elementary manifold vacuum question

[Mike Morrin]

At 12:28 pm 11/8/98 -0800, Stowe, Ted-SEA wrote:
>
>I have a question for the collective. I think I have forgotten vacuum 101. I
>am trying to make sure everything is hunky dory before I strap my gm
>throttle body onto my car.I have a 77 mgb 1800 cc.
>
>this has siamesed intake and exhaust ports unfortunately.
>
>when it is cold and I hook up a vacuum gage to the intake manifold, at about
>600 rpm the needle fluctuates between 15-19 inches. meaning that the needle
>itself is nearly invisible I can see the trend. if I pinch the hose to the
>gauge, I can dampen down the 'pulses' and get it to stay steady. at higher
>rpms this effect goes away completely.

If you are only worried about the effect of the pulsing on your MAP sensor
(and the software vbehind it), then I can see no problem in providing a
"low pass filter" in the pressure signal.

Restricting the path (as you have done by pinching the tube is one way,
another is to provide a resonator (e.g. a plastic bottle) in the line
between the manifold and sensor.

You want the cut point of the filter to be low enough that the amplitude of
the pulses is reduced to a few percent. but not so low that the signal is
delayed too much.  If you use a plastic bottle as a resonator, you should
be able to "tune" it by varying the volume of the bottle.

Have fun.

Mike