many of your are taking as constant. With the pedantry out of the way, here’s the simple truth:

There’s a practical reason higher flowing fuel injectors are a supporting upgrade for a larger turbo. There’s a reason one chooses a turbo based on the compressor map and VE. You can’t bench race fluid dynamics because the system isn’t ideal. Compressor maps are experimental data. VE is an assumption. And larger injectors are a necessary practicality. As has been pointed out, if you are moving more air, more fuel is required to react. Fuel injection measures starting state (MAF) and final state (O2 sensor) and connects the dots to adjust the fuel ratio. If it runs out of ceiling...

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nawww man. psi is psi. You get the same airflow at a given psi regardless of whether the turbo has a 35mm inducer or a 100mm inducer. pv=nrt is all you need to know. All engines of the same displacement move the same amount of air. I mean how could it be any different, "V doesn't change dawg"


Dynamics and chemistry, pssshhhawww. Like those things change. 2.3 liters at 6000 rpms is the same 2.3 liters at 3000 rpms. 15 psi is 15psi.


in all seriousness, where I was going with my comments and remarks was to make things obviously disproportionate enough to force a re-think. Part of the reason is I feel like some amount of effort needs to be exerted on the part of those inquiring, and the other part is cynical in nature--this very subject has been beaten to absolute death on this forum and others, and yet the insipid notion remains that the *only* reason bigger turbos exist is to run higher boost and/or cooler charge temps, and that that is the *only* reason they make more power. Its a discussion so old and worn out at this point I have a hard time not having fun at other people's expense. May not be the right way to deal with that, but horses and water and what not.

Horsepower and fuel consumption are both directly related to airflow versus rpms. It matters not if you're cramming air in under pressure or sucking it in, THAT is the basis of everything. More air, more power. More fuel required. Otherwise, we would not need bigger fuel systems. Larger exhaust systems would matter not, changes to intercoolers would net minimal changes. You have to be able to get that air out as well as push it in.

At a glance, eh, it's not a wash, but a 15g is not a "big" turbo. It's about what volvo should've put on the cars from the get-go, but that's a subjective statement.. At any rate though, the differences between them are not just in the compressor wheel size and design, but also in the size of the turbine wheel and housing. So in addition to a higher ceiling of flow at a given PR looking strictly at the cold side, you have a larger volume turbine housing, and a larger turbine wheel which allows easier and more egress from the system. We're not talking 100hp worth of difference, it's still not THAT big, but more than one would expect from a nominal reduction in post-compressing temperatures.

You have to be able to get the stuff out, it's not just putting it in there that matters and forget about it.. If your little turbo is ripping along at say 15psi intake pressure, but you have 40 psi of backpressure in the exhaust pre-turbine, how much force are you robbing from the engine to push out the spent charge? how much of that spent charge is going to displace the incoming fresh air? What happens if you have a cam with overlap? What about the post turbo exhaust? Sure, it's getting forced out, but that's not free effort.

Exactly. People are throwing around false equivalenies and the ideal gas law, but still run a 3” exhaust? 3” 012 MAF? If all psi (bunch of closet MAP fappers) is equal, why bother???

GTFO with the ideal gas law...

The ideal gas law is an approximation anyway.

At higher pressures, the volume of the molecules can no longer be ignored in calculations. This leads to the ideal gas law giving inaccurate results. For this reason, I use the Redlich-Kwong equation of state in my calculations. You would be surprised in the difference this makes. I cannot believe people do not take this into consideration.

While in reality I'm just another drunk who bolts together mismatched parts of 1980's technology.

Now I vaguely want to hook a PSI gauge to my exhaust manifold and see how high the PSI gets when it's making 20 psi of boost.

Not enough to actually DO anything about it, mind you.

JohnMc said:

Now I vaguely want to hook a PSI gauge to my exhaust manifold and see how high the PSI gets when it's making 20 psi of boost.

Not enough to actually DO anything about it, mind you.

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I haven't done this yet either, but it's on the list of things to add to racecar V....whatever version it is.

Sjeng said:

The ideal gas law is an approximation anyway.

At higher pressures, the volume of the molecules can no longer be ignored in calculations. This leads to the ideal gas law giving inaccurate results. For this reason, I use the Redlich-Kwong equation of state in my calculations. You would be surprised in the difference this makes. I cannot believe people do not take this into consideration.

While in reality I'm just another drunk who bolts together mismatched parts of 1980's technology.

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At high pressures, no you cannot ignore the molecular volume, but then you’re existing in combustion dynamics. In an intake runner or all four overall? At the intake valve as it opens? The exhaust valve at full excursion? Between pulses, at a non-smooth curve in the exhaust manifold?

Without breaking a turbo charged gasoline engine into component parts—which is what this thread would love to accomplish, but cannot—one would be hard pressed to find an EOS that works from MAF to exhaust tip. This aint rocket science, literally. Unless we are talking about single stroke 240’s...

Broad strokes, experimental data, trial and error. As you said, 80’s fuel injection... not ideal. Y’all can go pervert equals nert quietly in the corner.

After a day of pondering, I came up with this:
Its a semi closed loop system. The amount of exhaust effects the amount of intake. How do we improve the efficiency of this system, to get more power? Find the current worst bottle neck and eliminate it. The goal is a balanced system.

If the intake is a drinking straw, it doesn't matter what size turbo is there, you can only move so much air at a given pressure. Increase the size of the straw and a point is reached where that no longer is the bottle neck in the system. Keep increasing the intake flow capabilities and you get no more results because something else is now the restriction. Perhaps the next bottleneck is the cam dynamics, and the cam is not suited to a performance turbo motor. Install a cam better suited to a turbo motor, flow goes up and now the bottleneck is the exhaust. Its not converting enough exhaust energy to intake charge. Port the exhaust to allow the exhaust turbine to better use the exhaust energy, and you will see improvement until the turbine is the new source of restriction to the new flow levels of the rest of the system. It can't handle the flow the engine is capable of, and instead of converting it to intake energy, it maxes out and heat is the result. Install bigger/higher flowing turbine and the restriction is now moved to the compressor. Up the size of the compressor to match the turbine, and you will now see the power gains. Balance.
If the components are sized correctly to each other, the system should be working in balance, at a new hp level. There will always still be a small bottleneck somewhere. Want more power? you need to adjust the whole system to a higher balance point.

I realize I never included heat, at least not directly. Too much heat is evident in a system that is not in balance. Something along the way is causing resistance. Heat is a product of inefficiency. Ideally all the exhaust energy directly equates to intake charge. This is not true and the difference is heat. Heat should not be a factor in a system designed in a balanced and efficient manner. Heat is an indicator of inefficiency. You can't get away from the heat that is created when you compress a gas, That is physics. You can however reduce any additional heat applied to the air by the inefficiencies of the system by achieving balance.
What are intercoolers for? To reduce the heat you can't get away from. It is not for reducing the heat created by mismatched components.
This requires looking at each part separately, and seeing if it is the current bottleneck in the system as a whole.

linuxman51 said:

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.

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The volume IS static. The volumetric efficiency varies. Those motors all have different stroke/bore ratio, head configuration, rotating assembly mass, compression ratio, valve size, cooling, oiling, etc.......All of these factors play into the VE of the engine and why those engines do or do not make power at a given displacement.

NotSoFresh said:

What are intercoolers for?

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So far as I can tell, they?re for collecting oil...

NotSoFresh said:

The volume IS static. The volumetric efficiency varies. Those motors all have different stroke/bore ratio, head configuration, rotating assembly mass, compression ratio, valve size, cooling, oiling, etc.......All of these factors play into the VE of the engine and why those engines do or do not make power at a given displacement.

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Which was my point. Swept volume may be static, but for what we're discussing, it's largely meaningless by itself. You don't consider the static displacement of the engine to be the end all be all when you're playing with the turbo calculators (whether you realize it or not).. on a very basic level you plug in VE, displacement to get a comparable number, and then plot it against various rpm points on the compressor map based on the calculated amount of air going through the engine. (be it cfm or lb/min, whatever). (that sounds contradictory eh)

swept displacement matters most if you're comparing two identical engines except for that, e.g. b21 vs b23 or b200 vs b230, that sort of thing.. where your VE based on head performance is going to be fairly similar.

(I'm not trying to suggest that displacement matters not at all, don't get me wrong)

SlowRide said:

So far as I can tell, they’re for collecting oil...

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yeah, sometimes it seems that way eh

that explanation is a bit problematic (I had to rush it, caught a cramp if you knowwhatImean)... anyway, you caught what I was getting at before.