Re: RE: Re: Vacuum 101

From: Terrible Tom (silvereightynine@aol.com)
Date: Sun Sep 23 2007 - 00:46:25 EDT


Bernd D. Ratsch wrote:

> Every plugged cat vehicle i've seen does the same thing: Vacuum goes higher
> (numerically). That's even one of the tests we do to check for a plugged
> cat...watch the vacuum go up as rpm's go up under a load.
>
> Normal conditions: Vacuum will drop as throttle blade angle increases (under
> a load)...vacuum will go higher under decel (closed throttle).
>
> Now, hang a few exhaust valves open (or larger overlap cams) and vacuum
> drops lower at idle (10-13" for an example). Plug the exhaust and watch the
> vacuum climb slowly back up.
>
>
>
>>"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.
>>
>
>
> jon@dakota-truck.net wrote:
>>
>> 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. :-)
>>

mmm I dunno.. something doesnt sound right. Can you have more than one
exhaust valve open at the same time? I guess what I'm thinking is that
a restriction in the exhaust will cause higher pressure in the exhaust
system before the point where its restricted - but can that have any
effect on INTAKE MANIFOLD vacuum? What Jon was saying was pretty much
along the lines that I was thinking. What effect does the restricted
exaust have on intake vacuum/pressure?

Basic fundimentals review here...

The suction of multiple pistons pulling air in through open intake
valves is what keeps vacuum in the manifold.

The throttle blade is what will decrease the amount of vacuum (negative
air pressure) when the throttle is opened

faster engine RPM with the throttle closed (accelerating and then
letting off the throttle) will cause more vacuum (higher values of
negative pressure)

Ok - all that said... I dont see how a restricted exhaust will have any
sort of effect on intake manifold vaccum?

I still think restricted exhaust would cause lower vacuum levels because
the engine has to fight and push the exhaust out - using more power and
choaking engine speed... less engine speed means the pistons are moving
slower and sucking air in slower meaning less vacuum in the manifold....

Right?

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