are we mesuring boost correctly?
Turbo Mini Forums

i've been having a little trouble with air/fuel ratio and boost control/pressure and thought about boost pressure in general.

i know pressure is messured in 3 ways. static pressure, velocity pressure and total pressure.

the way we normally take the pressure reading gives static pressure as we tap into the side of a pipe or plenum. velocity pressure would be messured if the tapping used turns 90deg into the air flow, and total pressure is the average of the two.

the point to this is that if fuel pressure is to rise with boost pressure, which boost pressure should be used?

as for controlling boost....
i know that for a given total pressure the static and velocity are different. so as revs climb and air flow increases so would the velocity pressure but may be not the static.

some people seem to be getting good power from say 10psi and others can run more boost but get less from it. is this due to a better flowing engine using more velocity pressure and thus need less static pressure?

yes i could go out and purchase half a dozen boost gauges and messure the differences, but thought it may be cheaper to discus the sience first and decide if i'm talking crap.

Velocity pressure only really comes into the equation where you have large changes of cross sectional area.

You can measure the static pressure in an inlet runner and a plenum and you will get different pressure readings. This is because some of the pressure energy available in the plenum is converted to velocity to get the air down the runner.

However, I'm not sure whether we are talking 1% or 10% of the total pressure being due to velocity.

Time to dig out the text books again.

Edited by Paul S on 30th Nov, 2007.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Right guys, after spending many hours on this forum and researching as much as I can for my Turbo 998 project I think i am at last beginning to understand the ins and outs of some of the problems and misconceptions.
Please bear with me as I give you my take on the above.
1) Gas Law P1,V1=P2,V2
2) Boost pressure is created by the turbo compressing air against a restriction, ie closed inlet valve
3)Valve opens and air flows with increasing velocity until it reaches equilibrium (see 1)
4)Velocity pressure does not factor in because it is only a function of the change from one equilibrium to the other. No additional energy has been added to the sytem as the equilibrium changes.
5)The method you describe to measure the "velocity pressure" is just a measure of the gas velocity in the port and is certainly not added to the measured Boost pressure.
6)Non of the pressure is due to velocity, the velocity is a function of the change from one state to another, hence the different pressure readings between inlet runner and plenum.
Regards
Dave

Edited by danboy on 22nd Dec, 2007.

Edited by Paul S on 22nd Dec, 2007.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Paul, you are dead right in what you say above regarding Boost vs quoted power. In fact I think it' totally pointless to quote x bhp @ y boost as a change in intercooler efficiency will give more power for less boost.
Add an intercooler to a given set up and you will see more power for less indicated boost.
You are right in identifying temperature as the key factor with anything to do with forced induction.
Regards
Dave

At the risk of being shot down, I also think that if you get an intercooler with 100% efficiency you could run 10.5:1 compression ratio on 15 psi!

Reality is that 100% is impossible, but the only reason to drop the compression ratio is due to temperature, not boost.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Also:

(P1*V1)/T1 = (P2*V2)/T2

So if the temperature goes up, the pressure also goes up, for the same volume of air.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

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Edited by danboy on 22nd Dec, 2007.

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Edited by danboy on 22nd Dec, 2007.

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Edited by danboy on 22nd Dec, 2007.

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Edited by danboy on 22nd Dec, 2007.

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Edited by danboy on 22nd Dec, 2007.

Pentuple post! Must be a record?! LOL.

have you got the shakes Dave??.lol

WWW.FORCE-RACING.CO.UK PLEASE CLICK HERE

Fookin hell I have only just hit the button.
But it' nice to know you are watching
Dave

website:- http://www.terryhunt.co.uk

Nice innovation this automatic posting !!!
Dave

WWW.FORCE-RACING.CO.UK PLEASE CLICK HERE

no I didn't, but I did wonder why I had to keep going back to continue.
Anyway how are you, it' a while since we spoke.
Going to have a good drink now unless Paul get's too technical.
All the best to you and yours for xmas and new year.
Dave

What you have to remember is that boost pressure all starts out as velocity energy in the compressor wheel, then converts to pressure energy as it slows down.

There will always be a velocity energy component to the total energy in the charge. So don't discount it because it may become significant at high gas speeds.

I think that you are compicating your own understanding by breaking down the sequence of events with valves opening and closing etc. Just think of it as a contiuous process.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Edited by Paul S on 22nd Dec, 2007.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

turbo tel, are you as pissed as I am about to become?
Dave

Edited by Turbo Tel on 22nd Dec, 2007.

website:- http://www.terryhunt.co.uk