How to Read Uncommon Gauges

[zerfetzen]

Hopefully anyone with pearls of wisdom can add to this:

Fuel Pressure
A fuel pressure gauge measures the pressure in the fuel line prior to the carburetor.  Although some enthusiasts mount these under the hood, it is more useful in the cabin.  Observe the fuel pressure gauge at the track in the upper RPM range of each gear, usually toward the end of the run.  If, at high RPM, the fuel pressure reading drops at all, then the fuel system is not delivering enough fuel, the engine is running lean at that moment, and engine damage may occur.  The fuel pressure gauge may tell you the highest RPM that you can run safely before a lean condition occurs, assuming as well that the air filter is large enough to continue to supply enough air at high RPM.  If the fuel pressure drops at low RPM, then a problem is indicated in the fuel delivery system, and may be the fuel filter, fuel hose, fuel pump, fuel line, or sending unit.


Intake Manifold Vacuum
The intake manifold vacuum gauge measures the difference in air pressure between the intake and atmosphere.  The difference is measured in inches of Mercury (in.Hg).

The reading should be steady, not fluctuating.  Vacuum fluctuation may be caused by one or more cylinders not contributing their share of power, a misfiring cylinder, incorrect ignition timing, and an air/fuel mixture that is too rich or too lean.  If one of the cylinders is not contributing its share of power, then this could be caused by incorrect valve adjustment, a worn camshaft lobe, collapsed lifter, leaky valve, worn cylinder bore or piston rings, a hole in the piston, or a leaking cylinder head gasket.

On most engines, the reading at idle should be between 15-22 in.Hg.  Altitude and cam design both affect manifold vacuum.  For instance, each 1,000 ft. increase in altitude lowers vacuum by 1 in.Hg.  Performance camshafts may have increased intake-to-exhaust overlap, causing lower vacuum readings.

A vacuum reading less than 15 in.Hg may indicate either incorrect ignition timing or a worn engine.  A worn engine may have an air leak due to a blown cylinder head gasket or a leaking intake manifold gasket.

Under acceleration, the vacuum reading should drop, but remain steady.  The drop indicates that the engine is under load.  If the reading is steady at idle and fluctuates under acceleration, then the valve springs may be weak, allowing the valves to float or not fully seat.

When you release the throttle, the vacuum reading should increase to about 5 in.Hg above the reading at idle.  If the vacuum reading does not increase that much, then the pistons and rings are not sealing well.

Knowledge of the intake manifold vacuum can allow the owner to tune the engine and adjust driving habits for maximum efficiency, including fuel efficiency.

[mysil bergsprekken]

Interesting reading.

So you were saying: "On most engines, the reading at idle should be between 15-22 in.Hg."
I have a 69 charger with a 440 4 barrel. The cam is just stock as far as I know. The altitude is about 0. (not exact 0 of course but you get the idea)
What should the vacuum be then exactly? (and at what idle rpm?)

Thanks

[John_Kunkel]

If, at high RPM, the fuel pressure reading drops at all, then the fuel system is not delivering enough fuel, the engine is running lean at that moment, and engine damage may occur.

Not necessarily true, a drop in fuel pressure "at all" doesn't mean the engine is running lean it merely means the fuel pressure has dropped...if there is sufficient fuel in the bowl(s) to feed the demand there is no leaning.

IOW, if the normal fuel pressure is 7 psi and it drops to 4-5 psi at WOT and maintains that there is no danger of leaning out.