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Just havin' fun!
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Discussion Starter · #1 ·
I know I don't fully understand EVERYTHING there is to know about turbos and boosting. I know there's alot to be said on how much a turbo flows air - CFMs.

So my question is how can a bigger turbo, say 400-500cfm, pull harder at the same psi setting, say 12psi, as a stock turbo and still only be 12 psi? I know bigger turbos flow more, thus more power, but how can it still be only 12 psi? My new turbos puller harder at 12psi than the stockers did at 15.:D

Any help on this is greatly appreciated as I'm having trouble comprehending the whole thing. :rolleyes: :)
 

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580 Posts
well the turbos bigger
meaning more room
which means you need more air to to fill up that room to produce that 12 psi
know what i mean?

and those new turbos are prolly more efficient too


here a link scroll about half way down
it shows diff. turbos @15psi and how much they flow(CFM)

http://www.stealth316.com/0-frames.htm

hope this helps
 

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No longer a 3/S owner.
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stealth316

im sure the issue is covered at www.stealth316.com...there is an excellent turbo information page, something every TT owner should read. I still must say though, I'm a little baffled myself:p There is only so much area, I wouldn't think it would change with turbo size, only so much displacement. If someone could link the page that answers such a question maybe it would be helpful to a lot of people!
 

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http://www.stealth316.com/0-frames.htm

go to garage
then to turbo upgrade guide the scroll about half way down....

it'll show you diff turbos @15psi and how much CFM the flow

the reason why you get more power is cuz there is more volume to fill to get the same pressure meaning in the end still 15psi but more air in the end.

see what im saying?
 

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Hellfire Master
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Hmmm I always thought bigger turbos pulled harder at the same psi as smaller ones because it was easier for them to compress the air and thus the pressurized air is much, much cooler.

The ability to flow high CFM of air translates to the ability to more easily compress the air. Basically the turbo is forcing air into a closed space, so the more air it can flow, the easier it is for it to meet its psi quota. You aren't necessarily getting more CFM of air, the CFM is the same but the pressure is different. I.E. 400CFM of air at 1psi=800CFM of air at 2psi. The larger turbos can flow more CFM and thus are more efficient and can produce cooler pressurized air.

devilmanVISA
 

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http://www.stealth316.com/2-3s-compflowmaps.htm
http://www.stealth316.com/2-turboguide.htm

Give a *static* situation (constant RPM in this case), in the same engine at the same intake manifold air pressure and air temperature, it makes no difference at all what turbo is compressing the air.

Now in a *dynamic* situation (WOT acceleration for example), it can make a difference which turbo is used (larger could be better or worse - just depends).

Let's assuming we have two different turbos can both produce X amount of airflow to reach the desired boost level at the highest usable RPM. Here is why I would guess one would perform better than the other.

1) I would have to assume the turbo that has a more efficient turbine section (less backpressure, better heat to rotation conversion) would spool faster and so be able to supply air faster for a faster increase in engine RPM --> better car acceleration.

2) The turbo that has the exhaust section and compressor section matched so that the turbine wheel rotation puts the compressor wheel rotation (they are the same) in the most efficient compression area would produce slightly cooler output air and so marginally better street performance.

3) The turbo that has the most-efficient (easiest spinning) center section bearings would spool faster --> better car acceleration.

There are certainly more examples I can't think of at the moment.

There are also the obvious - non-equal - differences. Like smaller turbos spool faster and so produce boost earlier. Like larger turbos flow more air and so hold higher boost higher in the RPM range. Compressor efficiency (affects temperature of compressed air leaving turbo) does not make much difference if an excellent IC is used. But if stock and poor ICs are used, it could be important.
 
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