Relative pressure vs absolute pressure
Moderators:R!C0, JonMan, RickS
Hi,
when comparing pressure to other peoples maps on other ecu's, one usually talkes about the kind of pressure you see on the boost-gauge. Which is relative to barometric pressure.
I just wonder how to correctly translate the Map RAW into this relative pressure.
Max pressure logging: 180kpa (190kpa spikes sometimes)
Engine off: 84kpa (+-)
Is my relative pressure then approx 100-110 kpa? I would think so. But my boostgauge shows 1.3 bars (130kpa) and stays on 0 with the engine off and minus on low throttle. But the needle is vibrating a bit at full boost. I think my boostgauge is not as accurate as the map-sensor, so I am probably at approx 1 bar then. But I cant help thinking that there is something that I have forgotten to calculate?
Thanks in advance for any help
Vids
when comparing pressure to other peoples maps on other ecu's, one usually talkes about the kind of pressure you see on the boost-gauge. Which is relative to barometric pressure.
I just wonder how to correctly translate the Map RAW into this relative pressure.
Max pressure logging: 180kpa (190kpa spikes sometimes)
Engine off: 84kpa (+-)
Is my relative pressure then approx 100-110 kpa? I would think so. But my boostgauge shows 1.3 bars (130kpa) and stays on 0 with the engine off and minus on low throttle. But the needle is vibrating a bit at full boost. I think my boostgauge is not as accurate as the map-sensor, so I am probably at approx 1 bar then. But I cant help thinking that there is something that I have forgotten to calculate?
Thanks in advance for any help
Vids
Re: Relative pressure vs absolute pressure
Hi,
Obviously we are expecting absolute pressure to be around 100kpa (1Bar). With you engine off you are reading 84kpa and this is your absolute pressure. Depending on which ECU you have you may have an option called 'Load Offset'. This can be used scale the MAP raw value meaning you can change the map raw value to relative by subtracting the value of map raw at absolute reading.
MAP Raw is then fed into the MAP sensor table which decides the MAP sensor output based on the input. I pressume your boost gauge is also using a MAP sensor similar to the one run by the ECU? if this is the case we have found the pressure sensors we use, the ones everyone uses, to be extremely reliable and accurate so i would start looking for reasons for this difference like location of sensor or gauge accuracy etc.
The gauge should indeed read 0 when the engine has stopped and may go to minus on small throttles however the negative figure you see means the turbo will be being drawn round by the engine rather then the exhaust gases driving it. This is obviously both ineficient and a power drain and is cured by antilag. however it depends on the application you are using the car for as antilag will shorten turbo and exhaust life when used regularly on the road.
Hope this helps
Ed
Obviously we are expecting absolute pressure to be around 100kpa (1Bar). With you engine off you are reading 84kpa and this is your absolute pressure. Depending on which ECU you have you may have an option called 'Load Offset'. This can be used scale the MAP raw value meaning you can change the map raw value to relative by subtracting the value of map raw at absolute reading.
MAP Raw is then fed into the MAP sensor table which decides the MAP sensor output based on the input. I pressume your boost gauge is also using a MAP sensor similar to the one run by the ECU? if this is the case we have found the pressure sensors we use, the ones everyone uses, to be extremely reliable and accurate so i would start looking for reasons for this difference like location of sensor or gauge accuracy etc.
The gauge should indeed read 0 when the engine has stopped and may go to minus on small throttles however the negative figure you see means the turbo will be being drawn round by the engine rather then the exhaust gases driving it. This is obviously both ineficient and a power drain and is cured by antilag. however it depends on the application you are using the car for as antilag will shorten turbo and exhaust life when used regularly on the road.
Hope this helps
Ed
Re: Relative pressure vs absolute pressure
Most helpful!
Thanks!
Thanks!