Boost Versus Temperature Question
Well I think cold air = more boost. What happened to me albeit only few times (in my 5 years of ownership) is that on some really crisp cold winter days (when I really floored it) high speed - max load, I spiked 1.3 bar,. When this happened car's limiter kicked in and the car went into limp mode. I switched the engine off and turned it back on whilst driving (CPU reset) and all was back to normal.
This also happened in my 944 Turbo S. I think this is because the tune is set right on the limit not taking into consideration ambient temps. I guess backing off 0.1 bar would eliminate this but in relation to only few cold days in 5 years - no way - I am giving up 5bhp for that. Ha ha ha!
This also happened in my 944 Turbo S. I think this is because the tune is set right on the limit not taking into consideration ambient temps. I guess backing off 0.1 bar would eliminate this but in relation to only few cold days in 5 years - no way - I am giving up 5bhp for that. Ha ha ha!
I also had a 944 Turbo S,loved that car..I had it modified and it made around 325 rear wheel HP/TQ back in the day..
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I am sorry if I misunderstood you. I reacted to some of your statements like:
“The question is I know cooler air = higher pressure. But I wonder if really cold air means less pressure? He is basically saying he went from 1.3/1bar to 1.2/0.9bar. 0.1 bar loss. Could this be leak? Or would the drop be more dramatic?”
That is a wrong assumption under a pressure controlled environment. Meaning, boost pressure in this case. Cooler air does not mean higher boost.
Later on you went on to say:
“Would explain though: colder air less pressure for same bhp”
It is incorrect but can be taken as correct depending how you want to look at it.
This is exactly what I tried to explain. Colder air will give more HP at the same pressure and therefore you can conclude that in order to get the same HP with colder air you can indeed say less pressure but be sure that is not how your waste gate works and is not valid in the example you used. Your waste gate only sees kPa (bar) and not colder or warmer air.
The ambient temperature has no influence on the pressure (boost) controlled by your waste gate.
You mention”
“Aperently 996TT takes as much boost as is necessary in the conditions to give you 420 bhp. Hence one day booting it in 3rd gear it takes .7 bar and the next day on the same road it will be .6.bar. I say "apparently" because there is a lot of hearsay on this topic".
This is a completely inaccurate statement and yes you say it is hearsay.
Why your pressure (boost) drops as your engine revolutions goes up has nothing to do with controlling your horsepower either.
I am sure there are many of our knowledgeable friends here (Like Tim941NYC “please read his threads” to name one) that can explain this.
One such an explanation is.
Just like any naturally aspirated engine, turbo charged engines also does not like back pressure caused by exhausts piping issues as well as silencers. The higher the revs the more pressure drop will occur over the piping as well as the exhaust leaving less pressure drop “available” across the turbo which will result in lower boost pressure (reduction of turbo revs as a result of less pressure drop available across the turbo). Remove your silencer and re-do your test and see how your turbo will loose less boost at higher revs with whatever the ambient temperature is.
To conclude, ambient temperature has no influence on your boost pressure.
If I am wrong with modern day cars (ECU controlled waste gates) then please don’t shoot me.
Again, my 2c worth.
I am sorry if I misunderstood you. I reacted to some of your statements like:
“The question is I know cooler air = higher pressure. But I wonder if really cold air means less pressure? He is basically saying he went from 1.3/1bar to 1.2/0.9bar. 0.1 bar loss. Could this be leak? Or would the drop be more dramatic?”
That is a wrong assumption under a pressure controlled environment. Meaning, boost pressure in this case. Cooler air does not mean higher boost.
Later on you went on to say:
“Would explain though: colder air less pressure for same bhp”
It is incorrect but can be taken as correct depending how you want to look at it.
This is exactly what I tried to explain. Colder air will give more HP at the same pressure and therefore you can conclude that in order to get the same HP with colder air you can indeed say less pressure but be sure that is not how your waste gate works and is not valid in the example you used. Your waste gate only sees kPa (bar) and not colder or warmer air.
The ambient temperature has no influence on the pressure (boost) controlled by your waste gate.
You mention”
“Aperently 996TT takes as much boost as is necessary in the conditions to give you 420 bhp. Hence one day booting it in 3rd gear it takes .7 bar and the next day on the same road it will be .6.bar. I say "apparently" because there is a lot of hearsay on this topic".
This is a completely inaccurate statement and yes you say it is hearsay.
Why your pressure (boost) drops as your engine revolutions goes up has nothing to do with controlling your horsepower either.
I am sure there are many of our knowledgeable friends here (Like Tim941NYC “please read his threads” to name one) that can explain this.
One such an explanation is.
Just like any naturally aspirated engine, turbo charged engines also does not like back pressure caused by exhausts piping issues as well as silencers. The higher the revs the more pressure drop will occur over the piping as well as the exhaust leaving less pressure drop “available” across the turbo which will result in lower boost pressure (reduction of turbo revs as a result of less pressure drop available across the turbo). Remove your silencer and re-do your test and see how your turbo will loose less boost at higher revs with whatever the ambient temperature is.
To conclude, ambient temperature has no influence on your boost pressure.
If I am wrong with modern day cars (ECU controlled waste gates) then please don’t shoot me.
Again, my 2c worth.
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I found a thread dealing with the same issue which sheds some light onto things. Huh!
https://www.6speedonline.com/forums/...t-control.html
https://www.6speedonline.com/forums/...t-control.html
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