The effects of ambient temp V boost temp/engine power
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PassionFord Post Whore!!
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The effects of ambient temp V boost temp/engine power
I know Im no expert, but I like to think I have a basic grasp that a warm day means you are not getting as much power/higher air charge temps/IC not working as effeciently etc etc
Ive been trying to explain this to someone on another forum and he just cant see it, anyone care to set the facts straight/feel free to comment on James' ideas too
http://www.vauxhallownersnetwork.co....d.php?t=107484
Ive been trying to explain this to someone on another forum and he just cant see it, anyone care to set the facts straight/feel free to comment on James' ideas too
http://www.vauxhallownersnetwork.co....d.php?t=107484
Testing the future
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From: W. Sussex
there's 2 ways in which high temps reduce power.
1) lower density.
that means for the a given engine size, say 2 litres, with an assumed volumetric efficiency of 100%, the engine will always get 2 litres of air into it.
at a cold temperature, 2 litres of air has a mass of say 200g (that's just a figure plucked from thin air). to get the air to fuel ratio right, of 14 parts air to 1 part fuel by weight, that means a mass of about 15g of fuel. there is a certain amount of energy that can be extracted from that fuel that will give a certain power.
increase the temperature, and the density comes down. now 2 litres of air may only way 150g, to which you only need to add 10g of fuel to keep the same AFR. 10g of fuel can obviously release less energy than 15g.
2) detonation
a hotter mixture of air/fuel will burn easier than a cold one. to prevent that happening uncontrollably, the spark has to be retarded to later in the cycle, which means getting less energy out of the mixture
hope that helps
1) lower density.
that means for the a given engine size, say 2 litres, with an assumed volumetric efficiency of 100%, the engine will always get 2 litres of air into it.
at a cold temperature, 2 litres of air has a mass of say 200g (that's just a figure plucked from thin air). to get the air to fuel ratio right, of 14 parts air to 1 part fuel by weight, that means a mass of about 15g of fuel. there is a certain amount of energy that can be extracted from that fuel that will give a certain power.
increase the temperature, and the density comes down. now 2 litres of air may only way 150g, to which you only need to add 10g of fuel to keep the same AFR. 10g of fuel can obviously release less energy than 15g.
2) detonation
a hotter mixture of air/fuel will burn easier than a cold one. to prevent that happening uncontrollably, the spark has to be retarded to later in the cycle, which means getting less energy out of the mixture
hope that helps
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PassionFord Post Whore!!
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From: Deep in the underground
As I thought
He is trying to say that the ambient temp has no effect over power on a turbo car because the boost pressure will always be the same.
According to him, if you have 12psi, then it will always be 12psi and therefore never give anymore power
He cant grasp the fact that the denser the air, the more air gets into the engine and makes more power
He is trying to say that the ambient temp has no effect over power on a turbo car because the boost pressure will always be the same.
According to him, if you have 12psi, then it will always be 12psi and therefore never give anymore power
He cant grasp the fact that the denser the air, the more air gets into the engine and makes more power
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PassionFord Post Whore!!
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His other main theory is that the differences in power are more noticable in an NA car than a turbo.
Trying to explain that the IC only magnifies the differences is pointless lol, Ive given up with him now, I just wanted to make sure I wasnt talking bollocks too
Trying to explain that the IC only magnifies the differences is pointless lol, Ive given up with him now, I just wanted to make sure I wasnt talking bollocks too
PassionFord Post Troll
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edited - misread 
Maybe he needs a practical demonstration? my s2 just felt dead flat to drive, instead of it's spritely self
I remember hearing about the ACT's at event being too high and no one could get good times in as it was a hot day?? think it was totb or something?
Maybe he needs a practical demonstration? my s2 just felt dead flat to drive, instead of it's spritely self
I remember hearing about the ACT's at event being too high and no one could get good times in as it was a hot day?? think it was totb or something?
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PassionFord Post Whore!!
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From: Deep in the underground
got it ... a simple example ...
it's 40 degrees, your motor is producing Xbhp at 15psi, it's limit.
the temp goes down to 5 degrees ... your motor, everything else being equal, wants to produce 16psi due to the knock on effect of the increased air density/ mass ... but is limited to 15psi.
it produces more power, cos the charge is cooler, but not as much as it otherwise could, because its limited ...
therefore, the effect of ambient temperatures potentially has less overall effect on blown motors than NA motors which are open to the full effect of the change.
it's 40 degrees, your motor is producing Xbhp at 15psi, it's limit.
the temp goes down to 5 degrees ... your motor, everything else being equal, wants to produce 16psi due to the knock on effect of the increased air density/ mass ... but is limited to 15psi.
it produces more power, cos the charge is cooler, but not as much as it otherwise could, because its limited ...
therefore, the effect of ambient temperatures potentially has less overall effect on blown motors than NA motors which are open to the full effect of the change.
Can someone put an answer into words better than I can
Testing the future
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From: W. Sussex
smash the teeth out of his head with a claw hammer then. they're only £1.99 in homebase. will be the best £2 you ever spent
or a not so serious answer to him is that yes there will still be 15 psi of boost, but that will have put in say 300g of air at a cold temperature, but only 275g at a hot temperature due to the difference in density.
on a n/a engine, you will be running at lower volumetric efficiency than a turbo one, so any change in density will have less of an effect (even ignoring the reduced performance of the i/c at high ambients on the turbo engine). a 10% reduction in density on a volume of 70% of 2 litres is much less than a 10% reduction in density on 100% of 2 litres
or a not so serious answer to him is that yes there will still be 15 psi of boost, but that will have put in say 300g of air at a cold temperature, but only 275g at a hot temperature due to the difference in density.
on a n/a engine, you will be running at lower volumetric efficiency than a turbo one, so any change in density will have less of an effect (even ignoring the reduced performance of the i/c at high ambients on the turbo engine). a 10% reduction in density on a volume of 70% of 2 litres is much less than a 10% reduction in density on 100% of 2 litres
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