Trucho-951

It all depends on the gauge, some gauges. For example, tire pressure gauges ignore atmospheric pressure. So when you read your tire pressure at 32 psi, it really means atmospheric + 32 psi for an abosulte pressure inside the tire of 14.7 + 32 = 46.7 psi total.

In engineering we identify gauge pressure with a small "g" in the units, for example 32 psig = 46.7 psia (where "psia" means psi "absolute" or total).

951Boost

Ok, so in layman's terms when folks say "I'm running 15" (or 20 or 25) "pounds of boost" they are refering to the reading on their gauge or 15/20/25 psig. In other words like with an Autometer gauge when someone sez my max boost is 15 pounds what they mean is 15psig and not 29.7psia, right??? (Even though it is 29.7 absolute if they are @ sea level)

Trucho-951

Yes. If you notice most aftermarket "boost guages" have two different scales. Below zero, vaccum measured in inches of Mercury (in. Hg.) for when the engine is running without boost.

And above zero measured in psi. for when the engine is running under boost.

So by defenition: Vaccum = pressure less than atmospheric, and Boost = pressure above atmospheric.

On our 951 boost guage, boost pressure is how far the needle swings above the 1 bar mark..

Mighty Shilling

Wow, i didn't know leadville had people! that's kinda wierd.... let alone, a Porsche? wtf?

just kidding man... Hey, if I ever make my way up there, we should go to Frisco's back country brewery and grab some brew... Also, if you see a 924S in the dead of winter with a ski rack and snowtires, it's me...

dime1622

Wow, I didn't know a boost pressure thread could go into the double digits of posts...

Btw, the conversion factor of 14.5 is slightly off - it's actually ~14.7

951Boost

Porschephile 924, sounds good PM me if you are planning to come up, love the brews at BC . You are right, not many people up here (thats why I like it ) and I have only seen 2 other Porsches that are locally owned. Trucho-951, thanks for your patience and thoughroug explanation.

APOAPR33

Ok i have a question for yall or opinions welcome, I know all of you have Porsche's but I'm in Japan right now and im looking at your conversion issue, I dont really mess with my car as far as tuning it but I don't understand the whole atmosphere pressure deal, i know the whole bar psi 0.5 bar is 7.2 psi and so on up the chain so how do I figure in the other factors like Trucho was talking about?

CPR

Simplify it.

Look at 1 bar as ZERO on the gauge. Everything past that is boost, everything under that as vac.

If you are referring to the AP conversions, simply insert your value for the standard 14.7psi (1bar). Let's say your specific AP value is 11 (instead of 14.7), you would be "creating" boost at roughly .75bar OR if your AP was higher, say 20 you would not build boost until roughly 1.4bar.

Skip Wolfe

Some confusion in the conversion - 1 bar = 14.5 psi, 1 atmosphere - 14.7 psi. 1 bar does not equal 1 atmosphere.

schip43

Wow this is timely, I just got my A$$ burned on Pelican over the stupid freaking factory gauge and 1 bar not being 14.7 lbs of boost! I guess the factory gauge reads in absolute pressure. I don't see that being used as a reference for boost anywhere but on Pelican where they say 1 bar and no boost!

But I get it now, I use the Multi meter in my Zetronix, so I don't care what the factory gauge says. It's just freaking weird! Uhh not much help there,just saying.

CPR

Sort of I suppose.

Bar was rounded down to fit sequentially, hence the 14.5 in lieu of atmosphere pressure of 1.0132. Although adopted unilaterally as correct @ 14.5 in the 30's IIRC, scientifically the correct number is 14.664. Which equates to the proper (although not uniform) round of 14.7

mikey_audiogeek

Industrial terminology can be useful.

Atmospheric pressure = 0 bar(g) or bar gauge; = 1 bar(a) or 1 bar absolute.

You sometimes see them written as barg and bara. In imperial, psig and psia.

Whoops - Trucho already pointed this out. Sorry for the double post.

Skip Wolfe

As long as we are going to get specific:

1 atm = 1.013 bar = 14.696 psi (not 14.664)
1 bar = 0.987 atm = 14.504 psi

Gotta love being an engineer - can't leave stuff like this alone.

333pg333

So basically each 10th of a bar is 1.45 psi approx.

mikey_audiogeek

and 1 psi = 3.742 buckets per cubic samurai

depending on humidity, of course