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.

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!

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.

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.

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

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.

YES, exactly, those schysters...

just for clarity, I was wrong, it is not a made up term but a virtual force, not a real force.

HOWEVER, all our mechanical distributors are use Centripetal force.

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

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?

Ok, guys, a dumb question:

Who originally started the "You can't turbocharge a 914" statement?

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.

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

Andy

I honestly don't know. Maybe some one that is against turbo's? 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.

Andy

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.

Slight other hijack for this thread that is some-what on topic;

do the hijackings of this thread suck, or do they blow?

Depends which way your piston travels ...

Andy

That you can't turbo a 914?

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.

Hum, our large vacuum pumps create a vacuum when the piston goes down... I think you mean a piston in a car...

Uhm, yeah. I thought we were talking motors.

Your vacuum pump isn't open on the other end to let air in. That's a big difference, me thinks.

A piston in an engine does not create a vacuum when moving down.

Dumb question then, with a vacuum gauge hooked up, what is it measuring?

The difference between ambient pressure and cylinder pressure in mm of mercury?

Then the piston is creating a vacuum. Yes?

it's creating a lower pressure zone. for the ambient air to blow into.

As the piston moves away from the valves, volume increases which leads to a pressure drop. Ambient air at a higher pressure moves into the cylinder.

No. A vacuum by definition is void of (almost) any matter.
http://en.wikipedia.org/wiki/Vacuum


The piston in your motor sucks in ambient air, thus your cylinders are not void of matter on the downstroke, they always have air & gasoline in them.
The whole purpose of your motor is to pump air, not to create a vacuum. It's not a vacuum pump, it's a air pump.

............So to summerize ..The engine is an air pump....and when the Exhaust valve and Intake valve are both open, at T.D.C. overlap position.....the exhaust gas scavenging cycle (lets call it)....CAN create a Vaccum in the combustion chamber. This is because the exhaust gases blowing down the exhaust pipes (from the HIGH pressures of ignition, and combustion),Can create a (small) Vaccum. This vaccum can be enhanced by an expertly designed exhaust system, and exhaust port work,(head work) valve guide shaping, valve seat angles,piston dome shape, etc.etc. ..and a proper camshaft precisely timed. The ENGINE in operation can/ will have a negative pressure in the combustion chamber at ..X...engine RPM,s based on the camshaft and many other factors, some mentioned above.

I think the confusion comes from the fact that a lot of people (incorrectly) substitute negative pressure with vacuum.

As long as you are moving matter, you don't have a vacuum. A vacuum is the absence of any matter.


The moving pistons and the expanding hot exhaust gases create negative pressure which aids in pumping large amounts of air through your engine.

At no point in the process are you creating any significant amount of empty space that is void of all matter.

Hence, no vacuum.

.........YES....my post revised....I have substituted vaccum for negative pressure, in the past myself.

Check your physics books (or google some info now-a-days)...

There is no such force as 'suction'

Go look up why a well pump can only pull up water equal to the atmospheric pressure....

Fark Fark .... now I have to take apart all the vacuum gauges I have and remark them "negative pressure" gauges. I wonder if there is enough room on the gauge face to be politically correct.


Gad, I think I will go

If the cylinder doesn't produce suction on the downstroke, why do I feel suction on my finger when I stick it in the spark plug hole? Is there a pair of lips in there? Is there a more politically correct single word?

These and all other questions of this nature will be answered in tomorrow's episode .... "Why We Can't Suck; Only Create Negative Pressure"

Oh, and a bumble bee cannot fly!

Do you have a "Negative Pressure Cleaner" at your house?

No, I have a mechanical device that moves the dirt and garbage from the viewing area to underneath the rug. From there, by some mystical & unknown force, it moves to the moon, where it can be found in great piles. On a dark night, if you look carefully with a telescope, you will see the piles.

So yellow cars arn't really faster?

i remember my shop teacher flogging the term "negative pressure". hows that for a P.C. compromise?

There have been a couple of turbo'd 914's I've seen on here, so search the topics on that & read on, but you'd have to resolve the higher compression ratio issues with a turbo to make it all run reliable I would think.

Careful Ron, or you'll have some guys sticking all sorts of thangs in there!


Nice to meet you & chat at GPR Saturday!
.... your string of various past/present "professional endeavors" reminds me of that limerick from the old Dick Van Dyke Show that only us old fartz will remember!

Cheers!
Tom T
///////

Wait....

Wait....

Wait....

WAIT!!!

Ok, I am a little confused here. I keep reading that there is a vacuum of sorts created by the piston on the down stroke. That this area of low pressure, causes or induces the intake charge to help fill that volume that was evacuated by the piston, right?
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My pistons move sideways, so how does it work then?

You must have Ornamental pistons .... in their part of the world, most things are sideways.

I had the heads off this spring. Pretty sure that there were no teeth in there either.

Does anyone else see the irony that "Dead Air" asked a question about the existence of "Vacuum" in the engine cycle?

Can't say because I wasn't there for the whole discussion with your shop teacher, but I am guessing that beyond trying to get his students to use proper scientific terms, he was probably trying to get across the idea that the amount of air in the cylinder depended on the present ambient air pressure because the amount of suction/negative pressure/vacuum remains the same for each stroke of the piston. Beyond this basic idea one can then start building on how changing things like the exhaust overlap can change some of the suction characteristics of the downstroke.