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Stallion
05-29-03, 10:48 AM
I know a lot of Corvettes (like mine) run off of the vacuum for many of tasks needed, and C3s are very vacuum oriented. But where does this vacuum come from? What creates the vacuum in the Corvette?

Thanks!! :)

TR

cntrhub
05-29-03, 11:22 AM
Vacuum is created by the down stroke (intake) of the piston. Tubes are placed on the carburetor, sometimes the air cleaner housing, and the intake manifold. Hoses are then T'd off to components that need constant vacuum. Those would be parts like the brake booster, EGR valve, igniton advance unit, heater blend doors, or the vacuum tank for the headlights...just to name a few. When the intake valve opens, the suction of the piston going down creates the vacuum. Multiply this by 8 times and the vacuum stays pretty much constant.

Stallion
05-29-03, 01:21 PM
Okay, I see what you mean!! So as the revs get higher, the vacuum gets higher, advancing the ignition (provided it's a vacuum advance)? I think I understand now. Thanks!! :)

TR

cntrhub
05-29-03, 10:35 PM
No, the vacuum gets "lower." Go back to your high school physics. "For every reaction, there is an opposite and equal reaction." Nature always wants to return to 14.7 PSI. So, say you suck a milk shake through a straw. It will take more vacuum to pull the shake up the straw. You'll suck so hard, you collapse the straw. Thus pulling the vacuum under 14.7 PSI. Release it and the straw will most likely return to it's normal shape... i.e. 14.7 PSI. So the faster the engine speed, the more the vacuum pulls (a lower pressure) on the diaphram inside the distributor advance. Just thought I'd throw that in to make it more understandable with the straw and shake analogy.

Stallion
05-30-03, 08:24 AM
Originally posted by cntrhub
No, the vacuum gets "lower." Go back to your high school physics. "For every reaction, there is an opposite and equal reaction." Nature always wants to return to 14.7 PSI. So, say you suck a milk shake through a straw. It will take more vacuum to pull the shake up the straw. You'll suck so hard, you collapse the straw. Thus pulling the vacuum under 14.7 PSI. Release it and the straw will most likely return to it's normal shape... i.e. 14.7 PSI. So the faster the engine speed, the more the vacuum pulls (a lower pressure) on the diaphram inside the distributor advance. Just thought I'd throw that in to make it more understandable with the straw and shake analogy.

Good analogy. :) I think I understand what you mean now. Thanks! :)

TR

LTdash1
06-27-03, 02:10 AM
You have to remeber that the Vacuum is stored in a resivor cannister (on the drivers side of an early C3 and as I recall in the front bumper crossmember on 1973-up cars.) The "system" uses a "check valve" to capture and store the vacuum in the cannister because of the volume required to open/close the headlights (and winsheild wiper doors for us fortunate early Shark owners.) There should be enough residual vacuum in a system to close or open the headlights with the engine completly off in a properly operating system or, you have a check valve, hose, or seal problem with the system. My expiereince, Eric B.

Stallion
06-27-03, 08:21 AM
Originally posted by LTdash1
You have to remeber that the Vacuum is stored in a resivor cannister (on the drivers side of an early C3 and as I recall in the front bumper crossmember on 1973-up cars.) The "system" uses a "check valve" to capture and store the vacuum in the cannister because of the volume required to open/close the headlights (and winsheild wiper doors for us fortunate early Shark owners.) There should be enough residual vacuum in a system to close or open the headlights with the engine completly off in a properly operating system or, you have a check valve, hose, or seal problem with the system. My expiereince, Eric B.

When my engine is off the headlights don't go down. :( They might budge a little bit, but they don't go down all the way. A problem?