I have a boost gauge still in place from when I had an FI system that I ended up removing(long story). The gauge reads directly off the intake manifold. My question is this:

In a normally aspirated vehicle, can I use the boost gauge as an indicator of throttle position. More specifically, when the boost gauge is reading 0 psi (no vacuum), can I assume that the throttle plate is wide open?

 

Assuming that '0 psi indicated' = 1 atmosphere = 14.7psi, that's backwards. I think that boost guages indicate the difference between manifold pressure and atmospheric pressure (or at least 14.7psi).

In a NA car, the faster the air flowing through your manifold, the lower the pressure is going to be. It's not actually possible to have 0psi indicated in a NA motor; the cylinders create a vacuum when they go down, which sucks air into the cyl via the throttle body. this means that if the car is running, you will have a negative manifold pressure reading.

If this is all correct, you could do some trial and error testing (or find a smart engineer) and figure out what vacuum level indicates what throttle position. I would assume that WOT = lowest indicated psi.

Or it could turn out that I'm totally wrong, so take it for what it's worth.

 

When your at WOT the TP is as wide open as it get's..If the gauge reads o psi at the same time at WOT then you have effectively calibrated the gauge at this Throttle Position. However, relative to intermediate Throttle Positions as a function of gauge pressure I don't think it's a simple thing to do..Probably would have to get into the ECU and if possible read the TP position angle as a function of manifold press.

 

While I understand your logic in a certain way, there are certain facts that lead me to believe otherwise. The pressure indicated on my boost gauge most definitely correlates with accelerator position. When I put the pedal to the floor, the boost gauge goes firmly to 0psi, and when I back of the accelerator the pressure becomes negative. When the pedal is almost to the floor, I am at -1, -2, -3 psi, etc. The manifold pressure definitely corresponds to accelerator position. I am simply wondering if the throttle position sensor is sensing WOT at the same time as my boost gauge is seeing 0psi

 

Well, that'll learn me!

So what are you going to use this for, anyway?

 

Wide open throttle should show some vacuum but only a little. The intake tube is not large enough to eliminate all vacuum. Max vacuum would be at throttle closed position....so theoretically you could use your vacuum amount on the boost gauge to relate throttle position. Max amount-min amount = total amount of vacuum. Half that amount on the gauge would be yor 50% throttle position.

 

I basically want to use the boost gauge as a "throttle position gauge" in conjunction with my Nitrous system. It is dangerous to run nitrous on anything less than WOT, and although there is a WOT switch in the system for the nitrous activation, the boost guage can act as a monitor for WOT. With my Nismo intake, I do see full atmospheric pressue in the intake manifold at WOT. I suspect a boost gauge, or manifold pressure guage, which is, in effect, what the gauge is in a NA vehicle, can be a good measure of intake restictiveness. The least restrictive intake system should give atmospheric pressure at full throttle.

Infact, one could argue that if you are seeing atmospheric pressure at WOT in any given area within the intake system, you really can't improve on that part of the intake system. This would hold true from the the filter all the way into the intake runners. Does this make sense?

 

Keep in mind unless you had the limited ECU reflash WOT is really 80/85% of TP.