Oops, got boost gauge w/only press psi...
12-26-03, 09:33 AM
#1
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Oops, got boost gauge w/only press psi...
I was going to buy a VDO gauge that had vacuum and press, but I accidentally bought the same design I was looking for off of ebay but didn't look at it for more than to verify that it was the same model I was looking at, so I bought it, but it doesn't have mmHg vacuum reading on it. I was told that I should get one with the vacuum too. Just wondering if it really makes that big of a difference, and what could I use the vacuum reading for? I can always just turn it around and sell it back on ebay or something, but if it'll do, why not.
12-26-03, 01:49 PM
#4
Just a brief overview I found:
A vacuum gauge shows the difference between outside atmospheric pressure and the amount of vacuum present in the intake manifold. The rotors in the engine serve as suction pumps and the amount of vacuum they create is affected by the related actions of:
Seal
Ignition system
Fuel control system
Other parts affecting the combustion process (emission devices, etc.).
Each has a characteristic effect on vacuum and you judge their performance by watching variations from normal. It is important to judge engine performance by the general location and action of the needle on a vacuum gauge, rather than just by a vacuum reading. Gauge readings that may be found are as follows:
Normal Engine Operation
At idling speed, an engine at sea level should show a steady vacuum reading between 14" and 22" HG. A quick opening and closing of the throttle should cause vacuum to drop below 5" then rebound to 23" or more.
General Ignition Troubles
With the engine idling, continued fluctuation of 1 to 2 inches may indicate an ignition problem. Check the spark plugs, spark plug gap, primary ignition circuit, high tension cables, or ignition coils.
Intake System Leakage, Or Low Compression
Vacuum readings at idle much lower than normal can indicate leakage through intake manifold gaskets, vacuum brakes or the vacuum modulator. Low readings could also be very late timing or worn rotor seals.
Exhaust Back Pressure
Starting with the engine at idle, slowly increase engine speed to 3,000 RPM, engine vacuum should be equal to or higher than idle vacuum at 3,000 RPM. If vacuum decreases at higher engine RPM's, an excessive exhaust back pressure is probably present.
A vacuum gauge shows the difference between outside atmospheric pressure and the amount of vacuum present in the intake manifold. The rotors in the engine serve as suction pumps and the amount of vacuum they create is affected by the related actions of:
Seal
Ignition system
Fuel control system
Other parts affecting the combustion process (emission devices, etc.).
Each has a characteristic effect on vacuum and you judge their performance by watching variations from normal. It is important to judge engine performance by the general location and action of the needle on a vacuum gauge, rather than just by a vacuum reading. Gauge readings that may be found are as follows:
Normal Engine Operation
At idling speed, an engine at sea level should show a steady vacuum reading between 14" and 22" HG. A quick opening and closing of the throttle should cause vacuum to drop below 5" then rebound to 23" or more.
General Ignition Troubles
With the engine idling, continued fluctuation of 1 to 2 inches may indicate an ignition problem. Check the spark plugs, spark plug gap, primary ignition circuit, high tension cables, or ignition coils.
Intake System Leakage, Or Low Compression
Vacuum readings at idle much lower than normal can indicate leakage through intake manifold gaskets, vacuum brakes or the vacuum modulator. Low readings could also be very late timing or worn rotor seals.
Exhaust Back Pressure
Starting with the engine at idle, slowly increase engine speed to 3,000 RPM, engine vacuum should be equal to or higher than idle vacuum at 3,000 RPM. If vacuum decreases at higher engine RPM's, an excessive exhaust back pressure is probably present.
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