So I had an 04 F150 with the 5.4 and i installed a vacuum gauge, basically just something to fill the other hole because I also installed a trans temp gauge.

I noticed with that truck that the vacuum would routinely drop to 0 under acceleration or towing etc...

So my question is this. If I had bought a gauge that was vacuum and boost. Without forced induction of course. Would the needle actually move into the boost range? however slight?

I guess I don't understand how vacuum can drop to 0 totally, and still be accelerating.

Does this make sense? I'm wondering because now I have an 06 with the 5.4 and was thinking of doing the same setup and if it would move into the boost range why not just get that gauge instead of the regular vacuum gauge and watching it bottom out routinely.

Anyway let me know what you guys think.

Thanks
Casey

 

i have a 2005 XLT 5.4, i was wondering where and how you attached your trans temp gauge i just bpught one. thanks for the help it will be much apprecaited.

 

A vacuum gauge measures vacuum in the intake plenum between the cylinders and the throttle butterfly. With the butterfly fully open (full throttle) there is no restriction in the intake plenum, hence zero vacuum. The engine is pumping in all the air it can due to piston action. There is a lot of FLOW, but no vacuum. The other extreme is with the butterfly fully closed. Now imagine going down hill and the throttle is completely closed (foot off the go pedal). The butterfly is closed but the pistons are still trying to pump in air. Maximum vacuum is being created in the intake plenum. Without a turbocharger or supercharger, there will be no boost. Nothing there to create artificial positive pressure (boost). The real utility of a vacuum gauge is to drive the vehicle with the maximum vacuum you can. That means the butterfly is closed to the extent possible and you are using the least gas possible. Remember, more go pedal, more gas is dumped in. More go pedal, less vacuum or a gauge reading closer to zero. In the old days of carburators, you could use a vacuum gauge to tune the carb. No so much any more. Hope this helps to clear up the questions.