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. ;)
- Bernd
-----Original Message-----
From: jon@dakota-truck.net [mailto:jon@dakota-truck.net]
Sent: Saturday, September 22, 2007 8:55 PM
To: dakota-truck-moderator@bent.twistedbits.net
Subject: Re: DML: Was: MAP sensor; Now: Vacuum 101
"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.
Picture a sealed container with a vacuum gauge in it as well as a
couple of ports. Hook a vacuum cleaner to one of the ports and block
off the other one, and you will be able to see the level of vacuum
inside the container on the gauge. Now if you unblock the other port,
the vacuum level will drop, this is basically a vacuum leak. If the
vacuum cleaner is strong enough to overcome the leak, the vacuum level
won't drop down to zero, but it will drop down by some amount,
depending on the strength of the vacuum cleaner and the size of the
leak. Now, instead of that second port just being a vacuum leak,
picture hooking the hose from your air compressor to the second port.
The vacuum will drop even further, and you may reach zero vacuum or
even positive pressure, depending on the relative strengths of the
vaccum cleaner and the air compressor. This is basically what is
happening when you have a clogged cat or other restriction in the
exhaust system. Essentially, your engine *is* a big air compressor,
so when the exhaust becomes restricted, the pressure will go up inside
it almost like the engine is an air compressor which is treating the
exhaust tubing as an air tank and is trying to pressurize it.
That's probably as clear as mud, but it was fun to think about so
I'll hit "send" anyway. :-)
--
-Jon-
.- Jon Steiger -- jon@dakota-truck.net or jon@jonsteiger.com -.
| '96 Kolb Firefly, '96 Suzuki Intruder, Miscellaneous Mopars |
`-------------------------------- http://www.jonsteiger.com --'
This archive was generated by hypermail 2b29 : Tue Oct 02 2007 - 15:23:18 EDT