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PSI vs. CFM and fueling upgrade

gsellstr said:
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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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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NA built V8. Slightly different scenario, still plays into the intake tract volume to an extent.
 
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.
 
Threads like this continue to underline the fact that none of yall are ever actually going to catch me.
 
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.
 
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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.
 
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.
 
JohnMc said:
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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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.
 
EivlEvo said:
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.
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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?
 
you're trying to disconnect two things that are intrinsically connected: intake and exhaust.

Seriously. Stop.
 
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.


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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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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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.
 
2manyturbos said:
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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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.
 
2manyturbos said:
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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Moles per minute. Pounds are weight and weight is dependant on gravity. Moles will give you number of molecules, regardless of gravitational pull.
 
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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