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> help me settle a debate @ my mechanic's shop, sort of a technical question
Dead Air
post Sep 20 2010, 08:31 AM
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My shop teacher way back told me many times that "ambient air pressure" pushes air into a cylinder as it descends on the intake stroke rather than the engine creating a "vacuum that pulls air" into it on the down stroke. I sort of understand the difference but it's been a topic of conversation @ beer:30 .I think my shop teacher was right.
can anyone clarify?
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ldsgeek
post Sep 20 2010, 08:34 AM
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QUOTE(Dead Air @ Sep 20 2010, 06:31 AM) *

My shop teacher way back told me many times that "ambient air pressure" pushes air into a cylinder as it descends on the intake stroke rather than the engine creating a "vacuum that pulls air" into it on the down stroke. I sort of understand the difference but it's been a topic of conversation @ beer:30 .I think my shop teacher was right.
can anyone clarify?

By moving down the piston creates a vacuum (an area of diminished pressure) which ambient pressure then tends to fill. So in reality both ways of looking at it can be right, it just depends on your point of view.
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GeorgeRud
post Sep 20 2010, 08:35 AM
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I believe that your shop teacher is right. Also explains why a turbo (which you can't use on a 914) will fill more charge into the cylinder as it's under more pressure. The air is moving from high pressure areas to low pressure areas in an attempt to equalize pressures.

The above is assuming that I remember my physics from 40 years ago!
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Cupomeat
post Sep 20 2010, 08:47 AM
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The real reason is pressure differential. Ambient air has a pressure associated with it, so anywhere that has a lower pressure will create a differential that the air will try to remove (by moving into or out of it).

If I were to be explaining it to someone, I'd go with the "Vacuum (partial) that pulls the air in" as "ambient air" is really just a baseline pressure of the surrounding air.

You can give them crap that it is not a "vacuum", but a "partial vacuum" but I'd go with their explanation first. Yours would be my choice if the piston was not creating a partial vacuum, but that one already existed.

I hope this helps.

BTW, see if you can get them to fight you on "Centrifugal" forces of a distributor weight, as a lot of places use that made up term.
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Dead Air
post Sep 20 2010, 09:01 AM
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QUOTE(GeorgeRud @ Sep 20 2010, 06:35 AM) *

I believe that your shop teacher is right. Also explains why a turbo (which you can't use on a 914) will fill more charge into the cylinder as it's under more pressure. The air is moving from high pressure areas to low pressure areas in an attempt to equalize pressures.

The above is assuming that I remember my physics from 40 years ago!


I had heard a rumor somewhere that you can't turbocharge a 914 (I don't remember where) so, thanks for confirming that!

I think a portion of my shop teachers point might have been intake charge variables regarding altitude above sea level and their effect on fuel ratios.
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RJMII
post Sep 20 2010, 09:34 AM
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QUOTE
"Centrifugal" forces of a distributor weight, as a lot of places use that made up term.


including merriam-webster's dictionary: http://mw1.merriam-webster.com/dictionary/centrifugal
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aircooledtechguy
post Sep 20 2010, 09:48 AM
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It's more complicated than that though. Because you also have valve overlap at work depending on the cam profile. This is where the exhaust valve is still slightly open as the intake valve is opening. The highly pressurized exhaust gasses rushing out, actually helps to pull the intake charge in. Exhaust design effects this greatly.

It's not just as simple as ambient pressure filling a void under vacuum.
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Cupomeat
post Sep 20 2010, 09:51 AM
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QUOTE(RJMII @ Sep 20 2010, 11:34 AM) *

QUOTE
"Centrifugal" forces of a distributor weight, as a lot of places use that made up term.


including merriam-webster's dictionary: http://mw1.merriam-webster.com/dictionary/centrifugal


YES, exactly, those schysters... (IMG:style_emoticons/default/lol-2.gif)

just for clarity, I was wrong, (IMG:style_emoticons/default/chair.gif) it is not a made up term but a virtual force, not a real force.

HOWEVER, all our mechanical distributors are use Centripetal force.
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Bartlett 914
post Sep 20 2010, 10:03 AM
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QUOTE(Dead Air @ Sep 20 2010, 10:01 AM) *

QUOTE(GeorgeRud @ Sep 20 2010, 06:35 AM) *

Also explains why a turbo (which you can't use on a 914)



I had heard a rumor somewhere that you can't turbocharge a 914 (I don't remember where) so, thanks for confirming that!



Don't take the "can't turbo charge a 914" too seriously. It is up there with "yellow cars are faster"
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jcambo7
post Sep 20 2010, 10:10 AM
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QUOTE(Bartlett 914 @ Sep 20 2010, 11:03 AM) *

QUOTE(Dead Air @ Sep 20 2010, 10:01 AM) *

QUOTE(GeorgeRud @ Sep 20 2010, 06:35 AM) *

Also explains why a turbo (which you can't use on a 914)



I had heard a rumor somewhere that you can't turbocharge a 914 (I don't remember where) so, thanks for confirming that!



Don't take the "can't turbo charge a 914" too seriously. It is up there with "yellow cars are faster"

(IMG:style_emoticons/default/agree.gif) and their is someone on this board that turbo charged his 4 cyl 914. He was recently a COTM winner too. I think in May?
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Cupomeat
post Sep 20 2010, 10:42 AM
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Ok, guys, a dumb question:

Who originally started the "You can't turbocharge a 914" statement? (IMG:style_emoticons/default/stirthepot.gif)

It has been thrown about for longer than I've been on the forum and I wonder where it came from.

Sorry for the hijack. (IMG:style_emoticons/default/chair.gif)
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SirAndy
post Sep 20 2010, 10:48 AM
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QUOTE(Cupomeat @ Sep 20 2010, 07:47 AM) *
You can give them crap that it is not a "vacuum"

(IMG:style_emoticons/default/agree.gif)

A piston, even at high rpm, does not actually create a real vacuum when moving down.

(IMG:style_emoticons/default/shades.gif) Andy
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jcambo7
post Sep 20 2010, 10:54 AM
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QUOTE(Cupomeat @ Sep 20 2010, 11:42 AM) *

Ok, guys, a dumb question:

Who originally started the "You can't turbocharge a 914" statement? (IMG:style_emoticons/default/stirthepot.gif)

It has been thrown about for longer than I've been on the forum and I wonder where it came from.

Sorry for the hijack. (IMG:style_emoticons/default/chair.gif)

I honestly don't know. Maybe some one that is against turbo's? (IMG:style_emoticons/default/confused24.gif) I honestly doin't understand why you would not be able to turbo charge a 914 at all. Especially when some one has done it. ottox914's revenna green 73 turbo that won COTM for May/June/July 2010
http://www.914world.com/bbs2/index.php?sho...hl=turbo+charge
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SirAndy
post Sep 20 2010, 11:00 AM
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QUOTE(jcambo7 @ Sep 20 2010, 09:54 AM) *
QUOTE(Cupomeat @ Sep 20 2010, 11:42 AM) *
Ok, guys, a dumb question:

Who originally started the "You can't turbocharge a 914" statement? (IMG:style_emoticons/default/stirthepot.gif)

It has been thrown about for longer than I've been on the forum and I wonder where it came from.

Sorry for the hijack. (IMG:style_emoticons/default/chair.gif)
I honestly don't know. Maybe some one that is against turbo's? (IMG:style_emoticons/default/confused24.gif) I honestly doin't understand why you would not be able to turbo charge a 914 at all. Especially when some one has done it. ottox914's revenna green 73 turbo that won COTM for May/June/July 2010
http://www.914world.com/bbs2/index.php?sho...hl=turbo+charge


It's a J.O.K.E.

(IMG:style_emoticons/default/rolleyes.gif) Andy
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RJMII
post Sep 20 2010, 11:02 AM
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QUOTE
Who originally started the "You can't turbocharge a 914" statement?


Sounds like something Mike Z would say.

I've actually seen several Turbo versions of the 914-4 since I started playing on the forums in 99. (i took a break from end of 02 to sometime in 05.
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RJMII
post Sep 20 2010, 11:04 AM
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Slight other hijack for this thread that is some-what on topic;

do the hijackings of this thread suck, or do they blow? (IMG:style_emoticons/default/lol-2.gif)
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SirAndy
post Sep 20 2010, 11:15 AM
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QUOTE(RJMII @ Sep 20 2010, 10:04 AM) *
do the hijackings of this thread suck, or do they blow? (IMG:style_emoticons/default/lol-2.gif)

Depends which way your piston travels ...

(IMG:style_emoticons/default/shades.gif) Andy
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jcambo7
post Sep 20 2010, 11:27 AM
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QUOTE(SirAndy @ Sep 20 2010, 12:00 PM) *

QUOTE(jcambo7 @ Sep 20 2010, 09:54 AM) *
QUOTE(Cupomeat @ Sep 20 2010, 11:42 AM) *
Ok, guys, a dumb question:

Who originally started the "You can't turbocharge a 914" statement? (IMG:style_emoticons/default/stirthepot.gif)

It has been thrown about for longer than I've been on the forum and I wonder where it came from.

Sorry for the hijack. (IMG:style_emoticons/default/chair.gif)
I honestly don't know. Maybe some one that is against turbo's? (IMG:style_emoticons/default/confused24.gif) I honestly doin't understand why you would not be able to turbo charge a 914 at all. Especially when some one has done it. ottox914's revenna green 73 turbo that won COTM for May/June/July 2010
http://www.914world.com/bbs2/index.php?sho...hl=turbo+charge


It's a J.O.K.E.

(IMG:style_emoticons/default/rolleyes.gif) Andy

(IMG:style_emoticons/default/dry.gif) That you can't turbo a 914?
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Cupomeat
post Sep 20 2010, 11:58 AM
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QUOTE(SirAndy @ Sep 20 2010, 01:00 PM) *

QUOTE(jcambo7 @ Sep 20 2010, 09:54 AM) *
QUOTE(Cupomeat @ Sep 20 2010, 11:42 AM) *
Ok, guys, a dumb question:

Who originally started the "You can't turbocharge a 914" statement? (IMG:style_emoticons/default/stirthepot.gif)

It has been thrown about for longer than I've been on the forum and I wonder where it came from.

Sorry for the hijack. (IMG:style_emoticons/default/chair.gif)
I honestly don't know. Maybe some one that is against turbo's? (IMG:style_emoticons/default/confused24.gif) I honestly doin't understand why you would not be able to turbo charge a 914 at all. Especially when some one has done it. ottox914's revenna green 73 turbo that won COTM for May/June/July 2010
http://www.914world.com/bbs2/index.php?sho...hl=turbo+charge


It's a J.O.K.E.

(IMG:style_emoticons/default/rolleyes.gif) Andy


Just in case it was not known, I know it is a joke, but I wondered who came up with it. I think it is funny, as if a 914 is built on different engineering principles than other cars.
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underthetire
post Sep 20 2010, 12:05 PM
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QUOTE(SirAndy @ Sep 20 2010, 09:48 AM) *

QUOTE(Cupomeat @ Sep 20 2010, 07:47 AM) *
You can give them crap that it is not a "vacuum"

(IMG:style_emoticons/default/agree.gif)

A piston, even at high rpm, does not actually create a real vacuum when moving down.

(IMG:style_emoticons/default/shades.gif) Andy



Hum, our large vacuum pumps create a vacuum when the piston goes down... I think you mean a piston in a car... (IMG:style_emoticons/default/biggrin.gif)
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