RE: RE: Re: Vacuum 101

From: Bernd D. Ratsch (bernd@dodgetrucks.org)
Date: Sun Sep 23 2007 - 01:10:12 EDT


Again, refer to the plastic bottle post. There are things such as positive
and negative pressure.

Can you have more tan one exhaust valve open at once....yup - sure can.
Either through cam timing, carbon buildup, worn springs, etc.

The pistons pulling the air into the cylinders (via the intake) does keep
vacuum in the intake stream. The opening of the exhaust valves releases
that though the exhaust piping. Plug the pipe and you can't release the
vacuum. Rich or lean mixtures also cause higher, lower, or swaying vacuum
readings...misfires, carbon, higher or lower compression, cam timing, and
exhaust tuning all have a play in the actual engine vacuum.

The engine doesn't really "push" the exhaust out by itself, scavenging
effects from cam overlap and manifold tuning all come into play on that.

You don't check a plugged exhaust at idle anyway - you have to put it under
a load.

- Bernd

-----Original Message-----
From: Terrible Tom [mailto:silvereightynine@aol.com]
Sent: Saturday, September 22, 2007 11:46 PM
To: dakota-truck-moderator@bent.twistedbits.net
Subject: Re: DML: RE: Re: Vacuum 101

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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