What is PSI??

[Guest Sypher Dyas]

Forgive me for being such a newbie but what is psi?

I have seen it mentioned a lot and I dunno what it stands for.

I do know that a NA car has 0psi, i think??

[Robo]

It's just pressure...

Just like you have in your tyres... eg. they run around 35 psi... (amount of air squuzed into a certin object or for a turbo the amount of air is being pushed through the engine).

[Guest Sypher Dyas]

Ok thanks Robo:)

[GaryD]

pounds per square inch. when you talk about a turbo it how much air is being compressed into a square inch of volume. the more air, the more to go boom when it goes into the engine

[AndyMac]

Pounds per Square Inch.

PSI is new

Bar is old

1 Bar of Boost equals 14.7 psi of boost.

[Guest Sypher Dyas]

I think I saw somewhere that a naturally aspirated engine would have 0 psi. Is that right?

[GTS-t VSPEC]

The other guys have most of it right.

Yes it does mean pounds per square inch, this was the old imperial measurement for pressure. Bar has been since we moved over to metric. Americans still use imperial mesurements on somethings, and so psi is still commonly used.

Pressure is a measure of force over area.

Most engines will run at a vacuum while idling, an NA engine will go to 0psi at full throttle, a turbo engine will create a positive pressure over and above atmospheric pressure.

See'ya:burnout:

[diduc]

now that was ps(i) ezy to understand:)

[sparc]

Originally posted by PeakRPM

Pounds per Square Inch.  

PSI is new

Bar is old

1 Bar of Boost equals 14.7 psi of boost.

What makes you think Bar is old and PSI is new?

Pounds per square inch? doesnt sound too new

Bar is metric....

[AndyMac]

Sorry. I goofed up.

[rs73]

Actually I think atmospheric pressure is 1 bar or about 14psi?

That is the amount of all air above us pressing down on all objects on earth (air has mass and weight as any other materials). This is accepted as the standard pressure. We don't feel the air pressing against our skin or eyes coz our body is designed to adapt to it thru our evolution. Otherwise we'll all b crushed to death by now.

So actually NA engine actually runs the same pressure as the atmosphere at full throttle. Turbo engine runs more than the atmospheric level, depending on boost controller.

However to make things easier, the atmospheric pressure is set as 0 psi on boost gauges. So if your factory boost gauges shows 7 psi, that means the air pressure going into the engine is the 7 psi above atmospheric pressure... you are running positive pressure....

Sounds confusing, I read that thing from howstuffworks.com, however to avoid further confusion, let's accept that NA engine runs 0 psi (on boost gauge) on full throtle, and factory R33 boost is around +6-7 psi on the boost gauge.

[BOOSTMEISTER]

psi, psia, psid, psig

traditional symbols for pressure units used in hydraulics and plumbing. psi is a symbol for pound per square inch (see above). psig is a symbol for pound per square inch gauge; this means that the pressure has been read from a gauge which actually measures the difference between the pressure of the fluid and the pressure of the atmosphere. psia means pound per square inch absolute, which is the total pressure including the pressure of the atmosphere. psid, pound per square inch differential, is a symbol for a difference between two pressures, neither of which is atmospheric pressure. Corresponding symbols for pound per square foot (psf, etc.) are also used.

[BOOSTMEISTER]

OH and while I am at it

PS = pferdestärke

German word for horsepower, meaning the metric horsepower. The symbol ps is used for horsepower in both the Japanese and German automotive industies.

HP = Horse power (Metric Horse power)

a unit of power, defined to be the power required to raise a mass of 75 kilograms at a velocity of 1 meter per second. This is approximately 735.499 watts or 0.986 32 horsepower. The unit is also known by its French name cheval vapeur or its German name pferdestärke.

[rs73]

There's also BHP, I think japanese use BHP unit rather than HP, they're almost the same but there's a slight difference, can't remember the factor.

[MegaGTS4]

The japanese like to use PS as their measurement for horsepower! Conversion is 1 horsepower = 1.01387 PS

Brake horsepower (bhp), sometimes called shaft horsepower, is the amount of power available for useful work. Bhp is less than flywheel hp because of the various power losses which occur during engine operation.

To determine the brake or shaft horsepower that is delivered as useful work by an engine, the sum total of all mechanical losses must be deducted from the total HP (initial or flywheel horsepower). BHP is the reading that dyno's use! This is also sometimes called RWHP or Rear Wheel Horsepower!

Here is an online conveter so if you know your HP and want to find out the PS/KW

http://locost7.info/converter.php

MEGA

[GTS-t VSPEC]

rs73,

Your right that atmo is about 1 bar, that's why I said that turbo engines read over and above atmospheric, while NA reads 0psi relative to atmo.

So we are in fact using psig, relative to gauge pressure, that is we take all measurement relative to 1atm.

See'ya:burnout:

[B-Man]

Geez - u guys are good - I knew what PSI and Bar were (I also dive)... but 4 the rest , I gotta say learned a 5hitload !!!

Thanks !