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Old 02-08-2018, 04:55 PM   #51
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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.
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Old 02-08-2018, 05:11 PM   #52
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Quote:
Originally Posted by apachechef View Post
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
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?
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Old 02-08-2018, 05:21 PM   #53
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Originally Posted by 2manyturbos View Post
Looks to me like you "get it".
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.
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Old 02-08-2018, 05:42 PM   #54
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Originally Posted by 2manyturbos View Post
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.
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Originally Posted by apachechef View Post
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.

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share the wisdom, Please

edit:
not asking about power, just OP's Q re: fuel
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Originally Posted by linuxman51 View Post
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****
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Originally Posted by apachechef View Post
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
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.
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Old 02-08-2018, 05:44 PM   #55
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Originally Posted by linuxman51 View Post
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?
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If this is your view on things... and T is the *only* factor that changes (in your opinion),
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

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.
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Old 02-08-2018, 05:49 PM   #56
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Quote:
Originally Posted by apachechef View Post
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

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.

there has been no unzipping or stretching for measurement over here
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Old 02-08-2018, 05:52 PM   #57
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Originally Posted by linuxman51 View Post
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.
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?

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Old 02-08-2018, 05:53 PM   #58
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Originally Posted by linuxman51 View Post
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.
Quote:
Ergo, volume cannot be considered static.

Quote:
. More mass, more fuel, more power


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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Old 02-08-2018, 05:55 PM   #59
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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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Old 02-08-2018, 05:55 PM   #60
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there has been no unzipping or stretching for measurement over here
Well, factor in T up north vs T down south and shrinkage...
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Old 02-08-2018, 06:03 PM   #61
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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 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.
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Old 02-08-2018, 06:12 PM   #62
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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.
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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Old 02-08-2018, 06:33 PM   #63
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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.
you know exactly what you speak of, and understand it fully.
from OP,
Quote:
but we're talking difference in HP produced
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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Old 02-08-2018, 07:47 PM   #64
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I, for one, have enjoyed this thread and learned a lot. Good discussion.
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Old 02-08-2018, 09:55 PM   #65
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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
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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Old 02-08-2018, 10:57 PM   #66
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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.
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Old 02-08-2018, 11:07 PM   #67
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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.
should i repeat my last sentence again?
Quote:
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...
WE AGREE
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Old 02-08-2018, 11:23 PM   #68
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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.
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Old 02-09-2018, 07:57 AM   #69
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Originally Posted by apachechef View Post
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
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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Old 02-09-2018, 08:54 AM   #70
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Old 02-09-2018, 10:40 AM   #71
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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...

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.
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Old 02-09-2018, 10:50 AM   #72
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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...

Last edited by SlowRide; 02-09-2018 at 10:58 AM..
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Old 02-09-2018, 11:11 AM   #73
Sjeng
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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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Old 02-09-2018, 11:13 AM   #74
JohnMc
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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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Old 02-09-2018, 11:32 AM   #75
linuxman51
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Quote:
Originally Posted by JohnMc View Post
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.
I haven't done this yet either, but it's on the list of things to add to racecar V....whatever version it is.
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