So i think someone needs to correct me if im wrong but i keep reading all this stuff about how two different turbos can create wildly varying amounts of power at the same boost pressure but everything i have been taught tells me that pumps create volume and not pressure. The pressure is created by a restriction (the engine) and the turbo only moves air. So the way i see it is if you take a small turbo and a big turbo and wastegate the big turbo down to the same pressure the small one makes the volume will be same and so will the power. Now i get that a smaller turbo would be spinning faster and creating more heat but the loss there is small compared to some of the claims. I dont claim to know it all these are just the pricipals i was taught in college and through my automotive training. Came someone explain this to me?
Remember... One test is worth a thousand expert opinions.
Sorry for posting twice but i forgot to add another important note. If pressure dosent mater like everyone says then shouldnt we all throw away our pressure gauges and install maf sensors and get a gauge that reads it. Atleast that is my understanding of what everyone is saying and it dosent make sense to me.
Remember... One test is worth a thousand expert opinions.
I don't really have an indepth scientific explaination but I'm sure if my buddy reads this he will have something to add but this is my simplified version (correct me if I'm wrong). Power is determined by how much air can flow. So when you compare a small turbo to a large one the large will obviously flow more air. Power is influenced by how much air can flow rather than how much psi it is at. Each turbo has a rating of how much CFM it can flow and each is different. So if both turbos are set at a predetermined psi the larger will flow more air resulting in more power. I'm not sure if this helps you but I'm sure people with have a more scientific explaination. But how i see it is if turbo A (small turbo) flows 300cfm and turbo B (larger turbo) flows 600cfm and if they are both boosting the same amount (psi), turbo B will flow more cfm than turbo A resulting in more power. Say you had a GT20 on a LD9 (for example) and compared it to a GT35 and both set at the same psi you are going to have a more powerful motor with the GT35 because it will flow more air. Anyways, hope that helps a bit but like I said someone will probably have a more scientific explaination for you.
Here is a post I made a while ago:
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Typically, if your engine flows X CFM naturally aspirated, you will need 2X CFM at 15 PSI and 3X CFM at 29 PSI and so on. Lots of people think that it is possible for a bigger turbo to flow more CFMs than a smaller turbo at the same PSI. In reality the only reason a bigger turbo makes more power at the same PSI as a smaller turbo is because it has less backpressure on the exhaust side, and heats the air less on the compressor side (and cooler air means more horsepower). PSI is a measure of intake restriction, and when you double your intake restriction it is because you are trying to add more air than the engine can suck in by itself.
Think of a garden hose. If you have a hose of a certain size, it will flow X gallons per hour at Y pressure. As long as the density of the fluid stays the same (IE, not a non-newtonian fluid, http://en.wikipedia.org/wiki/Non-Newtonian_fluid) than the pressure vs flow will follow a mathematical curve. All this is true as long as the hose does not change in diameter. If the diameter changes, than the flow vs pressure curve will change also.
The same is true for turbos. They have charts (commonly called turbo maps) that show us the relationship between flow and pressure. You can almost look at a bigger turbo as a bigger diameter garden hose. It will take less pressure to flow the same amount of air when compared to a smaller turbo. This is why most people think that a bigger turbo can flow more air at a given PSI than a smaller turbo at the same pressure. However, what they don't understand is that PSI is measured in the intake manifold. As long as everything after the turbo on the intake side stays the same between two different turbos, the bigger turbo will only make more power because it has less backpressure and the air coming out is cooler (and maybe it spins easier because it has better bearings). Obviously, if you port the heads and manifold it will flow more air at the same pressure.
It's kinda like having a 1" garden hose that goes into a 0.5" garden hose. Ultimately, the flow in that system is limited by the smaller hose. And cars are the same way, if your turbo can flow 1000 CFM and your engine can only flow 500 CFM than you will make about 15 PSI. If you get another turbo, it can't magically cram more air in at the same PSI. It is simply impossible. And if you don't believe me take Differential Equations and Fluid Mechanics.
I know that was a bit long-winded, but I think some people here needed to hear it.
Some more reading: http://www.ls1tech.com/forums/street-racing-kill-stories/1074657-i-finally-raced-subaru.html
Also, this picture helps understand: http://upload.wikimedia.org/wikipedia/commons/7/78/VenturiFlow.png
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From this thread:
http://www.j-body.org/forums/read.php?f=40&i=172794&t=172753#17279
That should clear up some basic turbo theory.
"PSI is a measure of intake restriction, and when you double your intake restriction it is because you are trying to add more air than the engine can suck in by itself"
A good way to compare pressure vs cfm is to think of a roots style sc. If you had a stock motor you would see more boost pressure from the air not being moved fast enough and building up more pressure in the sc. Now if you opened up the motor/exhaust to breathe better there would be less pressure but flow more cfm and make more power. When I switched from a t3 super 60 to the same size turbo but had a larger to4b compressor, I would see typically 2-3 more psi of boost, due to it flowing alot more air and pressure getting built up going into the motor. Turbochargers are driven off exhaust gas and a larger compressor side helps spool the turbine and decrease heat.
icemike89 wrote:"PSI is a measure of intake restriction, and when you double your intake restriction it is because you are trying to add more air than the engine can suck in by itself"
A good way to compare pressure vs cfm is to think of a roots style sc. If you had a stock motor you would see more boost pressure from the air not being moved fast enough and building up more pressure in the sc. Now if you opened up the motor/exhaust to breathe better there would be less pressure but flow more cfm and make more power. When I switched from a t3 super 60 to the same size turbo but had a larger to4b compressor, I would see typically 2-3 more psi of boost, due to it flowing alot more air and pressure getting built up going into the motor. Turbochargers are driven off exhaust gas and a larger compressor side helps spool the turbine and decrease heat.
That is the correct way to look at a bigger turbo. I don't know why so many people have a problem understanding that concept...
Wagonwes wrote:And if you don't believe me take Differential Equations and Fluid Mechanics
I am familiar with both...may I say something?
Wagonwes wrote:Here is a post I made a while ago:
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Typically, if your engine flows X CFM naturally aspirated, you will need 2X CFM at 15 PSI and 3X CFM at 29 PSI and so on. Lots of people think that it is possible for a bigger turbo to flow more CFMs than a smaller turbo at the same PSI. In reality the only reason a bigger turbo makes more power at the same PSI as a smaller turbo is because it has less backpressure on the exhaust side, and heats the air less on the compressor side (and cooler air means more horsepower). PSI is a measure of intake restriction, and when you double your intake restriction it is because you are trying to add more air than the engine can suck in by itself.
Think of a garden hose. If you have a hose of a certain size, it will flow X gallons per hour at Y pressure. As long as the density of the fluid stays the same (IE, not a non-newtonian fluid, http://en.wikipedia.org/wiki/Non-Newtonian_fluid) than the pressure vs flow will follow a mathematical curve. All this is true as long as the hose does not change in diameter. If the diameter changes, than the flow vs pressure curve will change also.
The same is true for turbos. They have charts (commonly called turbo maps) that show us the relationship between flow and pressure. You can almost look at a bigger turbo as a bigger diameter garden hose. It will take less pressure to flow the same amount of air when compared to a smaller turbo. This is why most people think that a bigger turbo can flow more air at a given PSI than a smaller turbo at the same pressure. However, what they don't understand is that PSI is measured in the intake manifold. As long as everything after the turbo on the intake side stays the same between two different turbos, the bigger turbo will only make more power because it has less backpressure and the air coming out is cooler (and maybe it spins easier because it has better bearings). Obviously, if you port the heads and manifold it will flow more air at the same pressure.
It's kinda like having a 1" garden hose that goes into a 0.5" garden hose. Ultimately, the flow in that system is limited by the smaller hose. And cars are the same way, if your turbo can flow 1000 CFM and your engine can only flow 500 CFM than you will make about 15 PSI. If you get another turbo, it can't magically cram more air in at the same PSI. It is simply impossible. And if you don't believe me take Differential Equations and Fluid Mechanics.
I know that was a bit long-winded, but I think some people here needed to hear it.
Some more reading: http://www.ls1tech.com/forums/street-racing-kill-stories/1074657-i-finally-raced-subaru.html
Also, this picture helps understand: http://upload.wikimedia.org/wikipedia/commons/7/78/VenturiFlow.png
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From this thread: http://www.j-body.org/forums/read.php?f=40&i=172794&t=172753#17279
That should clear up some basic turbo theory.
I'm not sure why you contradict yourself. Smaller exhaust pressure at the same intake manifold pressure
IS cramming more air into the cylinder.
In any case, intake manifold pressure (as any pressure) is a measurement of potential energy and nothing more. Some devices use the energy of the fluid itself as a propelent, and in the case of an engine, it uses the internal energy mixed with a fuel. Pressure itself does not account for system losses (including the energy required to produce that potential) or even the amount of air ingested per stroke (a result of higher exhaust manifold backpressure and reversion). I don't mean to pick on you, but many people oversimplify turbo systems all the time, which is a mistake. A turbocharged vehicle is a fairly complex system that is heavily dependent on the type of turbocharger you run.
If you want an extreme example to isolate only the induction side, you can run two completely different compressor wheels with the same exhaust wheel and A/R at the same pressure level and produce very different results. There are multiple reasons for this, which include differences in wheel inertias and the big one:
COMPRESSOR EFFICIENCY MAPS. If you can maximize the efficiency islands to your engine, then the wheel you chose is optimal. Likewise, a smaller wheel will spin beyond it's efficiency range, producing not much more than heat (and if the wheel is too small, it'll produce a drop in pressure). There is also the other extreme of using too large of a Compressor wheel and too small of a Turbine wheel and/or AR which leads to surge and eventually damage to your turbocharger. Smaller exhaust wheels also require more energy (higher pressure) to spin a larger wheel, which can in turn lead to high backpressure/reversion, which raises cylinder temps and increases the chance for detonation.
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I must apologize. I didn't realize you were getting most of your information from that "SonofaBish" guy on LS1tech. I must admit that he makes a pretty convincing argument for the average joe, but he is a classic case of "Blind leading the Blind." He even admitted to researching turbos for only a few months before he bought his.
If you want real information about turbochargers, either get a book, read
Garrett's online tech pages and/or frequent some forums for vehicles that come with turbos stock (WRX, Skyline, Supra, EVO, etc.) and see what they have to say.
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Ok well thanks everyone for confirming what i knew " If you get another turbo, it can't magically cram more air in at the same PSI. It is simply impossible." I just wanted to make sure that i wasnt just missing something and was wrong. I was begining to wonder why i kept seeing statements about bigger turbos craming more air at the same pressure than smaller ones in "faqs" when it just didnt seem to make sense. Thank you everyone for chiming in.
And whale sac isnt having backpressure maintaining exhaust gas in the cylinder just like having a e.g.r valve open and wouldnt that have the effect of lowering combustion chamber temps since less combustable materials are being burned?
Remember... One test is worth a thousand expert opinions.
Yes and No. Turbo gasoline exhaust gases are ~1400+ degrees farenheit...pumping that back into the cylinder heats the intake charge going into the cylinder, drastically increasing cylinder temp and pressure which increases dynamic compression and the chance for detonation. An EGR is used for increasing the pressure of the cylinder when VE is low, effectively increasing the otherwise low compression in the cylinder to increase power. An EGR is crap for running WOT, where VE is maximized, because the gases that recirculate are inert and have no useable energy content.
However, there is some regulation of exhaust manifold pressure (or pulses) that goes back to your choice of turbine housing A/R and wastegating. Perhaps I should have mentioned more about A/R choice as well. When you increase the area to radius ratio of the turbine housing, this increases the volume of gas that passes over the turbine blades, effectively increasing the overall efficiency of the turbocharger and increasing the power output of the engine at the expense of increased turbo lag. As a matter of fact, with technologies like VGT, you technically don't even need a wastegate anymore (depending on the boost/power level of course).
If you guys still believe that intake manifold pressure tells the whole story, then read this and scroll down to the dyno comparison...
http://www.honda-tech.com/showthread.php?t=2221092&page=2
It gives an overlayed dyno comparison between two completely different turbochargers (Garrett GT42 and a BW/Bullseye s368) at the same 10psi spring pressure.
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"When you increase the area to radius ratio of the turbine housing, this increases the volume of gas that passes over the turbine blades" You
How do you increase the volume of gas? the engine puts out a fixed amount no mater what shape your exhaust housing is.
egr valves were introduced to lower the level of Nox exiting the combustion chamber at part-throttle load. nox is created when combustion temps are to high thats why egr is used to dump "cold" exhaust gas back into the combustion chamber since you cant burn exhaust gas twice. Exhaust gas from a egr valve dosent pressurize a cylinder to up ve who told you that?
"In internal combustion engines, exhaust gas recirculation (EGR) is a nitrogen oxide (NOx) emissions reduction technique used in most petrol/gasoline and diesel engines.
EGR works by recirculating a portion of an engine's exhaust gas back to the engine cylinders. In a gasoline engine, this inert exhaust displaces the amount of combustible matter in the cylinder. This means the heat of combustion is less, and the combustion generates the same pressure against the piston at a lower temperature. In a diesel engine, the exhaust gas replaces some of the excess oxygen in the pre-combustion mixture.
Because NOx formation progresses much faster at high temperatures, EGR reduces the amount of NOx the combustion generates. NOx forms primarily when a mixture of nitrogen and oxygen is subjected to high temperature." Wikipedia
Done lie to me.. its not nice
Remember... One test is worth a thousand expert opinions.
Sorry, wanted to add some more helpful information
At 2500 degrees Fahrenheit or hotter, the nitrogen and oxygen in the combustion chamber can chemically combine to form nitrous oxides, which, when combined with hydrocarbons (HCs) and the presence of sunlight, produces an ugly haze in our skies known commonly as smog.
How to reduce NOx NOx formation can be reduced by:
-Enriching the air fuel (A/F) mixture to reduce combustion temperatures. However, this increases HC and carbon monoxide (CO) emissions.
-Lowering the compression ratio and retarding ignition timing; but this leads to reduced performance and fuel economy.
-Recirculating some exhaust gases.
How EGR systems work The EGR valve recirculates exhaust into the intake stream. Exhaust gases have already combusted, so they do not burn again when they are recirculated. These gases displace some of the normal intake charge. This chemically slows and cools the combustion process by several hundred degrees, thus reducing NOx formation.
Remember... One test is worth a thousand expert opinions.
Josh A wrote:"When you increase the area to radius ratio of the turbine housing, this increases the volume of gas that passes over the turbine blades" You
How do you increase the volume of gas? the engine puts out a fixed amount no mater what shape your exhaust housing is.
You're confusing cylinder volume with what I'm referring to. When you increase the A/R of a turbine housing, your increasing the volume of exhaust gas that flows through the turbine. The reason we use wastegates, is because we must have a way to regulate the excess exhaust pressure to maintain a constant compressor pressure ratio. If your A/R is too large, you may never build enough pressure on the compressor side and your wastegate will never open. This is the concept behind a Variable Geometry Turbocharger.
Josh A wrote:egr valves were introduced to lower the level of Nox exiting the combustion chamber at part-throttle load. nox is created when combustion temps are to high thats why egr is used to dump "cold" exhaust gas back into the combustion chamber since you cant burn exhaust gas twice. Exhaust gas from a egr valve dosent pressurize a cylinder to up ve who told you that?
"In internal combustion engines, exhaust gas recirculation (EGR) is a nitrogen oxide (NOx) emissions reduction technique used in most petrol/gasoline and diesel engines.
EGR works by recirculating a portion of an engine's exhaust gas back to the engine cylinders. In a gasoline engine, this inert exhaust displaces the amount of combustible matter in the cylinder. This means the heat of combustion is less, and the combustion generates the same pressure against the piston at a lower temperature. In a diesel engine, the exhaust gas replaces some of the excess oxygen in the pre-combustion mixture.
Because NOx formation progresses much faster at high temperatures, EGR reduces the amount of NOx the combustion generates. NOx forms primarily when a mixture of nitrogen and oxygen is subjected to high temperature." Wikipedia
Done lie to me.. its not nice
Exhaust gases are not "cold." They are "cooled" through a heat exchanger, such as a radiator or even the head, but they are far from cold. The flame front is also slower with recirculated exhuast. I stand corrected though. My understanding of an EGR is not entirely accurate. However, there is no denying that there is an increase in VE, when the inert recirculated gases bypass the throttle plate. This would require less intake charge to produce the same VE, but I don't see how it is possible for
Josh A wrote: the combustion generates the same pressure against the piston at a lower temperature.
when the engine has
- less oxygen
- smaller dynamic compression
- slower flame front.
These are all fundamentals for power from an Otto Cycle Engine. I think Wikipedia's explanation is a little too simplified, because it doesn't add up.
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Whalesac,
I would like to say first that I agree with everything you said. The whole purpose of my original post was to show that it is not possible to increase volumetric flow without an increase in pressure, when dealing with two different sized "pipes." I also understand how to read turbo maps, and how to calculate the theoretical points that one should be mapping on one. I have read both Garret's website and Corky Bell's "Maximum Boost" in their entirety, as well as many other websites and periodicals on the subject. The LS1 Tech link I provided was more or less there to help explain to the original poster in the old JBO thread that I also linked.
Ultimately what I was trying to get at was the fact that pressure and flow are related to each other. If you know what the manifold pressure and temperature are, you can calculate the mass flow of air going into the engine (speed density). If backpressure is lower, obviously there will be more room for fresh air after each exhaust stroke. Also, the ease of turning the wheels is taken into account (like you mentioned). The friction force on the bearings of the turbo is too.
I understand that a complete turbo system is indeed fairly complex, but I was trying to make it as easy to understand without going into great detail.
I believe that we are on the same page, I just left some of the story out.
Josh wrote: "When you increase the area to radius ratio of the turbine housing, this increases the volume of gas that passes over the turbine blades"
How do you increase the volume of gas? the engine puts out a fixed amount no mater what shape your exhaust housing is.
I believe the easiest way to look at A/R is with a pinwheel, like the ones that we used to play with when we were younger. If you take a small straw and blow onto the tip of the pinwheel, it will spin up very fast, but you are trying very hard to expel all of the air out of your lungs. If you use a large straw instead, it will take longer to spin up, but you will be breathing much easier to spin it. A small A/R is like a small straw, and a big A/R is like a big straw.
I understand how that part works that why i questioned what he said, everything you have been saying sounds right to me. Im just trying to figure out why everyone tolerates people supplying false information in forums when we all know people read this that have little to no idea about what is going on under the hood of their car and turn to resources like this for information and help. I dont want to argue with anyone i guess i just want help with some info. If you dont question the teacher noone will ever learn anything
Remember... One test is worth a thousand expert opinions.
I'm glad we are all in agreement
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Sorry i totally skipped over your last post whalesac. i used "cold" because compared to during a firing event the exhaust i much cooler. When the plug fires temps can go well over 2500 degrees and by the time the exhaust valve opens and the exhaust exits the combustion chamber its temp has dropped dramatically. so when the egr gas goes back into the chamber its temp may be down in the haundreds, so its not cold just much cooler. Sorry. I used wikipedia because i appreciate how it explains everything in a simple way but i do believe that its discription is correct. The same amount of air and fuel is required to produce the power to move a vehicle with or without egr operation. And if that is true Egr's primary job of cooling the exhaust down would create comparable power just with lower combustion temps.
Remember... One test is worth a thousand expert opinions.
Josh A wrote:Sorry i totally skipped over your last post whalesac. i used "cold" because compared to during a firing event the exhaust i much cooler. When the plug fires temps can go well over 2500 degrees and by the time the exhaust valve opens and the exhaust exits the combustion chamber its temp has dropped dramatically. so when the egr gas goes back into the chamber its temp may be down in the haundreds, so its not cold just much cooler. Sorry. I used wikipedia because i appreciate how it explains everything in a simple way but i do believe that its discription is correct. The same amount of air and fuel is required to produce the power to move a vehicle with or without egr operation. And if that is true Egr's primary job of cooling the exhaust down would create comparable power just with lower combustion temps.
I can accept that, but the wikipedia page says...
wiki wrote: In a gasoline engine, this inert exhaust displaces the amount of combustible matter in the cylinder.
...which is misleading.
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ah that is true sir, it is misleading. Agreed. this went way off topic...
Remember... One test is worth a thousand expert opinions.
Most/all of this is probably covered above but let me clear it up:
A larger turbo will move the same volume of air easier/more efficiently, meaning it will produce less heat in the process. With lower combustion temperatures you can run more boost and/or advance ignition timing to see the large power increases associated with a bigger turbo. There will be an increase in power solely from the increased air density and exhaust efficiency before any adjustments in tuning have been made as long as you don't wind up being too lean (there needs to be extra fuel to burn for the extra oxygen). In a sense, you are not producing more CFMs of air but you are putting more air in the chamber by increasing the density of the charged air coming in (same volume, more mass) and reducing the back pressure of the exhaust exiting the chamber leaving less inert exhaust particles in the next cycle.
Over all: A larger turbo DOES "cram" more air in at the same boost level, but simple (even sarcastic) terms to describe the event definitely leave something to be desired.
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