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Old 02-06-2018, 09:33 AM   #1
vwbusman66
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Default PSI vs. CFM and fueling upgrade

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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Old 02-06-2018, 09:38 AM   #2
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yes.
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Old 02-06-2018, 09:39 AM   #3
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but LH doesn't measure pressure, it only measures airflow, so.. if the overhead is there (or the tuning is there), it's handled somewhat transparently
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Old 02-06-2018, 10:52 AM   #4
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What ^^^^ said. 1b on a 15g will be pushing the stock injectors, but something in the #34-38 range should handle that without too much issue.
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Old 02-07-2018, 08:14 PM   #5
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The whatever color tops (orange/red?) off of the 850 on my 15G at 15psi with TLAO chips have no issue keeping up.

This post is relevant for this discussion on 15psi is 15psi across turbos, which is incorrect obv.

The TLDR is PSI is a measure of force per SQUARE inch on the intake manifold. CFM is a volume of CUBIC air the turbo moves.
https://www.rx7club.com/3rd-gen-arch...me-psi-645551/
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Old 02-07-2018, 08:27 PM   #6
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Quote:
Originally Posted by EivlEvo View Post
The whatever color tops (orange/red?) off of the 850 on my 15G at 15psi with TLAO chips have no issue keeping up.

This post is relevant for this discussion on 15psi is 15psi across turbos, which is incorrect obv.

The TLDR is PSI is a measure of force per SQUARE inch on the intake manifold. CFM is a volume of CUBIC air the turbo moves.
https://www.rx7club.com/3rd-gen-arch...me-psi-645551/
I actually read that same thing before posting here! Very interesting engine/physics stuff to understand.

Thanks for the help guys!
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Old 02-07-2018, 08:27 PM   #7
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yes.
Hmm.

At the same RPM, How does the engine tell the difference betwixt the same psi from different turbos?

The same mass of air will fill the cylinder from the same psi seen at the intake, right?
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Old 02-07-2018, 08:29 PM   #8
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Originally Posted by apachechef View Post
Hmm.

At the same RPM, How does the engine tell the difference betwixt the same psi from different turbos?

The same mass of air will fill the cylinder from the same psi seen at the intake, right?
PSI is a measure of pressure, not volume.
A turbo flowing more air (volume) at the same pressure (PSI) with inherently make the fuel injection compensate for the increase in airflow (I think )
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Old 02-07-2018, 08:29 PM   #9
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Originally Posted by apachechef View Post
Hmm.

At the same RPM, How does the engine tell the difference betwixt the same psi from different turbos?

The same mass of air will fill the cylinder from the same psi seen at the intake, right?
No, because the valve timing is constant. If the turbo flows greater CFM more air goes into the motor for that given time, but the PSI on the manifold is the same.
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Old 02-07-2018, 09:01 PM   #10
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I used to run into this when I was in the air compressor business...."Why can't I run this 90cfm sandblaster off of my crappy 4.5cfm air compressor, I've got 100 psi."
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Old 02-07-2018, 09:20 PM   #11
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Originally Posted by apachechef View Post
Hmm.

At the same RPM, How does the engine tell the difference betwixt the same psi from different turbos?

The same mass of air will fill the cylinder from the same psi seen at the intake, right?
When tuning a normally aspirated engine, how much difference does a restrictive exhaust make?

It takes a lot more pressure on the hotside of a small turbo for it to make the same PSI as a larger turbo. More pressure on the exhaust side rather directly reduces HP. Look at how much effort goes into tweaking headers and exhaust port volumes and runner diameters on normally aspirated, because even small differences there affect HP.
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Old 02-07-2018, 09:44 PM   #12
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Think of it this way...40psi of water pressure, you will get a LOT more water from a fire hose than a garden hose.
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Old 02-07-2018, 10:22 PM   #13
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Quote:
Originally Posted by vwbusman66 View Post
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.
A better description would be: A more efficient turbo producing the same PSI (1bar) is actually flowing a larger mass of air.

Power is mainly proportional to mass of air in the cylinder. Higher temperature air is less dense (e.g. hot air balloon) and has less mass. A lower efficiency turbo will heat the air more, while a higher efficiency turbo (at a given boost point) heats the air less. Less heat means denser air, means more power.

No matter what, compressing air increases the temperature, more so with an inefficient turbo. Hence improving the intercooler reduces the air temperature. Lower temperature air means denser, means more mass, means more power.

Quote:
Originally Posted by vwbusman66 View Post
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?
At 1bar, a 15g is more efficient than a 13c - the rise in boosted air temp at 1bar with a 15g will be less than a 13c. Thus lower temp 15g air means denser air, means more mass, means more power, and needs more fuel at full boost. The MAF - Mass Air Flow meter measure air mass, not CFM (it has temperature compensation built in) and will automatically adjust the fuel flow to match if the injectors can keep up.
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Old 02-07-2018, 10:27 PM   #14
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A better description would be: A more efficient turbo producing the same PSI (1bar) is actually flowing a larger mass of air.

Power is mainly proportional to mass of air in the cylinder. Higher temperature air is less dense (e.g. hot air balloon) and has less mass. A lower efficiency turbo will heat the air more, while a higher efficiency turbo (at a given boost point) heats the air less. Less heat means denser air, means more power.

No matter what, compressing air increases the temperature, more so with an inefficient turbo. Hence improving the intercooler reduces the air temperature. Lower temperature air means denser, means more mass, means more power.



At 1bar, a 15g is more efficient than a 13c - the rise in boosted air temp at 1bar with a 15g will be less than a 13c. Thus lower temp 15g air means denser air, means more mass, means more power, and needs more fuel at full boost. The MAF - Mass Air Flow meter measure air mass, not CFM (it has temperature compensation built in) and will automatically adjust the fuel flow to match if the injectors can keep up.
got it
ok, so answer isnt cfm, its T

same psi, same volume, lower T
more air mass, needs more fuel.
comparing cfm is just crappy proxy for temp

i think your explanation is best, no pump pumps more air into a closed chamber once it is at max psi

Last edited by apachechef; 02-07-2018 at 10:50 PM..
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Old 02-07-2018, 10:30 PM   #15
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Think of it this way...40psi of water pressure, you will get a LOT more water from a fire hose than a garden hose.
but, you can only drink a mouthful at a time.
a faucet will look the same hooked up to either.
lil different with compressible air.

im imagining 2 huge air tanks filling ballons, at same psi.
no matter the cfm of dif compressors, balloons fill same.
bottleneck of valve. and of vol dif betwixt tank/balloon
filling directly from compressor, cfm would matter, but matters less as valve gets smaller, as as size of balloon rel to tank dec.

im thinking the ratio of vol of intake tract air to cyl air matters, as well as restriction at valve.

Last edited by apachechef; 02-07-2018 at 10:48 PM..
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Old 02-07-2018, 10:35 PM   #16
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Originally Posted by EivlEvo View Post
No, because the valve timing is constant. If the turbo flows greater CFM more air goes into the motor for that given time, but the PSI on the manifold is the same.
only if temp is different.
same size container, same P, same T, same mass of air.
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Old 02-07-2018, 10:56 PM   #17
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Quote:
Originally Posted by apachechef View Post
but, you can only drink a mouthful at a time.
a faucet will look the same hooked up to either.
lil different with compressible air.

im imagining 2 huge air tanks filling ballons, at same psi.
no matter the cfm of dif compressors, balloons fill same.
bottleneck of valve. and of vol dif betwixt tank/balloon
filling directly from compressor, cfm would matter, but matters less as valve gets smaller, as as size of balloon rel to tank dec.

im thinking the ratio of vol of intake tract air to cyl air matters, as well as restriction at valve.
Just watched an episode of Engine Masters testing the EXACT issue of intake volume tract. 3 different heads, virtually no difference in power output.

Not going to get into a pissing match with you on this one, given your stance at this point. Not worth my time.
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Old 02-07-2018, 11:26 PM   #18
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Just watched an episode of Engine Masters testing the EXACT issue of intake volume tract. 3 different heads, virtually no difference in power output.

Not going to get into a pissing match with you on this one, given your stance at this point. Not worth my time.
pissing match?

what do you disagree with?

link to vid? was it turbo?

to be clear, we agree that a higher flowing turbo is better, but perhaps differ as to why?

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Old 02-07-2018, 11:39 PM   #19
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I want to disagree that different turbos will have different fueling needs at the same pressure.

Because.

The motor is the flow restrictor, and 15psi is 15psi no matter what turbo is pushing it.
How and when it gets there will change as the outputs of the turbo are different.
Naturally the larger turbo is capable of pumping more air.
The only way I see a larger turbo having different fueling requirements at a set boost pressure is if the air flow is changed by a less restrictive exhaust side.

You would have to raise the motors volumetric efficiency in order to burn more fuel (make more power).
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Old 02-08-2018, 12:08 AM   #20
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That old Saab commercial claiming that the turbo takes wasted energy from the exhaust and recycles it as boost isn't quite completely correct. Exhaust backpressure reduced engine output. The turbo sitting in the exhaust stream restricts flow. You pay for the boost with a reduction in power produced, it's just that you lose a lot less than you gain. But if you got the 20 psi of boost from some OTHER source (like a giant room sized tank of pressurized air) and had a wide open exhaust, you'd make more power.

Take a normally aspirated motor, and put a restrictor in the exhaust, say 1/2" in diameter. Then drive the car around. Floor it, and the intake manifold pressure goes up to 1 bar, 14.7-ish PSI worth of air. Each time the intake cycle happens the cylinder gets a gulp of that 14.7 psi of air. And then has to force it out into a highly pressurized exhaust system because the air can't get past that restrictor. 14.7 psi of air in the intake, full gulps of air in the intake cycle, and you're not making much HP at all.

Take that restrictor out and floor the throttle. Same exact pressure in the intake manifold - 14.7 psi. Same amount of air in the cylinder on the intake cycle. Only now on the exhaust stroke there's practically no pressure there, and the motor makes a lot of HP.

Nowe just mentally replace that restrictor with the turbo. A tiny turbo has to work a lot harder to pump air up to a certain PSI than a bigger turbo does. When you say 'work harder' it means that it requires more pressure in the exhaust system to spin the turbine faster to pump more air out of the compressor. The smaller the turbo, the more of a restriction it is in the exhaust, and the more HP it subtracts from the engine to create the boost. A larger turbo makes the same amount of boost with less backpressure, so it subtracts less HP.
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Old 02-08-2018, 12:14 AM   #21
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Quote:
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Think of it this way...40psi of water pressure, you will get a LOT more water from a fire hose than a garden hose.
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.
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Old 02-08-2018, 12:17 AM   #22
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NA built V8. Slightly different scenario, still plays into the intake tract volume to an extent.
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Old 02-08-2018, 12:23 AM   #23
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One time I had a cat melt down on my old VBR6 Jetta. The car ran perfectly fine, no codes were set (1995, so OBDI, not OBDII). Idled perfectly, ran 100% fine at part throttle. But put your foot down, and LOTS of missing power. Floor it, not much there at all. Hell, in neutral you could floor it and it would only rev up to about 3500 rpm and just sort of hang there, working hard but not making any power. No restriction in the intake, no fuel issues, no issues with the engine, just a massive restriction in the exhaust causing about 160 hp to vanish. When the cat cam off, there was a hole about 1/3" still flowing.

Turbos don't pump air for free, exhaust system pressure reduces power, a smaller turbo requires more exhaust pressure to make the same PSI.
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Old 02-08-2018, 09:53 AM   #24
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Threads like this continue to underline the fact that none of yall are ever actually going to catch me.
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Old 02-08-2018, 09:59 AM   #25
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