BAR vs #'s Boost
01-26-2005, 05:26 PM
#1
Track Day
Thread Starter
Join Date: Jul 2004
Location: West Chester, OH
Posts: 17
Likes: 0
Received 0 Likes
on
0 Posts
BAR vs #'s Boost
I'm new to this turbo stuff and need some explaining done.(In simple english please)
What is the difference between BAR, as it appears on the dash "turbo" gague, and #s Boost as in "17# boost pressure"? . How do they each effect performance?
Thanks for the help. As I said, I'm a Newbie.
What is the difference between BAR, as it appears on the dash "turbo" gague, and #s Boost as in "17# boost pressure"? . How do they each effect performance?
Thanks for the help. As I said, I'm a Newbie.
01-26-2005, 05:31 PM
#2
Drive-by provocation guy
Rennlist Member
Rennlist Member
Join Date: Apr 2002
Location: NAS PAX River, by way of Orlando
Posts: 10,439
Likes: 0
Received 0 Likes
on
0 Posts
Each BAR, (BARometric pressure) is 14.5 psi of boost nd 1,000 milibars 
On your stock guage, which you need to not consider as it has been proven to be very inacurate, shows 2 BAR total. I.e. 1 BAR of atmospheric pressure and 1 BAR, or 14.5 psi on top of that. As we are alsways at 1 BAR of pressure
On your stock guage, which you need to not consider as it has been proven to be very inacurate, shows 2 BAR total. I.e. 1 BAR of atmospheric pressure and 1 BAR, or 14.5 psi on top of that. As we are alsways at 1 BAR of pressure
01-26-2005, 05:37 PM
#3
Pro
Join Date: May 2004
Location: Dallas, TX
Posts: 537
Likes: 0
Received 0 Likes
on
0 Posts
Best explaination I found was on Clarks Garage
The 944 Turbo boost gauge reads the absolute pressure on the car's intake manifold. That means that the gauge not only reads boost pressure, but indicates vacuum in the intake manifold when the car is "off boost". So, when the gauge reads less than 1 bar, there is a vacuum in the intake manifold and the turbocharger isn't providing any forced flow to the cylinders. Above, 1 bar, the turbo is providing positive forced air flow to the engine. By comparison, the 911 Turbo indication is gauge pressure which means that it only indicates when the turbo is "on boost".
more at www.clarks-garage.com
The 944 Turbo boost gauge reads the absolute pressure on the car's intake manifold. That means that the gauge not only reads boost pressure, but indicates vacuum in the intake manifold when the car is "off boost". So, when the gauge reads less than 1 bar, there is a vacuum in the intake manifold and the turbocharger isn't providing any forced flow to the cylinders. Above, 1 bar, the turbo is providing positive forced air flow to the engine. By comparison, the 911 Turbo indication is gauge pressure which means that it only indicates when the turbo is "on boost".
more at www.clarks-garage.com
01-26-2005, 06:00 PM
#5
Banned
Join Date: Feb 2004
Location: In self-imposed exile.
Posts: 14,072
Likes: 0
Received 8 Likes
on
7 Posts
14.7 psi. Thus "1.5" on the bar gauge would be a manifold pressure of 14.7 x 1.5 = 22.0 psi.
People running "18 psi" setups are running 18 psi off their turbos on top of the ambient atmospheric pressure, so a manifold pressure of somewhere around 32.7 psi. Stock spec for the 951 is 1.75 bar +/- 0.05 bar, or 25.73 psi of manifold pressure.
Multiply that by the 951's 8.0:1 compression ratio to find "effective compression", somewhere in the neighborhood of 205.84 psi inside the cylinder. At 14.7 psi (atmospheric pressure, no boost) it's 117.6 - roughly the number you should see on a compression check. Hope this helps.
People running "18 psi" setups are running 18 psi off their turbos on top of the ambient atmospheric pressure, so a manifold pressure of somewhere around 32.7 psi. Stock spec for the 951 is 1.75 bar +/- 0.05 bar, or 25.73 psi of manifold pressure.
Multiply that by the 951's 8.0:1 compression ratio to find "effective compression", somewhere in the neighborhood of 205.84 psi inside the cylinder. At 14.7 psi (atmospheric pressure, no boost) it's 117.6 - roughly the number you should see on a compression check. Hope this helps.
01-26-2005, 06:10 PM
#6
Burning Brakes
Join Date: Mar 2004
Location: Seattle
Posts: 837
Likes: 0
Received 0 Likes
on
0 Posts
PSIA = Pounds per square inch absolute, includes atmospheric pressure. (944 factory gauge)
PSIG = Pounds per square inch gauge, pressure above atmosphere. (911 factory gauge)
Short version.
Lou
PSIG = Pounds per square inch gauge, pressure above atmosphere. (911 factory gauge)
Short version.
Lou
01-26-2005, 06:16 PM
#7
Pro
Join Date: Apr 2004
Location: Richmond, VA
Posts: 507
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by Porsche-O-Phile
14.7 psi. Thus "1.5" on the bar gauge would be a manifold pressure of 14.7 x 1.5 = 22.0 psi.
People running "18 psi" setups are running 18 psi off their turbos on top of the ambient atmospheric pressure, so a manifold pressure of somewhere around 32.7 psi. Stock spec for the 951 is 1.75 bar +/- 0.05 bar, or 25.73 psi of manifold pressure.
Multiply that by the 951's 8.0:1 compression ratio to find "effective compression", somewhere in the neighborhood of 205.84 psi inside the cylinder. At 14.7 psi (atmospheric pressure, no boost) it's 117.6 - roughly the number you should see on a compression check. Hope this helps.
People running "18 psi" setups are running 18 psi off their turbos on top of the ambient atmospheric pressure, so a manifold pressure of somewhere around 32.7 psi. Stock spec for the 951 is 1.75 bar +/- 0.05 bar, or 25.73 psi of manifold pressure.
Multiply that by the 951's 8.0:1 compression ratio to find "effective compression", somewhere in the neighborhood of 205.84 psi inside the cylinder. At 14.7 psi (atmospheric pressure, no boost) it's 117.6 - roughly the number you should see on a compression check. Hope this helps.
-Richard
Trending Topics
01-26-2005, 07:05 PM
#11
Banned
Join Date: Feb 2004
Location: In self-imposed exile.
Posts: 14,072
Likes: 0
Received 8 Likes
on
7 Posts
You are correct - subsititute 14.5 for 14.7. 1 "atmosphere" is 1013.3 millibars. Milli meaning "1/1000th" means that 1 atm = 1.0133 bar. The inverse of this is 0.98687. . . Taking this and multiplying it by the 14.7 psi that is 1 "atmosphere" gives you 14.5, so JFrahm is correct - thanks for the clarification. This multiplied by 8.0 = 116 psi, so THIS is the compression number for a turbo engine running no boost (such as on a compression check).
The numbers you're probably thinking of are for the n/a cars, which run a variety of compression ratios in the 9.5-10.9 range depending on year and whether it's a U.S. spec or ROW (rest of world) spec. Assuming a 9.5:1 c/r gives you 139.65 - pretty close to 140, so this is probably where you're seeing that number. It's for an n/a car.
The numbers you're probably thinking of are for the n/a cars, which run a variety of compression ratios in the 9.5-10.9 range depending on year and whether it's a U.S. spec or ROW (rest of world) spec. Assuming a 9.5:1 c/r gives you 139.65 - pretty close to 140, so this is probably where you're seeing that number. It's for an n/a car.
01-26-2005, 07:26 PM
#13
Addict
Rennlist Member
Rennlist Member
Can you get higher PSI numbers in a compression test due to heating of the air? I suppose you would have to take that into account.
The actual PSI numbers people get when doing a compression test vary quite a bit. That's why you want to compare the cylinders and look at leakdown rather than obsess over 160 vs 180 psi.
As for determining what HP you can get at x pounds of boost, that is a bit of a black art. A lot is determined by how much of the increased boost is due to actual airflow and not overheating of the intake charge. 20 psi from an overdriven K26 is not going to perform like 20 psi from a T04e. There are not as many air molecules in the intake (despite the pressure, which is higher due to the air being hotter) and the motor will not tolerate as much spark advance so the overdriven turbo does not make as much power.
-Joel.
The actual PSI numbers people get when doing a compression test vary quite a bit. That's why you want to compare the cylinders and look at leakdown rather than obsess over 160 vs 180 psi.
As for determining what HP you can get at x pounds of boost, that is a bit of a black art. A lot is determined by how much of the increased boost is due to actual airflow and not overheating of the intake charge. 20 psi from an overdriven K26 is not going to perform like 20 psi from a T04e. There are not as many air molecules in the intake (despite the pressure, which is higher due to the air being hotter) and the motor will not tolerate as much spark advance so the overdriven turbo does not make as much power.
-Joel.
01-26-2005, 07:44 PM
#14
Race Car
I see people posting here all the time stating compression numbers of over 130psi. And I've seem many claims for N/A's over 180psi. Now I don't feel so bad seeing such low numbers on my car.
I am getting 100-120psi on my 951, and according to your information, that is perfect. So, I have now adopted your way of seeing things as my own, so I can feel better about my cars
I bet some of it is to do with the heating of the air on the compression stroke though.
I am getting 100-120psi on my 951, and according to your information, that is perfect. So, I have now adopted your way of seeing things as my own, so I can feel better about my cars
I bet some of it is to do with the heating of the air on the compression stroke though.
01-26-2005, 10:56 PM
#15
Banned
Join Date: Feb 2004
Location: In self-imposed exile.
Posts: 14,072
Likes: 0
Received 8 Likes
on
7 Posts
Well, again that's "theoretically" perfect. Some heating and corresponding increase in pressure will certainly occur. I'll check the factory manuals for a compression # spec tonight, but I'm willing to bet it ain't a whole lot more than 120 psi. Your car is probably fine. How's the leakdown?