Turbo header design

[Clarence Wood]

  I was using MacInnes's statement (page 70): Turbocharged-engine exhaust systems are different (than NA).  They duct the hot, high-pressure, high-velocity gases from the engine to the turbocharger. ...Any velocity or pressure lost in the exhaust system must be regained to the turbine housing.
  Wouldn't scavenging help keep pressure and velocity up?
  Don't be afraid to tell me I am barking up the wrong tree!  For me, this is a learning exercise.  But, MacInnes goes on to say: Let's say the exhaust gas comes out of the exhaust port of the engine at about 300 ft per sec.  
  At 300 ft per sec where does the 15 lbs of port pressure come into play??  I am not being sarcastic, just trying to learn.  Should I try to imagine a column of air traveling at that speed with 15 lbs of pressure? I am confused.  Information on how turbochargers work is hard to get.  I would really like to know where the pressure at the ports info came from as I would like to read about it as I am having trouble relating the equalization of intake pressure to the exhaust port pressure.  I am not saying that you are wrong, just that I don't understand.    
  

At 04:44 PM 1/6/99 EST, you wrote:
>I am interested in this thread as I would like to understand better the
>dynamics of a turbocharged engine.  It was always my understanding that the
>reason for a header over a manifold (in a turbo application)was to equalize
>the pressures at the ports,,,not for any kind of scavenging effect because
>there is pressure in the exhaust before the turbo usually equalling the intake
>pressure(I'm not concerned with how fast the turbo spools up at this point of
>the thread,,,, only with what is happening at full boost)  I guess my question
>is how could there possibly be scavenging if the exhaust has 15lbs of
>backpressure(for a number)???????Greg???Anyone???
>-Carl Summers
>
>In a message dated 1/6/99 12:14:51 PM Pacific Standard Time,
>clarencewood at centuryinter.net writes:
>
>   Several years ago I read an article on the design of a collector for V8's
>that produced a bolt-on 20 hp increase.  I thought the idea was very slick:
>pair the tubes to their 180 out partner, baffle their exit into the collector.
>In other words:  as tube #1 was pushing out exhaust the cylinder that tube #2
>was servicing was on the power stroke;  as tube #2 started to extract it was
>helped by the vacuum created by the extraction of tube #1 which had just
>finished.
>   My question is: why can't this be applied to the header of a turbo'd
>engine: pre-turbo.  I know that the turbine is already turning and therefor
>creating a vacuum; but, at velocities reaching .75 mach, wouldn't any tuning,
>like using the 180 out exhaust arrangement help??  I don't think the header
>tubes would have to be lengthened to any great extent but, the baffling would
>have to be in place.  Also, wouldn't the pairing of 180 out tubes help to more
>efficiently direct the vacuum created by the turbine?
> 
> Just a thought....
>