I have been reading through Mark Warner's Street Turbocharging book, and I came across a section where he briefly discusses different types of pressure. He offers a diagram like the one below (I took the liberty of copying it to the best of my ability). For those who have access to the book, this is on page 23:





So as we can see, there are two different types of pressure. There are actually four different chief types in my understanding. The two others are gauge pressure and absolute pressure. Understandably, absolute pressure affects nearly everything because it doesn't seem to be absolutely critical. Let me digress for a moment.

For those who don't have the book, let me fill you in a little bit. Absolute pressure is ALWAYS 14.7psi higher than gauge pressure. Warner gives the example of a completely deflated tire. To most of us, there is no tire pressure inside the tire. However, if we were measuring the absolute pressure, we would see that it would be at 14.7psi. Now let us assume that the tire is filled with compressed air. So we take the pressure and see that our regular tire pressure gauge reads 30psi. This 30psi is the gauge pressure, and since absolute pressure is always 14.7psi higher than gauge pressure, that means that the absolute pressure in this case would be 44.7psi.

Anyways, what I am trying to find out is if knowing the total pressure (instead of static pressure) is "better" from a performance standpoint. For those wondering, what is written in the picture is Warner's exact words, not mine. Now obviously, with a turbocharger like the T-25, this probably doesn't matter, because the nipple is on the compressor-side housing. However, for those individuals with an upgraded turbocharger (such as a 16G or whatever), there is no appropriate nipple on the compressor-side housing. Thus, the consumer is able to place the nipple wherever he chooses.

We must also keep in mind that disrupting the flow of air will have an effect on the performance of the engine, which is exactly why I would be skeptical of having a pitot tube in place of an protruding nipple (like we see on most LICPs). One more digression:

Webster&#8217 said:

Pitot tube - a small, L-shaped tube which, when inserted vertically into a flowing fluid with its open end facing upstream, measures the total pressure of the fluid and hence, indirectly, the velocity of its flow.

Click to expand...

Again, for those wondering, air is technically a fluid. If you're confused, just replace the term "fluid" with "air" where appropriate.

So after all this gabble, where are we at? Well, I have been trying to come up with ways that could possibly give us the effect of total pressure without impeding on the flow of air. Keep in mind, you are not always boosting, so having this pitot tube pointing in one direction isn't going to as effective when the airflow is going in the opposite direction (aka compressor surge, which is why we have compressor bypass valves). So here is what I have come up with so far:







So what am I trying to get at here? Well, as we can see with the first picture, the line on the left (this would go to the MBC) seems to be at a disadvantage because the air has to take an abrupt 90 degree turn in order to follow the passage of the line. Therefore, putting the nipple at an angle would aide in this potential problem, allowing the air to move into the tube easier.

We can also look at the third photo that I am supplying. As you can see, the two nipples on the far right seem to have a slight advantage over the nipple on the left. Both nipples on the right are in the direct path of the flowing air, the bottom one more so.

So on to my question - am I missing a vital fact about air (and particularly about compressed air) that completely negates my main thought-process?

I don't think that static/total picture applies much to a forced-induction setup. Once you pressurize a vessel -such as, the intake between the tubo outlet and the backside of the valve or the top of the piston- the pressure should be the same throughout. I forget which law it is, Doyle's or somebody.

I thought about this before also. THen i went ahead and took a look at my stock nipple and decided that if i didnt change my aftermarket nipple too much from stock form that i should be fine. I have mine arranged the same way as stock and noticed nothing different.

Normally you use a pitot tube if you want to calculate velocity or airflow (volume, cfm). Like your picture there are 2 pressure taps, one for the static pressure & one point directly into the aiflow. What this does is creates a pressure differential between the 2, this can be measured and ploted on a calibration curve to find out the actual velocity. All we are trying to see is pressure so I see no reason for a pitot tube being needed.

Technically abosolute isn't always 14.7 psi higher then gauge pressure. Absolute is equal to gauge pressure + the atmospheric pressure. While atmospheric pressure is 14.7 psi at sea level, changes in elevation will change this atmospheric pressure as will weather at your specific location.

Defiant said:

I don't think that static/total picture applies much to a forced-induction setup. Once you pressurize a vessel -such as, the intake between the tubo outlet and the backside of the valve or the top of the piston- the pressure should be the same throughout. I forget which law it is, Doyle's or somebody.

Click to expand...

This applies when the fluid is not in motion.

Since the air in your ICP will be in motion, (your engine consuming, turbo compressing) there will be a difference between static and total pressures. There is also a difference between the pressure at the backside of the valves, and at the compressor outlet, during consumption. Points causing back-pressure in the system will cause pressure to build more rapidly before that point, causing a pressure differential within that vessel.

This can be seen at home, by taking some piping, putting a few bends in it, gauges at various points in pipe, and a large valve on one end. Apply 30psi (or whatever) to one side, and open the valve on the other side to varying degrees. The more you open the valve, the longer it takes for the backside of the tube to catch up. You can see how it pressurizes unequally within the tube. Back pressure at work. Though the issue is minimal and happens very fast, it still exists.

Anthony,
I dont see this much effecting engine performance, only how it is measured. If you want more air, you still just turn up the boost.
Why not buy some inexpensive precision low pressure gauges (http://www.mscdirect.com) and experiment for us.