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Old 02-08-2018, 11:07 AM   #26
NotSoFresh
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The simple explanation is that PSI is a measure of intake restriction. Once the smallest restriction in the intake reaches its max airflow naturally, and air goes from being pulled to pushed the PSI goes up. The more restriction there is, the higher the psi will be for a given cfm.
Psi is the resistance to airflow. If there was no restriction, the air would flow freely forward into engine and the psi would be low. Close the restriction, and the pressure goes up but less air goes through. In an engine, the intake tract will only flow so much air. At pressure, more flows through but the amount of increase is directly related to the size of the intake restriction.
Think of your intake like a garden sprayer and the turbo as the tap. If you restrict the sprayer, after a point it does not matter how much you open the tap at the wall, you don't get any more water. Open up the sprayer more, and you get more water at the same line pressure at the tap, until you reach the flow limit of the tap.
If you don't change the intake tract restriction the only way to get it to move more air is pressure. At 15psi, there is a fixed amount that can fit through the restriction regardless of pump size.
PSI is easier to measure than airflow, and works as a general turbo load indicator in a given engine with a non-changing intake tract.

Airflow cfm can be directly used to calculate power. Psi can not.

Now, all of this is just talking cold side stuff. The hotside, and its sizing issues, that is another thing, which JohnMC already touched on.

Last edited by NotSoFresh; 02-08-2018 at 12:40 PM.. Reason: Wrong john..
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Old 02-08-2018, 11:52 AM   #27
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In this scenario, though, there are no differences in the intake side. Well, the turbo compressor is smaller, but you've already worked your way around that problem by spinning the compressor a lot faster/harder.

There's one smaller effect (IMO) from the harder working turbo adding more heat to the compressed air, but the main difference here is the increased pressure on the exhaust side.

On a well tuned N/A engine, the exhaust stroke is a near freebie, the latent combustion pressure starts the exhaust whooshing out as soon as the exhaust valve opens, the piston moves up against practically no pressure. With significant pressure in the exhaust system, though, either through a tiny turbo or some other restriction, this becomes real work. The piston has to PUSH the exhaust out (pressure differential between the pressurized exhaust and the ambient air pressure in the crankcase). If the pressure in the exhaust gets high enough, the motor will stop making power. Hell, if it gets really high, the motor will turn backward - negative HP! (But you'd need an external source of pressure there).

Trading some power loss through exhaust pressure for power gain through cramming more air into the cylinders is a good tradeoff though. It just not a static tradeoff because of the varying efficiencies of different turbos. Some will take more HP out of the system to create the given PSI, others will take less.
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Old 02-08-2018, 01:55 PM   #28
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You guys. Stop putting "simple explanation" in front of things.

You can't just change physics.

PSI Pressure per square inch. This has nothing to do with air flow. At all. It has to do with the pressure that the air is exerting on a physical surface (in this case, the intake manifold or wherever your boost gauge is)

CFM Cubic feet per minute. This has nothing to do with pressure. At all. It has to do with the VOLUME of air that is able to flow for a given time.

If the time is constant. Then you have your cubic feet of air. If you have a turbocharger that can move 400CFM then in 1 minute, you get 400 cubic feet. That's VOLUME. If you have an 800 CFM turbo then in 1 minute you get 800 cubic feet.

This is the same circle jerk we all get into about intercooler piping and where to mount AMMs and BOVs and such.
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Old 02-08-2018, 02:07 PM   #29
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Quote:
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I'm not quite sure that analogy works completely here, because PSI is just a byproduct of the CFM. Well, the relative difference between how many CFM the turbo is moving and how many CFM the engine is consuming. If the little turbo was really moving less CFM, and the engine relatively the same, then it would result in less HP. The air would 'pile up' less in the intake.

What (I think) happens is that the volumetric efficiency of the engine changes based on the exhaust backpressure. The small turbo makes a lot more back pressure, thus the engine isn't as efficient at cycling air through, so the smaller CFM flow is matched by the reduced efficiency of the engine, resulting in the same PSI. Less air crammed in balanced by less air consumed.
Ding, ding, ding. We have a winner. That is exactly what is happening. Same PSI DOES NOT Equal same air flow in all instances. You have to look at pressure ratios. If you start driving a turbo out of its efficiency range it is less efficient and starts heating the charge more. Hotter air is less dense. It also takes more of a restriction on the exhaust side to drive that small turbo faster to create the higher pressure level. That's where your second penalty occurs. Turbocharging is not free horsepower. Bottom line, you can easily be moving much more air at the same boost pressure, one turbo vs. another.
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Old 02-08-2018, 02:51 PM   #30
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Originally Posted by EivlEvo View Post
You guys. Stop putting "simple explanation" in front of things.

You can't just change physics.

If the time is constant. Then you have your cubic feet of air. If you have a turbocharger that can move 400CFM then in 1 minute, you get 400 cubic feet. That's VOLUME. If you have an 800 CFM turbo then in 1 minute you get 800 cubic feet.
If you have a port that is big enough to flow 200cfm, it does not matter if the turbo can move 400 or 800.....If the turbo tries to push more than the port can handle, pressure rises. At a given pressure the port will only move so much air. A bigger turbo will not put more air in at a given pressure. It will allow you to run higher pressure without heating the air, and thus get additional flow through the port without adding heat to engine.

Now hotside size/exhaust restrictions and cam overlap all play together to influence air charge temp. That is the second part of the discussion, its all fine and dandy to say "hey i got a turbo that can flow 800cfm" but do you have enough exhaust gas flow to power it, and how efficiently are you using that flow. Ie: is the turbo hotside creating a restriction to the exhaust and thus reducing the max flow of the port under pressure? How much?
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Old 02-08-2018, 03:06 PM   #31
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you're trying to disconnect two things that are intrinsically connected: intake and exhaust.

Seriously. Stop.
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Old 02-08-2018, 03:18 PM   #32
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Originally Posted by EivlEvo View Post
PSI Pressure per square inch.
PSI is a unit of pressure, which is force per square inch.
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Old 02-08-2018, 03:21 PM   #33
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It's simple. Size the turbo that fit your goal.

Too small of a turbo, you won't get the HP you want.
Too big of a turbo, boost threshold will be at high RPM and your power band is narrow.


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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?
Maybe, maybe not. The thing is bar doesn't matter in your stock Volvo as your car's ECU uses MAF which uses air mass to calculate fuel needs. Use your wideband.
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Old 02-08-2018, 03:39 PM   #34
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PSI=/=CFM
PSI = pressure
CFM = volume
2 entirely different measures.
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Old 02-08-2018, 03:45 PM   #35
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CFM isn't even a complete picture. Pounds per minute is a complete picture. CFM without temperature and humidity still doesn't cut it.
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Old 02-08-2018, 03:52 PM   #36
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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.
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Old 02-08-2018, 03:53 PM   #37
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CFM isn't even a complete picture. Pounds per minute is a complete picture. CFM without temperature and humidity still doesn't cut it.
Which is why AMM and MAF mean what they mean. Air Mass Meter, Mass Air Flow. They're not measuring CFM, or PSI, they're measuring the mass of the air going in.

MAP (Manifold Air Pressure) driven injections systems can work pretty well, but they have some major assumptions built into the logic about the volumetric efficiency of the engine at a given RPM/MAP combination. If anything changes to affect the VE (like freer flowing exhaust, different cam, more displacement) then it isn't really accounted for (directly, only indirectly if there is O2 sensor feedback). But changes for stuff like that is accounted for by an AMM/MAF driven system, because it sees the extra air moving through the system.
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Old 02-08-2018, 03:58 PM   #38
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CFM isn't even a complete picture. Pounds per minute is a complete picture. CFM without temperature and humidity still doesn't cut it.
Moles per minute. Pounds are weight and weight is dependant on gravity. Moles will give you number of molecules, regardless of gravitational pull.
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Old 02-08-2018, 04:00 PM   #39
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Holy **** the level of stupid here
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Old 02-08-2018, 04:01 PM   #40
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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?
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Old 02-08-2018, 04:02 PM   #41
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Originally Posted by NotSoFresh View Post
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.
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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Old 02-08-2018, 04:11 PM   #42
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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).
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Old 02-08-2018, 04:13 PM   #43
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Moles per minute. Pounds are weight and weight is dependant on gravity. Moles will give you number of molecules, regardless of gravitational pull.

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)
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Old 02-08-2018, 04:15 PM   #44
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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?
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yes.
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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Old 02-08-2018, 04:17 PM   #45
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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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Old 02-08-2018, 04:17 PM   #46
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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?
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.
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Old 02-08-2018, 04:25 PM   #47
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Moles per minute. Pounds are weight and weight is dependant on gravity. Moles will give you number of molecules, regardless of gravitational pull.
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.
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Old 02-08-2018, 04:28 PM   #48
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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.

(PV)/T=(PV)/T

share the wisdom, Please

edit:
not asking about power, just OP's Q re: fuel
Looks to me like you "get it".
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Old 02-08-2018, 04:33 PM   #49
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Get Duder in here and he can describe the process much better. He lives turbos.
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Old 02-08-2018, 04:39 PM   #50
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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****
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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