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    Does Boost change with outside air temp ?

    Hi folks
    Has anyone noticed a drop in boost pressure as the outside air temp drops ?
    So far I have noticed ...with an Outside Air Temp of +8c .. a peak of 1.2 bar in overboost mode,and a peak of 1 bar std mode.
    Driving today with the OAT of -1 c ..overboost only 1 bar and std 0.8 bar !
    Its a shame if the ecu is written to do so because cold air =denser air =move power with the correct air fuel ratio !
    There is a formulae which proves that every drop in( x) degree c = + bhp .this is very evident in Turbo cars,,eg the importance of intercoolers etc etc..
    If anything you can RAISE boost levels as the OAT drops as long as you can squirt enough fuel in to keep the mix right etc !!

    Re: Does Boost change with outside air temp ?

    Quote:
    Chopper said:
    Hi folks
    Has anyone noticed a drop in boost pressure as the outside air temp drops ?
    So far I have noticed ...with an Outside Air Temp of +8c .. a peak of 1.2 bar in overboost mode,and a peak of 1 bar std mode.
    Driving today with the OAT of -1 c ..overboost only 1 bar and std 0.8 bar !
    Its a shame if the ecu is written to do so because cold air =denser air =move power with the correct air fuel ratio !
    There is a formulae which proves that every drop in( x) degree c = + bhp .this is very evident in Turbo cars,,eg the importance of intercoolers etc etc..
    If anything you can RAISE boost levels as the OAT drops as long as you can squirt enough fuel in to keep the mix right etc !!


    Although we never get to -1C here, I can tell you that the last few nights when we got down to 3-4 degrees C, my car felt much more eager but the 1.2 bars max boost remained the same as on those 20 degree days, maybe it is just spooling faster and certainly not the other way around. Did you use different octane gas?

    Re: Does Boost change with outside air temp ?

    In Sweden I use either 98 octane or v99 Shell which I suppose is 99 octane !!

    Re: Does Boost change with outside air temp ?

    Are you sure that you mean octane? I thought they used RON in Europe.

    Re: Does Boost change with outside air temp ?

    OK as far as I read below the difference is about -4 in the States.
    99 Europe = 94 State side
    Measurement methods
    The most common type of octane rating worldwide is the Research Octane Number (RON). RON is determined by running the fuel through a specific test engine with a variable compression ratio under controlled conditions, and comparing these results with those for mixtures of isooctane and n-heptane.
    There is another type of octane rating, called Motor Octane Number (MON) or the aviation lean octane rating, which is a better measure of how the fuel behaves when under load. MON testing uses a similar test engine to that used in RON testing, but with a preheated fuel mixture, a higher engine speed, and variable ignition timing to further stress the fuel's knock resistance. Depending on the composition of the fuel, the MON of a modern gasoline will be about 8 to 10 points lower than the RON. Normally fuel specifications require both a minimum RON and a minimum MON.
    In most countries (including all of Europe and Australia) the "headline" octane that would be shown on the pump is the RON, but in the United States and some other countries the headline number is the average of the RON and the MON, sometimes called the Anti-Knock Index (AKI), Road Octane Number (RdON), Pump Octane Number (PON), or (R+M)/2. Because of the 8 to 10 point difference noted above, this means that the octane in the United States will be about 4 to 5 points lower than the same fuel elsewhere: 87 octane fuel, the "regular" gasoline in the US and Canada, would be 91-95 (regular) in Europe.

    Re: Does Boost change with outside air temp ?

    Quote:
    Chopper said:
    Hi folks
    Has anyone noticed a drop in boost pressure as the outside air temp drops ?
    So far I have noticed ...with an Outside Air Temp of +8c .. a peak of 1.2 bar in overboost mode,and a peak of 1 bar std mode.
    Driving today with the OAT of -1 c ..overboost only 1 bar and std 0.8 bar !
    Its a shame if the ecu is written to do so because cold air =denser air =move power with the correct air fuel ratio !
    There is a formulae which proves that every drop in( x) degree c = + bhp .this is very evident in Turbo cars,,eg the importance of intercoolers etc etc..
    If anything you can RAISE boost levels as the OAT drops as long as you can squirt enough fuel in to keep the mix right etc !!


    Yes your boost will drop with reducing outside temps - your engine is mapped for DIN conditions 20DegC and will attempt to hit the target values whatever the ambient conditions. If the air mass is lower (ie warmer ambient) you will get higher boost to generate the mass) if the air is cold as you have observed it will ask for lower boost to give the required mass of air. It is unusual to be able to spot this working with the factory gauges as they are normally so inaccurate (can't speak for the 997tt )

    Re: Does Boost change with outside air temp ?

    Thanks ..I will not be rushing back to the dealer complaining yet !

    Re: Does Boost change with outside air temp ?

    Thanks chopper for the detailed fuel octane explanation.

    Re: Does Boost change with outside air temp ?

    Thanks chopper...now I get it.

    Re: Does Boost change with outside air temp ?

    Quote:
    TB993tt said:Yes your boost will drop with reducing outside temps - your engine is mapped for DIN conditions 20DegC and will attempt to hit the target values whatever the ambient conditions.



    It depends on where the boost pressure reading is picked up. On the 996 Turbo it's just before the throttle body, so the pressure is reflective of what is in the intake and not reflective of how hard the turbos are working to generate this pressure.

    Under low ambient pressure the trubos work harder (produce more output pressure) in an attempt to reach the ECU commanded target intake pressure (e.g. the ambient pressure in the intake= 14.0 psi, and the ECU target pressure is 29.0 psi, then the turbo has to generate a total pressure of 15 psi. But the boost as read in the cockpit would still be equal to 29 psi...presented in bar). Under high ambient pressure, the turbos don't have to work as hard to reach the target intake pressure (e.g. ambient =15 psi and the ECU target boost = 29 psi, the turbos only have to generate 14 psi of boost). The boost reading in the cockpit would be the same in both of these situations.

    Re: Does Boost change with outside air temp ?

    I have also noticed that since the weather turned cold (about 26F today) my max boost has been 12 and 15 psi (stock and sport, respectively) whereas earlier last fall the boost pressures were about 15 and 18. Suspecting a possible fault, I asked the tech at the dealer today. He believes it is a software adjustment to compensate for the greater density of the colder air. I'll have to wait for things to warm up again to prove him right or wrong.

    Re: Does Boost change with outside air temp ?

    maybe your altitude is the reason of O2 shortage .. are you driving in high areas like "everest" as an example ... i mean maybe its the reason ,,, maybe other thing affect your boost level , like forgetting to push the overboost button , or the humidity was extremely high , or your car is "lazy" (( not driven in autobahns )) so cant get high boost always ... in general , what you`re saying is 100% right but there is many reasons to have low boost .. good luck

    Re: Does Boost change with outside air temp ?

    Quote:
    Dock (Atlanta) said:
    Quote:
    TB993tt said:Yes your boost will drop with reducing outside temps - your engine is mapped for DIN conditions 20DegC and will attempt to hit the target values whatever the ambient conditions.




    It depends on where the boost pressure reading is picked up. On the 996 Turbo it's just before the throttle body, so the pressure is reflective of what is in the intake and not reflective of how hard the turbos are working to generate this pressure.

    Under low ambient pressure the trubos work harder (produce more output pressure) in an attempt to reach the ECU commanded target intake pressure (e.g. the ambient pressure in the intake= 14.0 psi, and the ECU target pressure is 29.0 psi, then the turbo has to generate a total pressure of 15 psi. But the boost as read in the cockpit would still be equal to 29 psi...presented in bar). Under high ambient pressure, the turbos don't have to work as hard to reach the target intake pressure (e.g. ambient =15 psi and the ECU target boost = 29 psi, the turbos only have to generate 14 psi of boost). The boost reading in the cockpit would be the same in both of these situations.


    Dock- we've had this discussion before
    The ECU is trying to hit air mass targets NOT pressure targets, it doesn't see boost pressure..... hence it allows less boost during denser air (cold) conditions to achieve the same air mass targets which would have required more boost in warmer (less dense) air conditions

    Re: Does Boost change with outside air temp ?

    The only reason your boost would be dropping is if the ECU is telling it to (sounds normal in this case) or the turbos had abnormally high exhaust resistance. Like i said though, since the ECU controls the turbo boost so much that there are no blow-off valves, it sounds normal.

    Re: Does Boost change with outside air temp ?

    Quote:
    Chopper said:
    Hi folks
    Has anyone noticed a drop in boost pressure as the outside air temp drops ?
    So far I have noticed ...with an Outside Air Temp of +8c .. a peak of 1.2 bar in overboost mode,and a peak of 1 bar std mode.
    Driving today with the OAT of -1 c ..overboost only 1 bar and std 0.8 bar !
    Its a shame if the ecu is written to do so because cold air =denser air =move power with the correct air fuel ratio !
    There is a formulae which proves that every drop in( x) degree c = + bhp .this is very evident in Turbo cars,,eg the importance of intercoolers etc etc..
    If anything you can RAISE boost levels as the OAT drops as long as you can squirt enough fuel in to keep the mix right etc !!



    I got different figures which made me sad... temp here is like 19c to 26... i get in standard mode no more than 0.7 and overboost mode 0.9 sometimes it touches 1.0 bar, thats it... can i know if any thing is wrong with my car or it is normal?

    Re: Does Boost change with outside air temp ?

    Quote:
    TB993tt said:
    Dock- we've had this discussion before
    The ECU is trying to hit air mass targets NOT pressure targets, it doesn't see boost pressure..... hence it allows less boost during denser air (cold) conditions to achieve the same air mass targets which would have required more boost in warmer (less dense) air conditions



    The question is not what the ECU is measuring, it's what's displayed in the cockpit. The bar reading on the tach is read just before the throttle body, and is read as manifold pressure, not air mass.

    Re: Does Boost change with outside air temp ?

    even with electronic "wastegates" I would be very surprised if the boost was lower in denser air. The boost should still read off manifold pressure after the TB not before it to get the most accurate determination for combustion chamber conditions - IC and TB conditions are not as reflective. With that being said, I think the reason the boost should be higher would be due to lower EGT's from lower ambient intake temps. On these cars the EGT will determine how much max boost will develop.

    Re: Does Boost change with outside air temp ?

    Quote:
    eclou said:
    even with electronic "wastegates" I would be very surprised if the boost was lower in denser air. The boost should still read off manifold pressure after the TB not before it to get the most accurate determination for combustion chamber conditions - IC and TB conditions are not as reflective. With that being said, I think the reason the boost should be higher would be due to lower EGT's from lower ambient intake temps. On these cars the EGT will determine how much max boost will develop.


    The programming targets for air mass determine how much boost will develop, not EGTs (elevated EGT's will affect the system but starting with retarding timimg and eventually lowering boost, but not by very much on a stock engine). Less boost (than DIN) IS a consequence of lower (than DIN) ambient temperatures. I have spent too much time watching this happen on a "proper" boost gauge.
    What is really amazing is how much MORE boost the programming demands when the ambient temperatures go higher - on my motor 1 bar @ 6000rpm can become 1.15 bar in 30DegC ambient !

    Re: Does Boost change with outside air temp ?

    Quote:
    TB993tt said:
    The ECU is trying to hit air mass targets NOT pressure targets, it doesn't see boost pressure..... hence it allows less boost during denser air (cold) conditions to achieve the same air mass targets which would have required more boost in warmer (less dense) air conditions



    TB993tt,
    What is your source for this information?

    I may be misunderstanding you, but I have a different take on this.

    My view is that:
    - air mass flow is primarly decided by throttle opening (or your right foot)
    - the ecu's job is to match the quantity of fuel injected to the air mass flow rate, as well as to match the spark timing to engine speed and load

    What makes you think that the ecu does not have a boost pressure sensor input?

    Re: Does Boost change with outside air temp ?

    Let's see if I can explain it to everyone's satisfaction.

    On page 41 of the US 997 Turbo brochure it listed what the ecu in the 997 Turbo measures, of note, it measures

    -pressure upstream from throttle;
    -temperature of airflow upstream from throttle;
    -temperature of ambient air;
    -ambient air pressure

    Remember the throttle body is aft of the turbocharger/intercooler and is part of the pressurized track.

    That first item must be where the boost bar takes its measurements.

    From the chart it seems like the ecu mappings all work off pressure, not air mass. I imagine the ecu has a absolute, not relative, pressure target for the intake plenum and it bleeds off the excess pressure using the bypass valve which it also controls.

    In cold weather/denser air situation, the ecu knows the lower air temperature and higher ambient air pressure and to hit the target absolute pressure mark, a lower pressure is needed, hence the lower boost value we see on display. In other words the boost bar is a relative pressure, not absolute pressure.

    TB993TT is correct in his theory, just not the terms.

    NA cars are quite sensitive to change in air density/air temperature as there is no way for the engine to compensate for loss of air pressure, hence the large change in performance in varying air temperature/altitude. Turbocharged and supercharged cars on the other hand are relatively unaffected as the ecu works off a fixed target, the turbo/supercharger just have to work harder, but only up to the max ability of the turbo/supercharger. There will be a point where even the turbo/supercharger simply cannot keep up with the ecu target and that's when the performance of the car drops off.

    In theory a ecu can be programmed to use relative pressure as target and the turbocharger/supercharger programmed to produced a fixed boost value. A car like that would take advantage of colder denser air to produce more power but it would produce less in the hot weather, but such a car would have wildly difference performance for different markets, ie the same car will be much slower in Dubai than in Finland and THAT won't make the oil sheikhs happy

    Re: Does Boost change with outside air temp ?

    Quote:
    Whoopsy said:
    In cold weather/denser air situation, the ecu knows the lower air temperature and higher ambient air pressure and to hit the target absolute pressure mark, a lower pressure is needed, hence the lower boost value we see on display.



    Except that the pressure we read on the tach is system pressure...the pressure that exists just prior to the charge entering the cylinders.

    PCNA told me that boost is generated"above ambient". The turbos work harder to produce the ECU's desired target when the ambient pressure is low, and they don't work as hard to produce this same system pressure when the ambient pressure is high. But in both cases, the system pressure, and therefore the pressure read on the tach, should be the same.

    Re: Does Boost change with outside air temp ?

    That system pressure you are talking about is the absolute pressure the ecu is working off, as you said, it's the pressure in the plenum just before it enters the cylinder.

    Your second statement is exactly what I have typed and try to explain. We are working off the same page

    The only sticky point left is the pressure displayed on the boost guage, is it a reading of absolute pressure (system pressure) or relative pressure (above ambient).

    Are you sure the pressure displayed is the absolute pressure (system pressure) not the relative pressure (above ambient) one? Someone might want to check with PCNA on that.

    It would seems logical to assume the function of the boost guage is to guage how hard the turbo is actually working (relative pressure), not how hard it is suppose to work (absolute pressure).

    Re: Does Boost change with outside air temp ?

    I'll check with PCNA and see what they say.

    Re: Does Boost change with outside air temp ?

    Fritz
    My information is what Gerhard Schmirler (RS Tuning) have explained to me over the years and lots of observation in different conditions.
    Whoopsy, so you are saying the 997tt has a MAP type of system as opposed to the MAF (hot film) system in the 993tt/996tt ? That is quite a switch and I'm surprised I've not seen that discussed anywhere ?
    I would say that the stuff you are reading from the brochure is incorrect and they are using the terminology "pressure" as it is easier for punters to understand than air mass in kg/metre3 (which obviously varies with pressure and temperature) ?
    Regarding the boost readings - my analogue gauge is piped into the intake just in front of the intercooler so it is reading "real" boost pressure. I have engine dyno boost curve for reference and trust me when its cold the observed inlet boost pressure goes down and when its hot it goes up, I have driven this car for 12 years now and am on the 4th engine configuration. It does seem that the higher the boost requirement of the engine, the bigger these variations of boost which Iguess is a percentage thing ?

    Re: Does Boost change with outside air temp ?

    Although I don't have first hand info, I have came to that conclusion from what Porsche has published.

    The ECU is already measuring the few require readings for a MAP system, ambient air pressure (air density), ambient air temperature and MAT (manifold absolute temperature, ie the temperature reading upstream from throttle). With all the sensors and readings, the air mass into the intake manifold at any given instant is easily determined by the ECU as it's simply a function of air density and flowrate.

    It's just seems logical for me to assume a MAP system as all the require sensors are mentioned in the Motronic chart, especially the pressure sensors as it's a turbo engine and the sensors are all required.

    Now I could be wrong and you are right on the MAP/MAF thing, but we are on the same page as to why less boost is required on cold days

    BTW, since your guage is before the intercooler, do you know how much pressure is lost aft of the intercooler?

    Re: Does Boost change with outside air temp ?

    Sorry. I was not clear - my gauge is after the intercooler

    Re: Does Boost change with outside air temp ?

    hahaha oops

    so it's the same place as the stock pressure sensor, aft of intercooler but before the throttle body. It make sense for it to give a variable max reading as it measures the relative pressure the turbo is pushing in varying ambient air temperature and pressure, not the programmed absolute pressure ceiling.

    Re: Does Boost change with outside air temp ?

    Quote:
    Dock (Atlanta) said:
    I'll check with PCNA and see what they say.



    PCNA tells me that the boost displayed on the tach is system pressure...the pressure entering the engine.

    Re: Does Boost change with outside air temp ?

    hmm, on that bombshell, there really isn't anything i can say right. Let me see if I can fill myself in with any info I can dig up

     
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