I need to get an adjustable fuel reg for my Zetec but I am a little confused what the vacuum pipe is on the reg and why it is needed. Any explanation would be good....Are there adjustable valves with and without the vacuum connection..

thanks
Dave B

The idea is that the regulator holds a set fuel pressure across the injectors.

Without it, the reg will hold a set pressure in the fuel rail but as the vacuum in the manifold changes with engine revs and throttle position, the pressure accross injectors will fluctuate, affecting the fueling.

Personally, I would avoid any regulator that doesnt reference the manifold pressure.

I have a manifold with individual throttle bodies... I don't remember seing a hole in the manifod to connect to a regulator.

A lot of people don't bother with the vacuum reference (on a normally aspirated engine, boosted ones do need it), just leave it open to atmosphere.

If you find you have trouble getting a decent idle then you might want to consider connecting it up. With the vacuum reference working the regulator will maintain a constant pressure difference between the manifold and the fuel rail - i.e. across the two ends of the injector. This means that if the ECU commands the injector to open for 1ms, it will always deliver the same quantity of fuel no matter the manifold pressure.

If the vacuum reference is disconnected, then the fuel pressure remains at a constant value relative to atmosphere. At full throttle when manifold pressure is close to atmos, this means there is very little difference to having the vac reference connected. However at idle the manifold pressure is considerably lower, so the pressure difference across the injector is now greater, so for the same 1ms open time more fuel is delivered. This means that the ECU needs to be able to open the injector for a shorter time (and accurately) to deliver the same amount of fuel as it would with the vacuum reference connected.

This can become a problem if you have very large injectors, the required injector open time at idle can be so low that the minimum change the ECU is capable of (plus the time it takes the injector to actually open and close) is a significant proportion of the required open time, so fine control is lost. This usually results in a hunting idle.

As long as your injectors aren't very big either way will work OK. If you decide to switch between the two however you will need to completely re-map your ECU.

I've been advised that if you leave it vented to atmosphere you pipe the vac take off to underneath the car. Just in case the diaphragm in the reg goes fuel will come out of it....!