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JFairman JFairman is offline
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Join Date: Jul 2006
Location: S. Florida
Posts: 7,265
The benefit of the boost gauge with the vacuum gauge part at the bottom if you want to call it a benefit is so you can see what your intake manifod vacuum is at idle and steady cruise. That will help tell you how in tune the motor is and how efficiently it is running. The better the state of tune the higher and smoother the idle will be and the intake manifold vacuum wil be a little higher too.

At steady cruise it will give you an indication of how efficiently the motor is running.
The higher the manifold vacuum the stronger the engine is running at a given throttle position and it is overcoming intake mainifold vacuum pumping losses and gas milage will be better at steady cruise.

If vacuum is low and idle is weak the motor needs attention.
It usually means there is a vacuum leak somewhere, or the ingnition timing is retarded too far, piston rings are broken and leaking blowby, spark plugs are loaded up with burnt oil deposits, or the idle mixture AFR is way off.

The telltale needle is nice to have and looks cool. A couple few years ago they ran out of those and then they started getting them again and there was no big price diference for a gauge with the telltale needle.

It's just a needle mounted all by itself in the middle of the plastic lense on the face. It is mounted in a metal bushing and there are a couple very lightly compressed thin wavy washers and flat washers between it and the shoulder of the bushing to make very slight friction against it's movement.

The boost needle pushes it up as boost rises and the little wavy washers put just enough firction against it's movement to not hold back the boost needle but hold the telltale at the highest point it was pushed to by the needle.

I've taken these gauges apart and it's precision but very simple.
There is an approximately 2" long C shaped flexible brass pressure vessel inside that expands and straightens out a tiny bit as air pressure is applied to it.
One end of this brass vessel is fixed and the other end is free to move.
The free end has a little brass rack attached with gear teeth along one edge. Those tiny gear teeth engage with a round gear that is on the boost needles shaft. It's a rack and pinion set up.
As the C shaped brass pressure vessel expands and straightens just a tiny bit from boost pressure it pulls the rack gear with it and turns the round gear on the gauge needle shaft making it turn and display boost pressure on the gauge bezel.

Anyway, it's really simple and the parts for the telltale needle probably cost around $10 total and shouldn't cost a n y w h e r e near additional what was mentioned above.

The boost gauge is or was made by VDO not North Hollywood Speedometer so maybe Palo Alto Speedometer works on and modifies these too.
Old 08-14-2011, 06:17 AM
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