Cooler weather

[///1M]

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Can we expect much of a boost in engine output during the cooler months ahead?

What's considered to be the optimal temperature for tracking or racing, given all the variables... intake temperature, cooling capacity, braking and tire performance, etc.)?

[Simsims]

I feel my 135 is more powerful when it's around 24C outside (75F)

When it's -20 you don't wanna be driving fast anyway

[Schnorbitz]

1hp per degreeC I've heard quoted before?

Could just be bollocks though...

[blueshark]

Would be interesting to hear if people are feeling more power now that the weather is cooling off in parts of NA, Asia, Europe?

[1SerieStud]

Cooler air = denser air = better A/F mixture so the car runs like a champ, and this is definitely noticeable. There is no direct relationship between actual temperature and hp since certain parameters are constantly varying.

[yemski]

Originally Posted by 1SerieStud

Cooler air = denser air = better A/F mixture so the car runs like a champ, and this is definitely noticeable. There is no direct relationship between actual temperature and hp since certain parameters are constantly varying.

yup denser air means more oxygen per cubic cm/inch which means increased combustion. Also cooler ambient temperatures improve the effectiveness of the intercooler which means reduced IAT (intake air temperature) and that yields more power. When it is hot outside the engine ECU will pull or retard timing to prevent detonation (when the gas ignites before the spark due to high operating temperatures) thereby reducing power - aka heat soak.
In SF when it is cool and foggy you get natural misting on the intercooler which is a big plus, I have noticed increased power with my last three cars in these conditions - all have been turbo charged.

[tracer bullet]

Erm, I'm not so sure. I spent 10 years at a company doing engineering work around diesel engines. I didn't do turbo design but was around that kind of stuff for a long time. My recollection is that a turbo negates the ill effects of high altitudes having less dense air, up to a point anyhow, and that turbocharged trucks in Denver behaved the same as they did on the coasts.

Lag is much worse with less dense air, and it would affect the intercooler as well, but the actual peak power of the engine is relatively unaffected. Boost is boost, and it peaks at the same point either way.

I can't call this a fact as I didn't spend enough time on that subject. But don't right away assume that a turbo engine is nearly as affected as a N/A engine by the change in weather. I believe the density of the air coming out of turbo is approximately the same regardless of what goes in.

The 128i owners will see a noticeable change, the 135i guys not as much.

[DarthColin]

I'm not 100% sure, but doesn't the N54 have some sort of absolute pressure control to limit boost, so that it will make sea level power up to some very high altitude?

[1SerieStud]

Originally Posted by tracer bullet

Erm, I'm not so sure. I spent 10 years at a company doing engineering work around diesel engines. I didn't do turbo design but was around that kind of stuff for a long time. My recollection is that a turbo negates the ill effects of high altitudes having less dense air, up to a point anyhow, and that turbocharged trucks in Denver behaved the same as they did on the coasts.

Lag is much worse with less dense air, and it would affect the intercooler as well, but the actual peak power of the engine is relatively unaffected. Boost is boost, and it peaks at the same point either way.

I can't call this a fact as I didn't spend enough time on that subject. But don't right away assume that a turbo engine is nearly as affected as a N/A engine by the change in weather. I believe the density of the air coming out of turbo is approximately the same regardless of what goes in.

The 128i owners will see a noticeable change, the 135i guys not as much.

Actually you have it backwards. The 135i guys will see a bigger change than the 128i guys during cold weather. What you are neglecting to take into account is that in higher altitudes (less dense air), the reason force-inducted systems compensate better is that they have the ability to compress more air (oxygen), whereas NA systems only rely on the actual piston compression.

[Pig Farmer]

I seem to recall reading that 62* was the perfect temp at sea level. The cooler air improved the A/F ratio, but the air was not so dense that it produced excess aerodynamic drag. Only the seat of my pants can to confirm this theory.

[yemski]

Originally Posted by 1SerieStud

Actually you have it backwards. The 135i guys will see a bigger change than the 128i guys during cold weather. What you are neglecting to take into account is that in higher altitudes (less dense air), the reason force-inducted systems compensate better is that they have the ability to compress more air (oxygen), whereas NA systems only rely on the actual piston compression.

exactly, forced induction motors will see a bigger benefit from cooler temps than naturally aspirated motors.

[KIYO]

Agreed. When in cold rain, the intake air temperature will get to like 20 degrees Celsius, I feel more power.
Also, I feel powerful than the NA engine cars in high altitude in N54 engine as long as it does not exceed the capacity of the turbo. Normally I get 120 kPa (1.2bar) maximum intake manifold pressure, but at high altitude like Stelvio pass, I could not reach 120 kPa. So I think it was beyond the capacity of the turbo of N54 engines.

[mad chemist]

Originally Posted by DarthColin

I'm not 100% sure, but doesn't the N54 have some sort of absolute pressure control to limit boost, so that it will make sea level power up to some very high altitude?

DarthColin,

You are correct. Little difference on an N54 over a range of reasonable temps. Unlike my older STi which was weak when hot and much stronger in the winter.

Mad.

[tracer bullet]

Originally Posted by 1SerieStud

Actually you have it backwards. The 135i guys will see a bigger change than the 128i guys during cold weather. What you are neglecting to take into account is that in higher altitudes (less dense air), the reason force-inducted systems compensate better is that they have the ability to compress more air (oxygen), whereas NA systems only rely on the actual piston compression.

Right. I agree that FI "compensates better", as you put it, and NA can not.

And since FI compensates better, to me that means that what goes into it doesn't have as much of an effect as it does on NA.

In other words higher altitudes have less effect on FI than they do on NA (back to FI being able to compensate for it). And air density differences caused by temp changes (similar in result to air density differences caused by altitude changes) also have less effect on FI than they do on NA.

Again meaning that a 135i is less affected than a 128i, because the 135i has a FI system which compensates for the changes that really affect the NA guys.

[tracer bullet]

Originally Posted by HKG

exactly, forced induction motors will see a bigger benefit from cooler temps than naturally aspirated motors.

Why? Because the air is more dense with cooler temps?

By that reasoning, since sea level air is more dense than Denver air, FI motors see a bigger benefit by coming down out of a high altitude than NA motors do, when they get to the denser air. Flipping it around you'd have to believe that FI motors are more affected by high altitudes than NA motors are, and that's why it's so beneficial for them to get out of those high altitudes.

Having driven turbo and NA cars in Denver, and taking them up to Winter Park, I can tell you that sure wasn't the case in my experience. NA cars dogged, big time.

[yemski]

Originally Posted by tracer bullet

Why? Because the air is more dense with cooler temps?

By that reasoning, since sea level air is more dense than Denver air, FI motors see a bigger benefit by coming down out of a high altitude than NA motors do, when they get to the denser air. Flipping it around you'd have to believe that FI motors are more affected by high altitudes than NA motors are, and that's why it's so beneficial for them to get out of those high altitudes.

Having driven turbo and NA cars in Denver, and taking them up to Winter Park, I can tell you that sure wasn't the case in my experience. NA cars dogged, big time.

yes cooler air = more oxygen molecules per cubic inch, also cooler ambient temps means lower IAT's because the intercooler will be more effective further cooling intake air.
In high altitudes air is thinner aka less dense that is why forced induction cars have an advantage in these conditions because turbos/super chargers are essentially air compressors. The result is the air being forced into a forced induction motor is denser than the air that would be going into a NA motor.

[1SerieStud]

Originally Posted by tracer bullet

Right. I agree that FI "compensates better", as you put it, and NA can not.

And since FI compensates better, to me that means that what goes into it doesn't have as much of an effect as it does on NA.

In other words higher altitudes have less effect on FI than they do on NA (back to FI being able to compensate for it). And air density differences caused by temp changes (similar in result to air density differences caused by altitude changes) also have less effect on FI than they do on NA.

Again meaning that a 135i is less affected than a 128i, because the 135i has a FI system which compensates for the changes that really affect the NA guys.

Sure, but you're looking at it from a negative perspective only. Just like FI can run much better than NA during a hot day (negative), it runs THAT much better than NA during a cold day (positive). Again, a better compressed air-fuel mixture will always trump a better less compressed air-fuel mixture.

This is actually very common knowledge and assuming someone doesn't know a thing about forced induction, all they have to do is drive both cars during both conditions and it's easy to notice. Why do you think intercooler sprayers were invented? I don't see NA guys spraying anything at their intake pipes. Another supporting point: what does an NA car get out of a CAI? 1hp? Maybe 2-3hp? Now what does a forced-inducted car get out of a CAI? Typically better numbers, sometimes even in the range of double-digits.

Also, forced inducted cars run much safer mappings than NA cars, because damage to the engine can be detrimental due to detonation; so during hot days, they tend to run richer and timing gets retarded, which is why they can feel bogged down/slower. But when the cold weather comes around, advanced timing + a leaner mix wakes up the beast and this is why we experience the adrenaline rush.

[tracer bullet]

IC sprayers, meth injection, swap-outs of IC's, etc deal with the air after the turbo, not before. Agreed that cooler denser air on that side is good, no doubt. But that's a different subject, we're talking about the inlet side of the turbo and what the compressor can do. And CAI's can have at least as much affect on air flow for an engine that requires more CFM of it, than they do on actual temp of the air (some CAI's, especially short tube versions can be worse than stock). CAI's don't generally take in cooler air (same air supply source, and given a boundary layer inside the piping itself there's really no change in heat gain either), they tend to do it more efficiently.

Anyhow so if I understand you guys right, a FI car is less affected by less dense air than NA, but more affected by more dense air than NA.

In relative terms (less vs more density, regardless of the starting point) this seems conflicting. But you seem pretty certain about it so I'll roll with it.

Originally Posted by HKG

yes cooler air = more oxygen molecules per cubic inch, also cooler ambient temps means lower IAT's because the intercooler will be more effective further cooling intake air.
In high altitudes air is thinner aka less dense that is why forced induction cars have an advantage in these conditions because turbos/super chargers are essentially air compressors. The result is the air being forced into a forced induction motor is denser than the air that would be going into a NA motor.

I agree to all this. Sorry, wasn't asking a question, just laying groundwork to see where we diverge in how we think the rest works.

[1SerieStud]

How is it a different subject?! We are talking about compressing cooler air...which is exactly what a turbo does that an NA car cannot do - now that the air being compressed is denser (cooler), the better AF mixture results in a very noticeable increase in power. A turbo is nothing but a compressor placed between the intake and throttle body/intake manifold; take away the turbo and you have something similar to an NA system. You can view the intercooler and its piping as "an extension to the intake tubing", because that's really what it is since it simply connects from the turbo's compressor housing to the throttle body/intake manifold.

Originally Posted by tracer bullet

IC sprayers, meth injection, swap-outs of IC's, etc deal with the air after the turbo, not before. Agreed that cooler denser air on that side is good, no doubt. But that's a different subject, we're talking about the inlet side of the turbo and what the compressor can do. And CAI's can have at least as much affect on air flow for an engine that requires more CFM of it, than they do on actual temp of the air (some CAI's, especially short tube versions can be worse than stock). CAI's don't generally take in cooler air (same air supply source, and given a boundary layer inside the piping itself there's really no change in heat gain either), they tend to do it more efficiently.

Anyhow so if I understand you guys right, a FI car is less affected by less dense air than NA, but more affected by more dense air than NA.

In relative terms (less vs more density, regardless of the starting point) this seems conflicting. But you seem pretty certain about it so I'll roll with it.

I agree to all this. Sorry, wasn't asking a question, just laying groundwork to see where we diverge in how we think the rest works.

[///M1]

FWIW, the 4.2FSI in my RS 4 is a lot more sensitive to temps than the N54 (and altitude of course, which only makes sense as it is a NA engine). Haven't really noticed sizable performance difference in the N54 in my 135i (and I live in a 4-season climate with some extreme temps throughout the year). What is really telling is in the dyno room, regardless of how many pulls I've done in one session, the N54s numbers are very consistent while the RS 4s drop off a cliff given the heat soak.

[1SerieStud]

You're only talking about the negative case scenario...now take them back to the Dyno when it's really cold and you will see how forced induction benefits from cooler temps to an extend that NA never will.

Originally Posted by ///M1

FWIW, the 4.2FSI in my RS 4 is a lot more sensitive to temps than the N54 (and altitude of course, which only makes sense as it is a NA engine). Haven't really noticed sizable performance difference in the N54 in my 135i (and I live in a 4-season climate with some extreme temps throughout the year). What is really telling is in the dyno room, regardless of how many pulls I've done in one session, the N54s numbers are very consistent while the RS 4s drop off a cliff given the heat soak.

[///M1]

Originally Posted by 1SerieStud

You're only talking about the negative case scenario...now take them back to the Dyno when it's really cold and you will see how forced induction benefits from cooler temps to an extend that NA never will.

I have, in fact most of my dynos are in February (though do them also in the summer). The 4.2 FSI is notorious of its (overly) temp sensitivity, in fact some of the credible tuners have modified the (2) ECUs to decreases knock sensitivity due to higher IATs. Way more than the N54. Trust me, I have both and the difference in temps is a lot stronger felt on the Audi. I basically don't even drive it in hot and humid days.