Vacuum meter reading (1 Viewer)
- Thread starter mowog67
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I may not understand your question, but at idle my 40 produces 20hg of vacuum. At wide open throttle you will produce nearly zero vacuum. Your brake booster is plumbed into the intake and will drawn whatever vacuum your engine is producing at the time. So to answer your question, it varies.
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efficiency meter.....the old days
in the old days, some GM cars and aftermarket installs included a hg guage connected to the manifold. the idea was that you could maximize the gas usage by keeping your foot off the pedal as to keep the carburetor valves more closed during the average running of the engine. higher vacuum increases the O2 concentration and runs the engine as lean as possible. also allows the fuel (gas) to become completely gasseous before it hits the cylinder. i put one on my f engine before going with a 4 barrel carb. just to track the difference. i was running a high reading of 18 mmhg before and 17 mmhg after. i modified the offenhauser manifold by augging/boring out some key spots in the aluminum casting. after that i came in at 20 mmhg as well. thanks to an ole buddy who runs a vette with a racing engine for the clues, the tools, the pinger pointing help, and the tip off.
in the old days, some GM cars and aftermarket installs included a hg guage connected to the manifold. the idea was that you could maximize the gas usage by keeping your foot off the pedal as to keep the carburetor valves more closed during the average running of the engine. higher vacuum increases the O2 concentration and runs the engine as lean as possible. also allows the fuel (gas) to become completely gasseous before it hits the cylinder. i put one on my f engine before going with a 4 barrel carb. just to track the difference. i was running a high reading of 18 mmhg before and 17 mmhg after. i modified the offenhauser manifold by augging/boring out some key spots in the aluminum casting. after that i came in at 20 mmhg as well. thanks to an ole buddy who runs a vette with a racing engine for the clues, the tools, the pinger pointing help, and the tip off.
Just for the record, your standard vacuum guage will measure inches of mecury (inHg) not mm.
Normal is around 18 at idle, 25 or so when decelerating and anywhere down to 0 when getting into the throttle. Needle should be steady, if it floats, drops or is erratic there may be something else going on the the engine. It's an old time, but very useful diagnostic tool.
Normal is around 18 at idle, 25 or so when decelerating and anywhere down to 0 when getting into the throttle. Needle should be steady, if it floats, drops or is erratic there may be something else going on the the engine. It's an old time, but very useful diagnostic tool.
Better than words alone! Mike
