NEW TO FAST,TRYING TO FIGURE OUT HOW KPA'S CONVERT TO PSI I FOUND A FORMULA ON THE NET BUT IT DOES NOT SEEM TO ADD UP TO ME ,WHEN I CONVERT IT OVER IT SAYS I HAVE 37 PSI MY MECHANICAL GAUGE IN CAR SHOWS AROUND 11 I KNOW IT CANT BE 37 BECAUSE ITS A PUMP GAS MOTOR ,THEN ON THE FAST DASHBOARD I MADE IT READ 'PSIA' AND IT SHOWS AROUND 24 'PSIA' WHICH CANT BE RIGHT EITHER ,MY KPA GRAPH SHOWS 268 AT PEAK BOOST WHAT AM I DOING WRONG .THANKS IN ADVANCE CARLOS
Need Help To Convert Kpa To Psi
- Thread starter CZALDI
- Start date
Reginald West
Senior Member
- Joined
- May 25, 2001
There are two different ways the FAST unit reads pressure.
One KPA which the WB O2 has to have. It wont read without that measurement.
PSIA is PSI for what we look for which is boost. If it is reading negative ( vacuum ) on the motor you should see it. If it is reading positive that is boost.
Always trust the MAP sensor over the mechanical gauge. The ECM reads that sensor and all of the fuel calcs come from the info the MAP sends.
Hope this helps.
You can put both KPA and PSI on your dashboard and read them while the car is running. Just add the field to your dashboard and it should show up.
Make sure you are using a 3 bar MAP and not a 2 bar. It will mess up the map and wont read correctly.
One KPA which the WB O2 has to have. It wont read without that measurement.
PSIA is PSI for what we look for which is boost. If it is reading negative ( vacuum ) on the motor you should see it. If it is reading positive that is boost.
Always trust the MAP sensor over the mechanical gauge. The ECM reads that sensor and all of the fuel calcs come from the info the MAP sends.
Hope this helps.
You can put both KPA and PSI on your dashboard and read them while the car is running. Just add the field to your dashboard and it should show up.
Make sure you are using a 3 bar MAP and not a 2 bar. It will mess up the map and wont read correctly.
REGGIE,THANKS FOR THAT INFO HOWEVER THE MAP PSIA READS AROUND 24PSI DO YOU THINK A PUMP GAS MOTOR WOULD ACCEPT THIS WITHOUT PINGING OR KNOCKING ETC I JUST DONT BELIEVE IT CAN HAVE THIS MUCH MAYBE I HAVE THE WRONG MAP SENSOR ,IF IT IS A 2 BAR FOR EXAMPLE WOULD I GET THESE HI READINGS THANKS AGAIN CARLOS
The difference between absolute pressure and gauge pressure:
Absolute pressure starts from 0 at a perfect vacuum and goes up from there. You can only have a positive absolute pressure. An intake pressure at what we think of as vacuum (ie less than atmospheric) is still a positive pressure on this scale.
Gauge pressure starts with 0 at the the regular outside air pressure. With this scale a vacuum is a negative pressure while boost is positive pressure.
A boost gauge reads gauge pressure.
A MAP sensor reads absolute pressure.
The difference between the two depends on your local atmospheric pressure. At sea level the difference is about 14.7 psi. As you go higher the difference is smaller. In El Paso, TX, for example, the difference is about 12.8 psi.
If the FAST is showing 268 kPa then your manifold pressure is = 268 x 14.7/101.325 = 38.9 psia
And subtract 14.7 = 24.2 psig. So it sounds like you are calculating that correctly.
If you had a 2 bar sensor but the FAST thought it was a 3 bar, then the 2.68 bar reading (268 kPa) would really be 1.79 bar, or 179 kPa which is 25.9 psia or 11.2 psig. Could be the problem.
John
Absolute pressure starts from 0 at a perfect vacuum and goes up from there. You can only have a positive absolute pressure. An intake pressure at what we think of as vacuum (ie less than atmospheric) is still a positive pressure on this scale.
Gauge pressure starts with 0 at the the regular outside air pressure. With this scale a vacuum is a negative pressure while boost is positive pressure.
A boost gauge reads gauge pressure.
A MAP sensor reads absolute pressure.
The difference between the two depends on your local atmospheric pressure. At sea level the difference is about 14.7 psi. As you go higher the difference is smaller. In El Paso, TX, for example, the difference is about 12.8 psi.
If the FAST is showing 268 kPa then your manifold pressure is = 268 x 14.7/101.325 = 38.9 psia
And subtract 14.7 = 24.2 psig. So it sounds like you are calculating that correctly.
If you had a 2 bar sensor but the FAST thought it was a 3 bar, then the 2.68 bar reading (268 kPa) would really be 1.79 bar, or 179 kPa which is 25.9 psia or 11.2 psig. Could be the problem.
John
JOHN,FIRST OF ALL THANK YOU VERY MUCH YOU HAVE ANSWERED MY QUESTIONT TO THE TEE ,THAT IS EXACTLY WHAT I HAVE AS FAR AS BOOST PRESSURE ,I MUST HAVE A 2 BAR MAP NOT A 3 BAR MAP,AND I HAVE THE 'FAST 'SET UP FOR 3 BAR SO THATS EXACTLY MY PROBLEM ITS READING WRONG ,MY OWN FAULT ,THANKS AGAIN , AND THANKS TO REGGIE ALSO ,I WILL POST TOMORROW AND LET YOU GUYS KNOW WHAT I FOUND
- Joined
- May 28, 2001
Just to show you what I mean, we just dyno'd my Dads car.
The gauge read 25 psi and the MAP was reading 18. Big difference. We made 600 HP and 680 ft lbs torque and we knew the turbo was capable of much more. We felt a little better knowing that there was only 18 psi going through the motor instead of 25. Now as to what is causing that, we have no idea.
More work in progress.
There again the gauge is for our benefit so we can see what is going on.
The ECM uses the MAP sensor and that is what counts.
The gauge read 25 psi and the MAP was reading 18. Big difference. We made 600 HP and 680 ft lbs torque and we knew the turbo was capable of much more. We felt a little better knowing that there was only 18 psi going through the motor instead of 25. Now as to what is causing that, we have no idea.
More work in progress.
There again the gauge is for our benefit so we can see what is going on.
The ECM uses the MAP sensor and that is what counts.
Superskwrl
New Member
- Joined
- Nov 27, 2001
Slightly off topic and sorry to steal your thread
But.... recently saw a car with a FAST, it was a 100%stock mustang it made 18" of vacum @ idle, yet the map says only 10" any ideas????
, it's a pretty well known thing that a stock 5.0 will make 18" in fact some racing series use that as 1 of the rules
But.... recently saw a car with a FAST, it was a 100%stock mustang it made 18" of vacum @ idle, yet the map says only 10" any ideas????
, it's a pretty well known thing that a stock 5.0 will make 18" in fact some racing series use that as 1 of the rules
Czaldi,
Have a look here
http://www.turbobuick.com/forums/showthread.php?s=&threadid=56137
for a conversion table for PSI -> KPA
-Bob Cunningham
bobc@gnttype.org
Have a look here
http://www.turbobuick.com/forums/showthread.php?s=&threadid=56137
for a conversion table for PSI -> KPA
-Bob Cunningham
bobc@gnttype.org
Superskwrl, are you sure of the units? I think that FAST will display only MAP in kPa, psia, and psig (although I'm not 100% sure of that). If you saw -10 psig at idle, that would be 4.7 psia, or 32.4 kPa, or 20.4" Hg vacuum, which sounds about right. I didn't think the FAST would read out in inches Hg.
John
John
TurboDANDEE
New Member
- Joined
- Mar 16, 2003
Since you changed your map sensor to a 3 bar, does the MAP PSIA reading match with your mechanical gauge? if not, how much is it off?
Also, is there anyway you can change the Base VE Table to read in PSI?
TurboDANDEE
New Member
- Joined
- Mar 16, 2003
thanx man, im gonna change mine today to see if mine matches.
Turbodandee, instead of getting a new sensor and seeing if that helps, why not just change the software setup? If the software thinks you have a 3 bar and you really have a 2 bar, just change the software to 2 bar and see if the gauge reading now matches the MAP reading. Once you confirm the problem that way, THEN go buy a new sensor.
John
John
TurboDANDEE
New Member
- Joined
- Mar 16, 2003
oh no thats not what i meant. i already have a 3 bar sensor, i was just going to add the MAP PSIA on my dashboard to see if it matches my boost gauge. i tried it yesterday and it does match, it was off by a little bit and it wasnt as consistent as my gauge, but im gonna have to go with what my ecu reads. thanks anyways for the tip though.
