[Diy_efi] Re: Question about speed density, etc.

Brian Dessent brian at dessent.net
Thu Oct 31 04:01:54 GMT 2002


Erik Jacobs wrote:

> actually.. i think i'm beginning to understand... is it similar to water in
> hose?  like how you pinch off the end of the hose with your thumb and the
> pre-hose pressure is different than the post-hose pressure?  I'm having
> trouble with the fact that the manifold pressure is different than the
> system pressure...

Yes, very much so, that is the function of the throttle -- a restriction
that creates a pressure drop.  Think of the simple case of a NA engine
at idle, on the one side of the throttle is atmospheric pressume and on
the other side is 15-20"Hg of manifold vacuum.  The closed throttle is
what creates that vacuum, as a pressure differential is required to
force the flow of air across the restriction.

The same is true in a turbocharged car as well, say you have the
throttle wide open and at full boost.  Then you close the throttle to
shift gears or coast or whatever.  Suddenly the manifold pressure drops
from (say) 10psi boost to 22" Hg because of the closed throttle.  The
pressure on the upsteam side of the throttle is still 10psi, but it will
not remain that way forever.  So, if you are fast with your shift and
get back on the throttle, you will still have full (or near-full) boost
pressure in the manifold as soon as you open the throttle, depending of
course on how wide you open it.

Brian

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