DeTomaso Mailing List: February 98, Message #228
     



From: 	Andy Poling <andy@globalauctions.com>

Subject: Re: turbo vs blower pressure (was Re: Manns car)

Date: 	Wed, 4 Feb 1998 18:23:55 +0000



On Wed, 4 Feb 1998, Doug Mann wrote:

> Re: turbo vs blower pressure.

> 

> A turbo creates a high velocity of air, it's boost is created merely by the 

> engine not being able to except the air/fuel mixture at the same rate. 

> 

> A blower is a positive displacement pump.  It derives it's pressure by 

> positively displacing a larger amount of air than an engine can except.

> 

> Small little outlet that flows a high velocity of air vs. large outlet that 

> pumps a huge mass of air, with very little velocity.



I gotta jump in here D-Mann with some sobering fizix.  It sucks, but it

makes the world go 'round (literally).



First I need to correct your implication that all centrifugal superchargers

are exhaust-driven and all belt-driven superchargers are positive

displacement pumps.  There are also belt-driven centrifugal superchargers -

I'm installing one on one of my cars.  Some of the belt-driven centrifugal

superchargers are rather large too - the one I'm installing is about 9

inches inside diameter (and that's for a small engine).



All forced induction systems employ the same means: a pump (of some sort)

forces air into the intake manifold faster than the engine is consuming it,

thus generating positive pressure in the intake manifold.



It doesn't really matter whether it's a centrifugal pump or a positive

displacement pump - regardless of the velocity of the air in the pump or at

the outlet of the pump, the manifold pressure is directly related to the

_volume_ of air that the pump is pushing into the manifold.



The average velocity (i.e. the velocities measured in a plane perpendicular

to the direction of flow, averaged) of the air in the manifold (ignoring

surges such as when you slam the throttle shut) is determined by the section

area of the manifold (higher velocities in a smaller, narrower manifold) and

the rate at which the engine is consuming air.



There are differences between the thermal properties of positive

displacement pumps and those of centrifugal pumps.  Positive displacement

pumps cause greater heating of the air when they pump it.



There are differences between the thermal properties of exhaust-driven pumps

and those of crank-driven pumps.  Exhaust-driven pumps tend to leak heat

from the exhaust into the air they're pumping.



And there are differences between the pressurization capabilities of

centrifugal pumps and those of positive displacement pumps.  Positive

displacement pumps tend to be capable of maintaining higher output

pressurization than centrifugal pumps (given sufficient pump output to

attain the pressure).



But none of these differences lead to a different average air velocity in the

intake manifold.





> On a turbo, if you place a nozzle facing upstream in the center of flow 

> it will show you a higher psi than if you did the same thing under a 

> blower.  Assuming both are creating an equal static manifold pressure.



This may be true if the pressure transducer "nozzle" is very near the turbo

outlet, but that is just an invalid pressure sensor installation where

you're going to get some ram effect at the nozzle opening.  Such a sensor

location won't really provide useful information though...



-Andy

#3822



72 Pantera - Rocky         91 Miata - Steve (bionic)            96 A4Q - Rudolf

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