how boost is measured?

[absolut250]

hi all, i am new. just want to know how boost is measured is it by the pressure in the cylinder or in the turbo? as someone was telling me that 1bar reading on a small turbo is different from a 1 bar on bigger turbo.

 

[Battousai]

Boost is measured by the air pressure in your intake plenum. Again, just pressure, not mass of air. It's really a measurement of how hard the turbo is working.

10 cfm of air can be compressed to 1 bar.
100 cfm of air also can be compressed to 1 bar.

That's the diff between a big and small turbo. Big turbo may be flowing the same amount of air at 0.5 bar as a small turbo at 1 bar.

In simple terms, a big turbo doesn't have to work as hard as a small turbo to make the same power.

 

[dolly]

but then, for a given engine rpm and displacement, and pressure, any turbo flows the same amount of air. (assuming ur engine can make the turbo spool fast enough to generate the pressure in the first place)

http://www.stealth316.com/2-3s-compflowmaps.htm (scroll all the way down)

 

[Battousai]

>>At a given RPM and at the same plenum air pressure and temperature, the same amount of air flows regardless of which turbo is used.

But in the real world, at a given RPM and plenum air pressure, the temp is likely to be much higher if you use a smaller turbo right? Higher temp therefore less air is flowing.


I'm not a jedi yet. Also need to learn more :(

 

[dolly]

at least ur gonna be jedi, i still stuck at padawanabe lar :(

true oso battousai hehe, but must i guess oso must see what efficiency that thing is running too right?

 

[Battousai]

I hope to be a jedi some day lah. Now just a No0B. See if SDF sifu got any words of wisdom...

 

[SPEEDSPiRiT]

battousai,

can u teach me all ur knowledge :P
or should i print out the manual?
:(

 

[flamefox850]

let musang padawan tell u what i know..

what i know, cfm = cube feet per minutes that is how fast a turbo can supply the air. cube feet is like cm3 that is volume or in malay it is isipadu.
"berapa banyak isipadu udara boleh dibekalkan dalam 1 minit"

so, cfm in real world means how fast a turbo can reach it's full boost (that we set), . Turbo with higher cfm can compress more air (higher boost eg. 1.8bar) into the engine in just a blink of an eye, after it start spooling la..not including the lag time.
then, higher cfm turbo will get full boost faster and create max torque at a lower rpm..but lag is the problem. when it's start spooling then air will suddenly rush into the engine

but not to forget, bigger compressor blades tends to create less heat when compressing air compare to smaller blades..this is becos bigger blades needs only low speed spooling to compress given boost compare to smaller ones
less heat = more power

*the comparison of lower and higher cfm turbo should be on the same engine and same rpm range

correct me if i'm wrong

 

[dolly]

please keep this thing going, its a damn headache thingy forme to understand haha

 

[Yong_5290]

juz my 2 cent...try using nokia 6600 o those wit converter...

 

[Battousai]

let musang padawan tell u what i know..

so, cfm in real world means how fast a turbo can reach it's full boost (that we set), .
Actually, its how much air the turbo can flow at its peak efficiency. If, for example a turbo is at its peak at 1.2 bar, when you boost it up to 1.5 bar, you start losing power because the air is too hot, the turbo is spinning too fast and it simply cannot flow that much air. When you push the turbo too hard (high boost, high RPM) you'll find boost pressure starts to drop off.

Turbo with higher cfm can compress more air (higher boost eg. 1.8bar) into the engine in just a blink of an eye, after it start spooling la..not including the lag time. then, higher cfm turbo will get full boost faster and create max torque at a lower rpm..but lag is the problem. when it's start spooling then air will suddenly rush into the engine
I'm not sure about the getting full boost faster. It depends on whether the turbo is correctly matched to the engine. You can slap on a big-ass high CFM turbo to a L5 engine and it'll NEVER make full boost.

but not to forget, bigger compressor blades tends to create less heat when compressing air compare to smaller blades..this is becos bigger blades needs only low speed spooling to compress given boost compare to smaller ones
less heat = more power
It's not the compressor blades that create heat. Compression of air creates heat. The tighter you compress air, the hotter it gets. It follows that if you want a lot of air to flow thru a small turbo, you must compress it a lot, therefore causing heat.

*the comparison of lower and higher cfm turbo should be on the same engine and same rpm range
For comparison sake, yeah. But we're only talking about the compressor side, without even considering the exhaust side. A turbo that flows more, will flow more whether you put it on an engine or a hair dryer.

 

[flamefox850]

Originally posted by Battousai@Nov 24 2004, 00:03
let musang padawan tell u what i know..

so, cfm in real world means how fast a turbo can reach it's full boost (that we set), .
Actually, its how much air the turbo can flow at its peak efficiency. If, for example a turbo is at its peak at 1.2 bar, when you boost it up to 1.5 bar, you start losing power because the air is too hot, the turbo is spinning too fast and it simply cannot flow that much air. When you push the turbo too hard (high boost, high RPM) you'll find boost pressure starts to drop off.

ermmmm confusing here. cfm means cubic feet per minutes...then should be nothing to do with 1.2bar or 1.5bar.
i'm still with my theory cfm relates to how fast a turbocharger compress air aka how fast it reach full boost

Battosai: u can try downloading this video..showing how slow boost meter climb up. probabbly using low cfm turbo.
CLICK HERE

can see very slow climbing 1.8bar


p/s: of cos what cfm we r talking here is compressor side. put high cfm turbocharger with big turbine housing on L5 how come will boost la..cannot boost one. logical la hehe. that's lag. if want also can try medium compressor with smaller turbine housing...then lag is acceptable and noticeable decreasing of time to full boost

 

[Danny]

cfm in my understanding = airflow.

A small turbine, eventhough is spinning at the same speed as a big turbine, produces less airflow. That's why big turbines at the same bar measurement produces more power.

 

[Battousai]

How fast it reaches full boost depends on your turbine side and wastegate settings.

 

[flamefox850]

hmmmmm

cfm still cubic feet per minute means how much volume of air can be blow in a minute. higher cfm = faster air flow (same boost) or higher boost (same time)

about the small compressor VS big compressor and how small compressor generate more heat..u all can read here :

-----------------------------------------------------------------------------------------------
"Compressor
The compressor is the part of the turbocharger that compresses air and pumps it into the intake manifold. Air molecules get sucked into the rapidly spinning compressor blades and get flung out to the outside edge. When this happens, the air molecules get stacked up and forced together. This increases their pressure.
It takes power to do this. This power comes from the exhaust side of the turbo, called the Turbine. Not all of the power that comes from the turbine goes into building pressure. Some of the power is used up in heating up the air. This is because we lowly humans cannot build a perfect machine. If we could, all of the power would go into building pressure. Instead, because of the design of the compressor, the air molecules get "beat up", and this results in heat. Just like rubbing your hands together will warm your hands due to the friction between your hands, the friction between the compressor and the air and between the air molecules themselves will heat up the air.

If you divide the amount of power that goes into building pressure by the total power put into the compressor, you get the efficiency of the compressor.

For example, if the compressor is 70% efficient, this means that 70% of the power put into the compressor is used in building air pressure. The other 30% of the power is used heating up the air. That is why we like high efficiency compressors; more of the power is being used on building pressure and less is used heating up the air. Turbos, Paxtons, and Vortechs are all centrifugal superchargers. The are called this because the centrifugal force of flinging the air molecules from the center of the housing to the outside edge is what builds air pressure. The maximum efficiency of these kinds of superchargers is usually between 70% and 80%. Roots blowers, like the 6-71, work differently and have much lower efficiency, like about 40%! With those, when you try to build lots of boost you have to put in a lot of power and more than half of it gets used heating up the air instead of raising pressure."

-----------------------------------------------------------------------------------------------

so, conclusion is efficiency rate that decide a good or bad turbo in terms of heat generates not CFM. higher efficiency means less heat generate and this often occur with bigger compressor.

bigger compressor blades less 'grind' the air for given boost thus making more hp.

with high efficiency turbo, less heat generates then more air flow..bigger cfm means more air can be pump in less time


*revision : Compressor flow rate and efficiency

 

[flamefox850]

Originally posted by Battousai@Nov 24 2004, 01:35
How fast it reaches full boost depends on your turbine side

turbine side...ermmm this is more like LAG.

i dun wanna talk about lag here...i say bigger compressor boost faster when the boost starts to come in.

let say we have 1 engine with one stock D turbo (small) and aftermarket K turbo (bigger) boosting at 4th gear generating 1.5bar

1. stock D turbo starts boosting at 3k rpm (less LAG) and full boost at 5k rpm

2. aftermarket K turbo starts boosting at 4k rpm (more LAG) and full boost at also 5k rpm

then, which one is fast ? hehe

wastegate not include here also kuikuikui

 

[Danny]

the bigger turbo won't usually reach its peak at 5000 rpm, more likely 6000-7000. It might start spooling slower but it has better power at top end.

The slower spooling can be remedied with aftermarket engine management.

 

[Battousai]

turbine side...ermmm this is more like LAG.

i dun wanna talk about lag here...i say bigger compressor boost faster when the boost starts to come in.
How does the compressor boost faster without the turbine side? The compressor can't spin on its own.

let say we have 1 engine with one stock D turbo (small) and aftermarket K turbo (bigger) boosting at 4th gear generating 1.5bar

1. stock D turbo starts boosting at 3k rpm (less LAG) and full boost at 5k rpm

2. aftermarket K turbo starts boosting at 4k rpm (more LAG) and full boost at also 5k rpm

wastegate not include here also kuikuikui
Exactly how fast boost kicks in is entirely controlled by the wastegate. If you set your boost controller to maximum response, turbo D would probably start making boost by 2.5k rpm and be on full boost by 3000 rpm. Turbo K would start boosting at 3.5k rpm and be on full boost by 4.5k rpm. Who made boost faster?

 

[Battousai]

bigger compressor blades less 'grind' the air for given boost thus making more hp.

Very simply, a bigger compressor can flow more mass of air with less compression, resulting in the lower output temperature. The ideal gas law provides for this

PV=nRT

Increase the pressure, (P) and temp (T) must increase because number of air molecules cannot change (N) and ® is a constant number. This is where most of the heat comes from.

 

[flamefox850]

Originally posted by flamefox850@Nov 24 2004, 02:22
hmmmmm

cfm still cubic feet per minute means how much volume of air can be blow in a minute. higher cfm = faster air flow (same boost) or higher boost (same time)

about the small compressor VS big compressor and how small compressor generate more heat..u all can read here :

-----------------------------------------------------------------------------------------------
"Compressor
The compressor is the part of the turbocharger that compresses air and pumps it into the intake manifold. Air molecules get sucked into the rapidly spinning compressor blades and get flung out to the outside edge. When this happens, the air molecules get stacked up and forced together. This increases their pressure.
It takes power to do this. This power comes from the exhaust side of the turbo, called the Turbine. Not all of the power that comes from the turbine goes into building pressure. Some of the power is used up in heating up the air. This is because we lowly humans cannot build a perfect machine. If we could, all of the power would go into building pressure. Instead, because of the design of the compressor, the air molecules get "beat up", and this results in heat. Just like rubbing your hands together will warm your hands due to the friction between your hands, the friction between the compressor and the air and between the air molecules themselves will heat up the air.

If you divide the amount of power that goes into building pressure by the total power put into the compressor, you get the efficiency of the compressor.

For example, if the compressor is 70% efficient, this means that 70% of the power put into the compressor is used in building air pressure. The other 30% of the power is used heating up the air. That is why we like high efficiency compressors; more of the power is being used on building pressure and less is used heating up the air. Turbos, Paxtons, and Vortechs are all centrifugal superchargers. The are called this because the centrifugal force of flinging the air molecules from the center of the housing to the outside edge is what builds air pressure. The maximum efficiency of these kinds of superchargers is usually between 70% and 80%. Roots blowers, like the 6-71, work differently and have much lower efficiency, like about 40%! With those, when you try to build lots of boost you have to put in a lot of power and more than half of it gets used heating up the air instead of raising pressure."

-----------------------------------------------------------------------------------------------

so, conclusion is efficiency rate that decide a good or bad turbo in terms of heat generates not CFM. higher efficiency means less heat generate and this often occur with bigger compressor.

bigger compressor blades less 'grind' the air for given boost thus making more hp.

with high efficiency turbo, less heat generates then more air flow..bigger cfm means more air can be pump in less time


*revision : Compressor flow rate and efficiency

aihhh..tired of explaining la.

i say wastegate not included or turbine housing not included mean we r not going to take that 2 thing in our 'calculation' laaa
i know about WG setting makes different..it's stepping motor of elec.bc can do this.
i dun have any specific turbocharger's model to compare..so, hard to explain.

ok like this..one more example

we have one stock L2 turbo and one hybrid TD04/L2 turbo (Td04 compressor with L2 turbine housing) and using gear 4 also and aiming 1.5bar. other spec like engine, ecu, boost controller, wg are all the same
after the test, this car's result probably like this :

L2 stock turbo : boost start@2500rpm, peak boost@5500rpm

L2/TD04 hybrid turbo : boost start@3500rpm, peak boost@5300rpm

this figure could possibly happen. even though the hybrid turbo have bigger compressor housing but it can fill the engine with full boost in just seconds, becos the td04 compressor have better cfm than L2 compressor

now got it what i mean ?

p/s: only compare 2 things when other things not included