how boost is measured?

[flamefox850]

Originally posted by Battousai@Nov 24 2004, 10:34
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.

yerp..that's true.....IF we talking about same temperature generate by the 2 turbos and how much boost can they blow

but what i mean by "bigger compressor blades less 'grind' the air for given boost thus making more hp." is :

PV=nRT , P = constant (mean same boost between small and big turbo), eg. 1.5bar

V=same, R=same...so, n will be higher (more mass) as T is lower becos using bigger compressor

one more thing to remember, engine is not a big tank that u fill with pressure instead u r filling intake manifold with pressure with a 'hole' that leak. that 'hole' is combustion chamber. our turbo does not spin at constant speed to make full boost all the way...let say it spin 100k rpm to get 1.5bar@4500rpm (engine speed) then it speed up to 250k rpm to get same 1.5bar@6000rpm(engine speed)
so, to catch up engine rpm that leak so much air/pressure in intake manifold and to maintain the boost set, bigger turbo with higher cfm(high power blowing) in needed. it will spin at lower speed at lower engine rpm but medium speed at higher engine rpm.
small turbo can't do this job as it has it's own efficiency rate (lower eff, higher temp at higher boost). small turbo has to spin even more faster probably 300k or more rpm to get the same boost. and spining faster means more heat.

 

[Battousai]

Now that you explain it that way, I finally understand what you're getting at.

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

I suspect the reason why that happens is that the compressor side has been upgraded, but the turbine side has not. The bigger TD04 hybrid can input more mass of air into the engine, but the output (exhaust) flow is still the same. That causes choking in the engine resulting in pressure build up. That's why you'd seem to get full boost earlier with a big turbo.

 

[Battousai]

PV=nRT , P = constant (mean same boost between small and big turbo), eg. 1.5bar

V=same, R=same...so, n will be higher (more mass) as T is lower becos using bigger compressor


If the boost is same (1.5) between a big and small turbo, volume (V) cannot be the same.

A bigger compressor can flow the same mass (n) at a lower pressure. Resultingly, temperature will be lower.

 

[SPEEDSPiRiT]

Originally posted by Battousai@Nov 24 2004, 11:04
PV=nRT , P = constant (mean same boost between small and big turbo), eg. 1.5bar

V=same, R=same...so, n will be higher (more mass) as T is lower becos using bigger compressor


If the boost is same (1.5) between a big and small turbo, volume (V) cannot be the same.

A bigger compressor can flow the same mass (n) at a lower pressure. Resultingly, temperature will be lower.

the GURU have spoken!
the MASTER is awaken!

i dont give a fuck about wat u r talking about
i know ur car is fast can liow!

 

[flamefox850]

Originally posted by Battousai@Nov 24 2004, 11:04
If the boost is same (1.5) between a big and small turbo, volume (V) cannot be the same.

A bigger compressor can flow the same mass (n) at a lower pressure. Resultingly, temperature will be lower.

dude, that V i think is for engine volume la and not volume of turbo. engine volume can be calculate within the formula in that article as it consider engine CC and engine rpm at we calculate it. let that two thing same when we compare 2 turbochargers
this V is the 'hole' or leak that i told earlier

that's why bigger turbo have bigger O2 mass and making bigger power. we only taking care of oxygen..not the boost pressure. the engine V is the same, same P, same R but T is lower, then mass of oxygen/air molecules will be higher.
this is what we compare..."small turbo VS big turbo @ same boost..which one generate more power ?"

 

[Battousai]

Basically what we're both getting at is a bigger turbo can supply the engine's airflow requirements at a lower boost pressure right?

 

[flamefox850]

yerp..that's right.
same HP at lower boost compare to smaller turbo.

p/s: dude, i'm not a guru. still learning hehe. no hurt feeling ok..good debating makes good info

 

[Battousai]

Yea.. good discussion dude. Let me rephrase then

1. Power is directly relational to the number of 02 molecules you can cram into your cylinder. This is regardless of boost.

2. A bigger turbo can flow the same amount of air at a lower temp than a small turbo. Low temp = more 02 = more power

 

[polyxtroy]

Originally posted by Danny@Nov 24 2004, 02:58
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.

Besides misfiring, how else can an aftermarket help bigger turbos to spool faster. Jus curious. Does playing with ignition timing help? Of course not to the extent of a misfiring system which ignites when the exhaust valve is open la. Kindly enlighten.

 

[MrNismo]

Another thing to note here. Lag and boost threshold is 2 different issues.

Threshold is when the turbo start to produce noticeable boost.

Lag is how long the turbo need to achieve a desired boost.

 

[ch1ck3n]

21 psi of boost is how many bars..?

 

[dolly]

(21+14.5)/14.5

 

[SiGNAL]

Good writeup about the turbos. Could only roughly understand some part of it. :blink:

 

[dolly]

yalor when sifus talk can oni listen haha

 

[polyxtroy]

Originally posted by dolly@Nov 24 2004, 21:00
(21+14.5)/14.5

I tot 14.5 psi = 1 bar??

 

[dolly]

yes 14.5 psi is 1 bar. but our meter shows how much pressure above atmospheric pressure our turbo is producin

 

[polyxtroy]

So say I'm boostin at 15 psi, I am actually boosting at 2 bars??? But afc onli show 1 bar.

 

[dolly]

no you aren't. the pressure they show is how much over atmospheric pressure.

 

[flamefox850]

our atmosphere has pressure also..it is 1atm or 1 bar

ur boost meter only measure ur engine pressure..not including atmosphere pressure. if u unplug ur boost meter's vacuum hose, it shows 0bar is it ? hehe

 

[E V O L U T I O N]

ok i got 1 question...

how come the gforce we feel at lower rpm is greater than the g force we feel at higher rpm even though the same boost reading.

eg. boosting at 1 bar i can feel at 4k rpm

but boosting at 1 bar i cannot feel shit at 5 or 6 k rpm?