Static Test (4) - Manifold Pressure

Next thing I wanted to look at was the manifold pressure. This shows the pressure in the inlet manifold. The manifold produces a partial vacuum that is used to feed the ECU and also feeds the braking system to give some power assistance. I find this actually counter-intuitive. I used to think the more you revved the engine the better vacuum you produced. In fact it is the opposite, and the best vacuum is when the engine is idling. At idle we should be getting of the order of 5-6psi (Remember the ambient pressure is 1 bar = 15 psi). As you can see from the picture, it is working well. For the initial period I was getting 5psi and at the 9.5 min ‘step’ in the RPM it went to 6 psi. When I blipped the throttle the pressure went up to 9psi and on the overrun dropped to 3psi before recovering to 5psi. At the end of the session, when I turned off the ignition, it reverted to the ambient 15psi.

Again I have attached a screenshot of a more representative trip to see it working normally over the whole range.