Silly turbo question from someone who isn't very mechanically-savvy.

  • Thread starter Thread starter exigeracer
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exigeracer

exigeracer

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I'm a total car guy, but have never owned a car and have only done limited work/help on other people's cars. Lack of experience means I don't really know everything about how a car works. I've got a quick question that stems from my lack of understanding in compression ratios and turbos.

How come a turbo is providing "boost" at less than atmospheric pressure?

Intuitively I'd say it is because there is a lower pressure that 14.7 in the cylinder or else no air would enter to begin with. We see turbos spooling up to psi in the 20s, but sometimes they're running a conservative 12 or something. Is this the amount the turbo is providing in addition to the pressure in the cylinder? What is that pressure? At what part of the stroke is the pressure a turbo providing relevant (or measured?).

I guess I need a basic breakdown of compression ratios and turbocharging to get this, but a simple answer will satisfy my curiosity.
 
I think the deal is that they don't really use atmospheric pressure as a starting point. In their case, atmospheric pressure is 0 psi, and it goes up from there. It makes things simple. In the same way a tire gauge reads 0 psi when it's not doing anything.
 
This might help ya:
Turbocharger Compressor Calculations

Also, JIC, I've found this diagram helpful in picturing where all the components are relative to one another (WRX specific, but it generally applies for most setups I would gather):
wrx-intake-exhaust-diagram.gif
 
Because pressure gauges are in the atmosphere already they're tuned to use atmospheric pressure as 0. For instance, your tires can't realistically be full of anything less than normal atmospheric pressure, so why bother measuring below that? Anything less is a vacuum.

Naturally aspirated cars create vacuum in the intake manifold. During the intake stroke air rushes in to the cylinder to fill the void of the downward moving piston. To do that the air must accelerate through the intake tract, creating a low pressure zone that you can measure with a vacuum/boost gauge connected to the intake manifold. NA engines will never reach 100% volumetric efficiency however. The point of forced induction is to take volumetric efficiency over 100% by cramming more air into the cylinder than is naturally possible. That increases the intake manifold pressure and we call that boost.

Basically, just remember that atmospheric pressure at sea level will read 0 psi on a gauge. Anything less is vacuum.
 
What Keef said.

If the boost gauge on your car/truck is reading 12psi in the intake manifold, the absolute pressure is 12+14.7=26.7psi. To get absolute pressure, just add atmospheric pressure to whatever the gauge is reading.
 
The other thing to remember is that there's still pressure when a NA engine pulls the air in - the "Vacuum" is far from deep space. I'm not exactly sure how deep a vacuum it'd be, but it's nowhere close to absolute. You don't need much pressure difference to suck.

Even laboratory vacuums can't be absolute. We've gotten close, but no cigar...even deep space has some particles in it.
 

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