___________ BOOST: WHAT YOU SEE... MAY NOT BE WHAT YOU GET ________

When it comes to what your gauge is reading that is... ​

Some Acronyms:
IMO = In My Opinion

PSIG = Pounds Per Square Inch GAUGE
PSIA = Pounds Per Square Inch ABSOLUTE

EBC or EABC =
Electronic Boost Controller
or Electronic Altitude Boost Controller

MAP = Manifold Absolute Pressure

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hobbes said:

That is incorrect. The amount of oxygen molecules per square foot (or whichever area measurement you prefer) is going to be less at higher elevation. This is easily overcome by increasing the boost level as you climb in elevation. The MAP (manifold absolute pressure) sensor incorporated into the system will read the air pressure, which not only changes with altitude change but with temperature and weather changes as well, to the electronic boost control. The EBC will keep the pressure at the manifold the same by increasing or decreasing the boost accordingly. This is why it only takes @3 lbs of boost at sea level to make 200 hp but 6+ lbs of boost at 9000 feet to make the same 200 hp.

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hobbes said:

Either way, you are NOT MAKING THE SAME HORSEPOWER AT 9000 FT THAT YOU ARE AT 1500 FT unless you are adjusting your boost pressure.

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hobbes said:

Manifold absolute pressure per say, is the pressure measured at the manifold. Which in our case, describes air pressure available to the motor in the airbox or charge box. Boost pressure would be referring to the air pressure leaving the charge side of the turbine, and what most people put a gauge to. I've always been fond of your propensity to over complicate things MH, but sometimes the KISS approach works well LOL. Everything you have stated is perfectly correct IMO. Even if a bit over stated. Your bottle example is a good way to think about it, however, a bit misleading because you have the gauge on the sealed bottle and are noting the change there. We aren't dealing with a sealed bottle(or charge box) in our scenario. The question to ask is WHY did the pressure INCREASE in the bottle? It's because of the DROP in the ATMOSPHERIC pressure around it which causes the air in the bottle to expand and increase your bottle pressure. And in hook's case, the only way to replace the atmospheric pressure we've lost as we gain altitude is to increase the boost pressure to compensate for it. Restoring the manifold pressure to a similar pressure he saw at 1500 ft.

I completely follow your logic on wanting to see MPA pressure gauge. We throw so many variables at the manifold when boosting a motor it's almost beyond comprehension. Boost, temperature, air density, humidity, intake pulse etc. But again, I think we're over complicating the issue. Which is another reason having a simple low boost, no hassle turbo kit is all the more appealing to me.

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Good discussion... I sincerely like your presentations too Hobbes.

My take... in a presentation for our members here...and fodder for discussion... not trying to tell you stuff that I believe you already know.

In the case of the bottle example above, it is a 'sealed system' ... the pressure in the bottle remains constant and is unaffected by ambient pressure. Pressure inside the bottle, the force that the air pushes against the inside of the bottle, does not increase because of a drop in ambient pressure.

In order for the pressure inside the bottle to change, the bottle itself would have to deform (think a ballon rather than a rigid bottle) The air in the bottle will not expand unless the bottle itself expands.

The gauge reads differently because the atmosphere has dropped in pressure, not the gas expanding in the bottle. That is, using the most common, "needle type" boost gauges that we all see on sleds and pickup trucks.

If the bottle doesn't crush or rupture or deform... you could take that bottle to the bottom of the Marianas trench or mount it to the Hubble telescope... and the pressure inside the bottle would remain constant.

The pressure that is displayed on the gauge changes because most boost gauges are referenced against atmospheric pressure... if that atmospheric pressure drops, as does when you climb in elevation, then a fixed pressure (as in the bottle example above), will read against an atmospheric presure referenced by the gauge. A typical boost gauge actually measures differential pressures... not absolute.

The pressure that the engine 'sees" in a turbocharged sled is, in effect, that same sealed system... much like passengers in an airliner at 40,000 feet... the pump (turbo) maintains a pressure that the people/sled-engine feel.

The goal of the turbo is maintain or increase the amount of oxygen the engine can intake... which will allow you to burn more fuel than that engine in a N/A condition.

The EBC reads info from the actual pressure, using a MAP-sensor, on the charge side of the compressor (charge-tube/airbox/manifold)... and then directs the wastegate to spin the turbine faster or slower by adjusting how much is bypassed.... and relies on the BOV to bypass any pressure spikes when the throttle plates are snapped shut.

How quickly the waste gate responds, and how the spool rates of the turbo match demand, plays a large roll in maintaining consistent charge-side pressures with constantly varying throttle positions.

"How does the motor respond to the flipper"?
That is where thoughtful/experienced design plays a huge role in how the majority of consumers enjoy the ride !

In the end... the turbo system that is designed best will maintain consistent charge side boost pressures for constantly varying throttle position with no spikes or depressions in boost pressure.... Minimal lag as we like to call it.... "Linear response"

We've not touched on Temperature here either... which adds even more dimensionality to the equation..

Trying to keep it as simple as possible.


Here is a good one from AEM's site that elaborates better than I can.

Absolute (PSIa)
PSIa is referenced to absolute zero. Absolute zero is the pressure measurement when all the pressure exerted by the atmosphere has been removed. The most common use for absolute pressure sensors is reading manifold pressure. PSIa sensors are required of you wish to measure vacuum. A PSIa sensor will read about 14.7 PSI when the sensing element is exposed to atmospheric pressure at sea level however they can be recalibrated to read zero at atmospheric and a negative number at values less than atmospheric pressure.
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