Pressure Vs. Volume
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Senior Member
Joined: Jun 2004
Posts: 122
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From: Beaumont, Tx
Pressure Vs. Volume
I am new to this forced induction thing. I did just install a turbo on my truck and am very impressed with the extra power, and of course that really cool jet engine sound 
But anyway, I am wanting to learn more about pressure and volume on our trucks. I hear everyone say they run a certain psi, say 6 psi for example, which is what my wastegate is set at. But no one ever says what size thier intake pipe is, doesn't that part matter?
For example, a 4 inch line with flow a certain volume at 6 psi, but an 8 inch line with flow way more volume at the same 6 psi. Does this factor in at all on our trucks? I mean if you run a 2 inch line from the compressor (be it turbo or supercharger) and have a certain amount of horsepower, then double the size to 4 inch, with there be any horsepower gain at all?
I know some of you guys know a lot more about this than I do and hopefully can explain it where I can understand it. Thanks for the help.
Marlon
But anyway, I am wanting to learn more about pressure and volume on our trucks. I hear everyone say they run a certain psi, say 6 psi for example, which is what my wastegate is set at. But no one ever says what size thier intake pipe is, doesn't that part matter?
For example, a 4 inch line with flow a certain volume at 6 psi, but an 8 inch line with flow way more volume at the same 6 psi. Does this factor in at all on our trucks? I mean if you run a 2 inch line from the compressor (be it turbo or supercharger) and have a certain amount of horsepower, then double the size to 4 inch, with there be any horsepower gain at all?
I know some of you guys know a lot more about this than I do and hopefully can explain it where I can understand it. Thanks for the help.
Marlon
Last edited by fredman; Oct 4, 2004 at 12:38 AM.
Technical Article Contributor
Joined: Dec 1997
Posts: 9,417
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From: Windsor,Ontario,Canada
Fredman,
From what I understand by reading please correct me if wrong; If the inlet diameter of yout throttle body is 3 inches then you should not use anything larger than 3 inches. Turbos are sized by the amounf of air thay can flow by weight at a given rpm. So it is important to know the temperature of the air after it is compressed and the temperature just as it enters the engine. The colder it is the more air will flow.
All the waste gate does is allow the exhaust air to go around the turbine and lower the rpm. The turbo will still flow, for example 55 lbs/ minute of air at 10,000 rpm but opening the waste gate at 6 lbs of boost keeps the turbo from spinning past 9000 rpm. This is all using say a 2½ inch pipe. Now if you doubled the pipe diameter the turbo would have to spin a heck of a lot faster to creat 6 lbs of boost in that larger pipe. In fact it amy never reach 6 lbs so the waste gate will never open. So now you have a turbo spinning at 25000 rpm and it is moving 100 lbs/minute of air through a 5 inch pipe. More power becasue more air flow. More fuel needed. If you want foumuli have a look at; http://www.gnttype.org/techarea/turbo/turboflow.html
You can do the math.
JMC
From what I understand by reading please correct me if wrong; If the inlet diameter of yout throttle body is 3 inches then you should not use anything larger than 3 inches. Turbos are sized by the amounf of air thay can flow by weight at a given rpm. So it is important to know the temperature of the air after it is compressed and the temperature just as it enters the engine. The colder it is the more air will flow.
All the waste gate does is allow the exhaust air to go around the turbine and lower the rpm. The turbo will still flow, for example 55 lbs/ minute of air at 10,000 rpm but opening the waste gate at 6 lbs of boost keeps the turbo from spinning past 9000 rpm. This is all using say a 2½ inch pipe. Now if you doubled the pipe diameter the turbo would have to spin a heck of a lot faster to creat 6 lbs of boost in that larger pipe. In fact it amy never reach 6 lbs so the waste gate will never open. So now you have a turbo spinning at 25000 rpm and it is moving 100 lbs/minute of air through a 5 inch pipe. More power becasue more air flow. More fuel needed. If you want foumuli have a look at; http://www.gnttype.org/techarea/turbo/turboflow.html
You can do the math.
JMC
Senior Member
Joined: Feb 2002
Posts: 1,002
Likes: 1
From: Newark,CA
In the final analysis, provided your ducting is not restrictive (for our engines that would mean less than 2 1/2"), the amount of boos indicated will be predicated on how much air the compressor can put out versus how much the engine can ingest at WOT.
With a small, properly sized yurbo, we could create a situation where ther was never boost, just equilibrium of air supplied versus air consumed. However we oversize the compressor to the point where it produces more air than the engine can consume, the result is "boost".
It is the excess air delivered by the compressor that creates boost, not the converse.
With a small, properly sized yurbo, we could create a situation where ther was never boost, just equilibrium of air supplied versus air consumed. However we oversize the compressor to the point where it produces more air than the engine can consume, the result is "boost".
It is the excess air delivered by the compressor that creates boost, not the converse.