Power Versus Turbo Choice

[Branelly]

I was originally under the impression that turbo choice wouldn't effect power as long as the psi its putting into the engine is the same.

For instance two turbos, 1 reaches full boost at 3000rpm. The second which is slightly bigger reaches full boost at 3500rpm. Both are putting the same psi into the same engine. Are they both going to make the same power at 6000 rpm, because both are putting in the exact same amount of boost in at that time?

Hope you understand what im getting at, and can clear it up for me.

[STATUS]

I was originally under the impression that turbo choice wouldn't effect power as long as the psi its putting into the engine is the same.

For instance two turbos, 1 reaches full boost at 3000rpm. The second which is slightly bigger reaches full boost at 3500rpm. Both are putting the same psi into the same engine. Are they both going to make the same power at 6000 rpm, because both are putting in the exact same amount of boost in at that time?

Hope you understand what im getting at, and can clear it up for me.

no, think of turbos as pumps 15psi from a truck size turbo is going to pump alot more air into the engine than a small lawnmower turbo pumping 15psi

[GTScotT]

key words: volume and pressure.

two different units of measurement, and so begins your turbo saga .

[Branelly]

Hmmm weird, i just immediately thought that once the psi equals your set level your wastegate will open, there fore its not going to matter what your volume is.

In other words more volume into space = more pressure.

[Lukas 33]

Hmmm weird, i just immediately thought that once the psi equals your set level your wastegate will open, there fore its not going to matter what your volume is.

In other words more volume into space = more pressure.

Your going to need to do a fair bit of reading. No offense.

[Branelly]

Yeah none taken, just trying to get a grip on it all.

[SECURITY]

say for example you have 2 pipes, both flowing @ 15psi. one pipe is 50mm, the other is 100mm thick. you are getting more air out of the 100mm pipe than you are out of the 50mm one.

this is the same for turbos. the bigger the turbo, the more air it will push out. a bigger turbo can produce the same flow at a lower psi than a smaller turbo can produce at its highest psi. for example, a standard r33 turbo will cause a car to produce 200kw @ 12psi because it can flow a certain amount of air at 12psi. a larger turbo, for example a gt35r can produce the same 200kw but at a lower psi (say 7psi) because it can flow the same amount of air at that psi. at 12psi on the gt35 will see you pushing out almost 300kw because it can flow much more air at 12psi.

cars run on air and spark (basically). if you have more air flowing through the motor you can produce more power. it doesnt matter what the air pressure is, its the flow of air that will effect performance (ofcorse having lots of flow and high psi will produce lots and lots of power if your car can produce enough exhaust - but thats a whole other issue.

not sure if that makes sence - but i tried lol.

[Lukas 33]

say for example you have 2 pipes, both flowing @ 15psi. one pipe is 50mm, the other is 100mm thick. you are getting more air out of the 100mm pipe than you are out of the 50mm one.

this is the same for turbos. the bigger the turbo, the more air it will push out. a bigger turbo can produce the same flow at a lower psi than a smaller turbo can produce at its highest psi. for example, a standard r33 turbo will cause a car to produce 200kw @ 12psi because it can flow a certain amount of air at 12psi. a larger turbo, for example a gt35r can produce the same 200kw but at a lower psi (say 7psi) because it can flow the same amount of air at that psi. at 12psi on the gt35 will see you pushing out almost 300kw because it can flow much more air at 12psi.

cars run on air and spark (basically). if you have more air flowing through the motor you can produce more power. it doesnt matter what the air pressure is, its the flow of air that will effect performance (ofcorse having lots of flow and high psi will produce lots and lots of power if your car can produce enough exhaust - but thats a whole other issue.

not sure if that makes sence - but i tried lol.

Yeh thats a pretty good explanation. If I didnt already know that Im sure I would know that now Like everyone has said think of it as thought air in a pipe or water going through a pipe. A uni students definition. If you have beer bong with a straw at the bottom your going to need a lot of pressure in it to drink your bourbon in under 5 seconds. However if you had some garden hose on there the bourbon will flow into your mouth a lot faster and with a lot less pressure. Hence the idea of using a bigger hose when you have a 1L + beer bong.

Nah but seriously securitys and trents definition are good. Mine is just coz I have some spare time at work

[Lithium]

I sortof agree with the above explanations, however strictly speaking I'd say the engine is the "hose" bit of that explanation - the turbo is something different. The turbo has the capacity to pump a given amount of air at a given amount of pressure, but the engine has a natural capacity to move a certain amount of air at atmospheric pressure.

Any more air than that and the engine starts providing the turbo with resistance, which is (partly) where boost comes from. Pressure also increases with heat, so if the engine is able to breathe more than a turbo can move at a given pressure - then yes, you are likely to get more power on the same boost with a bigger turbo. If the turbo is able to pump more than the engine can breathe at a given boost level however, then going to an even bigger turbo will probably not make a lot of difference...

[JonnoHR31]

exactly, boost comes from the turbo pumping air against the backpressure of the motor, hook a turbo up to a pipe leading into thin air and it'll flow shitloads of air but make no boost. so with bigger cams and manifolds etc you'll flow the same amount of air on less boost with the same turbo aswell. the reason bigger turbos generally make more power is because of less restriction on the exhaust side and less effort from the bigger compressor to flow the same air, hence less heat, which means less pressure. also why heat makes such a big difference for turbo cars, we use boost as a control of airflow, so the amount of air flowing at that same boost level varies depending on the air temp.

[Lukas 33]

Do you know how the turbo and everything works? im not trying to be a smart ass. Im just curious as to how much you know.

[MrStabby]

exactly, boost comes from the turbo pumping air against the backpressure of the motor, hook a turbo up to a pipe leading into thin air and it'll flow shitloads of air but make no boost. so with bigger cams and manifolds etc you'll flow the same amount of air on less boost with the same turbo aswell. the reason bigger turbos generally make more power is because of less restriction on the exhaust side and less effort from the bigger compressor to flow the same air, hence less heat, which means less pressure. also why heat makes such a big difference for turbo cars, we use boost as a control of airflow, so the amount of air flowing at that same boost level varies depending on the air temp.

That's the best explanation i've read. I used to wonder why there wouldnt be a direct correlation between airflow and boost pressure, as i had thought that the engine would provided a fixed amount of resistance against a given flowrate. The difference is that we're talking about mass flowrate not volume flowrate. The bigger turbo = less temperature at the same pressure so more mass for the same volume. Is that it?

[Lithium]

Its not quite that easy, but as an overly simplified view of it - yes. This kind of thing is why we have compressor maps showing percentage efficiency at different flow/pressure levels. There are cases where smaller turbos have higher compressor efficiency at equivalent flow rates - this can be but isn't strictly at the higher pressure ratios/lower flow levels and could partly answer why some smaller turbos can be perceived as torquier.

Bigger compressors tend to make and hold their efficiency at higher flow rates, which is why they tend to be capable of making more power at lower boost. The turbine flow comes directly into it as well, as the easier the exhaust can leave the engine - the easier more fresh air/fuel mixture can enter the engine.

[Branelly]

I think im slowly learning something here, keep it up and i might retain some of it.

So basically boost (psi) is more a measurement of the air that's left before the engine uses it. If the engine can use the air the boost is lower, but the flow is the same or more.

@ Lukas, Yeah i understand basic turbo info. But still a little (a lot) clueless about compressor maps.

[Kwyjibo]

I think im slowly learning something here, keep it up and i might retain some of it.

So basically boost (psi) is more a measurement of the air that's left before the engine uses it. If the engine can use the air the boost is lower, but the flow is the same or more.

I don't think thats quite right. 'Boost' or the PSI is a measure of the pressure in the manifold piping created by the air. The measurement is Pounds per Square Inch so at normal atmospheric pressure its applying no force, but as you compress/pressurise the air it starts pushing on the inside of the manifold and piping. Basically the reason for this is that you now have forced a higher volume of air into the space than the atmospheric pressure at whatever altitude you are at. By increasing the air pressure(therefore overall density of air) you can force more air into the engine, without having to make larger pipes or increase port size.

So by putting in a larger turbo you are able to compress more air, it might only go to the same 'PSI' but its actually filling up a larger volume with the same compression.

An easier example to understand might be a water pump, a small water pump might be able to move around or 'pressurise' a volume of say 10. If you put that into a system with 20L you would need a bigger pump to pressurise the system to the same level. So the overall PSI might be the same but you are actually moving a much larger volume of water.

I'm by no means an expert, this is just what I've gathered so if anyone thinks I'm wrong please let me know.

[R31Nismoid]

So by putting in a larger turbo you are able to compress more air, it might only go to the same 'PSI' but its actually filling up a larger volume with the same compression.

Careful

Not filling a larger volume.

The volume of the piping/plenum/cooler is always a constant.

It is the amount of air flowing at that given pressure that differs, measured in lb's/min.

One turbo will flow 40lb's/min of air @ 15psi.

The other will be 52lb's/min of air @ 15psi.

[Shiraz200SX]

www.turbobygarrett.com has some really good explanations on it as well...

[Kwyjibo]

Careful

Not filling a larger volume.

The volume of the piping/plenum/cooler is always a constant.

It is the amount of air flowing at that given pressure that differs, measured in lb's/min.

One turbo will flow 40lb's/min of air @ 15psi.

The other will be 52lb's/min of air @ 15psi.

Good point. Am I correcting in saying that if you increase the amount of air you can compress that you could fill a larger volume at the same PSI with a larger turbo where a smaller one may struggle?