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Old 11-05-2006, 09:07 PM   #1
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Question CFM vs PSI vs HP

I've been thinking and now I have a question....

The way I understand it is.....(these numbers are only used for the example)

If you have 2 turbos, one of them has a flow of 400cfm and the other has a flow of 600cfm, at say 10psi they both should make the same power (all other factors are the same, displacement, tune, ect)

This is why I say this... when filling your cylinder its only going to be able to hold the 10psi ( only as an example) since the 10psi is a measure of pressure contained inside the cylinder and your boost controller/wastegate is only allowing this much pressure. The smaller flowing turbo will get to the 10psi limit faster since its spool is quicker due to pumping a smaller amount of air BUT the bigger turbo will eventually be able to create higher boost levels due to the higher flow capacity... but it will spool slower for the same reason.

If it takes 300cfm of flow to create 10psi then the excess will be diverted out via the waste gate.. whether its the 400 or the 600cfm turbo in use. As long as cylinder pressure is the same then the power output should be the same.

The way I see it is the higher the flow capacity of the turbo the higher pressure it is capable of producing, all things being equal at the same boost levels they should make the same power.

So... am I right?


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Old 11-05-2006, 09:19 PM   #2
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Your trying to confuse flow with boost

Saying a turbo can flow 600cfm has nothing to do with the amount of boost it may have

On a Focus that turbo at 10 psi may make 200HP same turbo on a honda at 10psi may make 280HP

Flow is how much air the turbo can or is moving not how much the engine can move at a given RPM

Keep in mind that boost (what you see on the boost gauge) comes from what CANNOT get into the Cyl.

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Old 11-05-2006, 09:39 PM   #3
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I was refering to the Zetec family of motors.

If you have 10psi on the gauge, thats showing that you have 10psi in the manifold. If you have 10psi in the manifold then shouldn't have 10psi in the cylinder as well?

Quote:
Flow is how much air the turbo can or is moving not how much the engine can move at a given RPM
Isn't flow required to build pressure and once you obtain the "set amount" the rest is just waste?
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Old 11-05-2006, 09:56 PM   #4
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what if the head flow sucks this would allow 10psi in the intake and maybe not as much in the Cyl

How well an engine (zetec) works with boost has a lot to do with how well the efficiency of the entire package is , intake , cams , head flow , valve size

The turbo i use makes 200HP at 10psi on the Zetec the same turbo on a Honda at 10psi made 288HP all because how much better the intake , head , cams , work

If a turbo can flow 600cfm but the engine cannot move but 400cfm that what makes the boost

These are just random numbers dont take them as working numbers so you have an idea

Tom
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Old 11-06-2006, 05:24 AM   #5
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Yea, I got some pretty descent responses over at the "other" forum.. basically I must have gotten some wires crossed somewhere in the old noodle.

Like I said there I'll chaulk this one up as a learning experience
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Old 11-06-2006, 05:56 AM   #6
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Did you see the post i started , he wasent exactly up to par on what he was saying either

Tom
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Old 11-06-2006, 10:09 AM   #7
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yes, I was refering to the same thread I started over there... but yes I did read your posts as well.
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