"Bernd D. Ratsch" <bernd@dodgetrucks.org> wrote:
> jon@dakota-truck.net wrote:
>> "Jamie Calder" <jcalder3@cfl.rr.com> wrote:
>>> Me too...It would seem to me that a clogged cat would create more pressure
>>> (back pressure) and not vacuum. I realize this isn't so...but why? How
>>> about Vacuum 101, The Why's and Why Not's.
>> A clogged cat would indeed create more backpressure, which
>> basically creates more pressure in the engine than would normally be
>> there with a free flowing system. Pressure is the opposite of vacuum,
>> so when you combine the two, naturally the vacuum will drop. The
>> clogged cat isn't creating vacuum, its decreasing vacuum.
> Hook up a DRB-III on a Dodge vehicle with a clogged cat....watch the vacuum
> at the MAP rise to 22-25". See this all the time. ;)
That's basically my point; if I am understanding you correctly,
that is... An engine which might normally be pulling 18" and now has
a clogged cat and is reading 22-25" at the MAP has shown an increase
in pressure and thus a decrease in vacuum. The MAP sensor doesn't
measure vacuum, only pressure, so if the MAP goes up that means the
intake manifold vacuum went down.
This whole subject has had me questioning my sanity ;-) so I did
some net searching to try to verify that I wasn't going completely
crazy. Although the methods for using a vacuum gauge to check for a
clogged cat or other exhaust restriction vary, they all basically boil
down to a restriction in the exhaust system will manifest itself as a
reduced vacuum level in the intake manifold.
--
-Jon-
.- Jon Steiger -- jon@dakota-truck.net or jon@jonsteiger.com -.
| '96 Kolb Firefly, '96 Suzuki Intruder, Miscellaneous Mopars |
`-------------------------------- http://www.jonsteiger.com --'
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