Cold Weather=More power. Why?

[trekkie]

You shoulda seen the hybrid forums the first summer. MPG went down because gas got warm and people were freaking out.

The tinfoil hats were being handed out at the door

[DoomsdayJesus]

Originally Posted by BANGER

I should really switch to a CAI.
I think I've been irrationally afraid of hydrolocking for too long.
I do live in SoCal after all...

Don't do it. Our stock intakes are more than adequate if you just get a decent drop in filter. Cold air intakes have to be tuned to get any real gain, and many of them have been forcing the car to throw codes and run like crap until they get tuned due to issues with the MAF sensor.

Also, I grew up in Temecula, still visit my parents there often. A city that used to be awesome for driving until they built thousands of houses and few extra or larger roads. Definitely couldn't complain about the weather, though.

[DoomsdayJesus]

Originally Posted by Suberman

Not only that but the compressor doesn't run at all in cold weather because the evaporator would freeze up. Plus 3C or about 40F is the lowest temperature at which the AC can operate. There's an anti frost switch which opens at that temperature and disconnects the drive pulley from the compressor.

When I turn on the auto climate control my A/C is constantly cycling on and off at idle regardless of temperature, even with the heat on. I don't know if it's a dehumidifying feature or what, but it noticeably affects my mileage even in cold weather when I just leave it on. It's really annoying.

[BANGER]

Originally Posted by DoomsdayJesus

Don't do it. Our stock intakes are more than adequate if you just get a decent drop in filter. Cold air intakes have to be tuned to get any real gain, and many of them have been forcing the car to throw codes and run like crap until they get tuned due to issues with the MAF sensor.

Also, I grew up in Temecula, still visit my parents there often. A city that used to be awesome for driving until they built thousands of houses and few extra or larger roads. Definitely couldn't complain about the weather, though.

Oh no, I don't drive a twin [yet]. I'm currently sitting on a Civic but a twin is in the foreseeable future. But consider it noted.

And there's still Rainbow and Highway 395 to mess around with if I happen to be coming up from the south, so it's not too bad; the streets are still well paved and the lanes wide. At least it isn't LA; absolutely hate LA driving.

[atledreier]

Originally Posted by Suberman

Does the BRZ have knock sensors? If it does then the engine won't make more power in cold weather.

Turbo engines do if they are intercoolers.

[Td-d]

Originally Posted by Suberman

Does the BRZ have knock sensors? If it does then the engine won't make more power in cold weather.

Turbo engines do if they are intercoolers.

What?

[Suberman]

Originally Posted by Td-d

What?

Knock sensor equipped engines develop peak bmep until they knock. Cold weather doesn't reduce the detonation point.

Intercooler equipped turbo engines can develop higher torque because the compressed intake charge is cooled effectively by very cold weather, but in the intercooler.

Cold ambient air doesn't affect combustion chamber temperatures by any significant amount. The higher the compression ratio the less the starting air temperature affects pre-ignition charge temperature. Detonation results from end gas spontaneous ignition prematurely caused by the compression wave resulting from the flame front. That in turn limits bmep which limits peak torque. Knock sensors permit combustion pressures to rise up to that point and not beyond.

Therefore, if this engine is controlled by knock sensors then it produces its highest bmep at the point the knock sensor operates.

It is true that colder intake air is denser and therefore an engine not already optimized will run better in cold temperatures than in warm temperatures. This is not the case off engines controlled by knock sensors unless turbo intercooled.

Remember that the key advantage of direction injection is charge cooling resulting from cool fuel sprayed directly into the hot compressed air in the combustion chamber. The starting temperature of that air is pretty much irrelevant now.

This is one reason "cold" air intake modifications won't produce any meaningful extra power out if these engines and because the stock air box is pretty efficient already there are no real flow advantages available either.

While an engine develops higher torque as it ingests more air mass it cannot exceed its designed detonation limit.

If you drove in the 70's and then when the first knock sensor equipped engines came out in the 80's you would realize this.

[Suberman]

Originally Posted by DoomsdayJesus

When I turn on the auto climate control my A/C is constantly cycling on and off at idle regardless of temperature, even with the heat on. I don't know if it's a dehumidifying feature or what, but it noticeably affects my mileage even in cold weather when I just leave it on. It's really annoying.

It stops when ambient temperatures drop below about 3C.

If you have auto climate control the AC light actually goes out.

You can't dehumidify freezing air using a conventional automotive AC unit because the humidity in the air blocks the evaporator with ice.

Also, unless the car interior is also freezing simply heating the air as it passes into the cabin also "dehumidifies" it by reducing the relative humidity, which is all that matters.

[DoomsdayJesus]

Originally Posted by Suberman

It stops when ambient temperatures drop below about 3C.

If you have auto climate control the AC light actually goes out.

You can't dehumidify freezing air using a conventional automotive AC unit because the humidity in the air blocks the evaporator with ice.

Also, unless the car interior is also freezing simply heating the air as it passes into the cabin also "dehumidifies" it by reducing the relative humidity, which is all that matters.

Going to try this right now, it's well below freezing this morning.

[Td-d]

Originally Posted by Suberman

Knock sensor equipped engines develop peak bmep until they knock. Cold weather doesn't reduce the detonation point.

I'm well acquainted with the reasons why cold air allows for greater air-mass, and the functioning of intercoolers - I just really struggled to follow your logic.

The problem with this theory lies in that first sentence - this is not how the Subaru knock strategy works. It's a more reactive process - see here. It's also a curious question as to whether the BRZ is knock sensor controlled, since I suspect every ODB2 compliant vehicle since 1996 is likely to have knock sensors (I stand to be corrected).

I agree that direct injection reduces in chamber temperatures, that's the whole advantage, but DI is not used at all load ranges.

[Suberman]

Originally Posted by Td-d

I'm well acquainted with the reasons why cold air allows for greater air-mass, and the functioning of intercoolers - I just really struggled to follow your logic.

The problem with this theory lies in that first sentence - this is not how the Subaru knock strategy works. It's a more reactive process - see here. It's also a curious question as to whether the BRZ is knock sensor controlled, since I suspect every ODB2 compliant vehicle since 1996 is likely to have knock sensors (I stand to be corrected).

I agree that direct injection reduces in chamber temperatures, that's the whole advantage, but DI is not used at all load ranges.

I think DI is always active. The port injection only supplements DI.

[SpeedR]

Originally Posted by Suberman

Knock sensor equipped engines develop peak bmep until they knock. Cold weather doesn't reduce the detonation point.

Intercooler equipped turbo engines can develop higher torque because the compressed intake charge is cooled effectively by very cold weather, but in the intercooler.

Cold ambient air doesn't affect combustion chamber temperatures by any significant amount. The higher the compression ratio the less the starting air temperature affects pre-ignition charge temperature. Detonation results from end gas spontaneous ignition prematurely caused by the compression wave resulting from the flame front. That in turn limits bmep which limits peak torque. Knock sensors permit combustion pressures to rise up to that point and not beyond.

Therefore, if this engine is controlled by knock sensors then it produces its highest bmep at the point the knock sensor operates.

It is true that colder intake air is denser and therefore an engine not already optimized will run better in cold temperatures than in warm temperatures. This is not the case off engines controlled by knock sensors unless turbo intercooled.

Remember that the key advantage of direction injection is charge cooling resulting from cool fuel sprayed directly into the hot compressed air in the combustion chamber. The starting temperature of that air is pretty much irrelevant now.

This is one reason "cold" air intake modifications won't produce any meaningful extra power out if these engines and because the stock air box is pretty efficient already there are no real flow advantages available either.

While an engine develops higher torque as it ingests more air mass it cannot exceed its designed detonation limit.

If you drove in the 70's and then when the first knock sensor equipped engines came out in the 80's you would realize this.

LOL wrong....

All modern cars have knock sensors.

Cold weather dose reduce the detonation point.

[Suberman]

Originally Posted by SpeedR

LOL wrong....

All modern cars have knock sensors.

Cold weather dose reduce the detonation point.

So, how much extra power is produced and at what temperature does this imaginary power show up?

This would be why your fuel economy improves in winter. Not.

[Suberman]

Originally Posted by Td-d

I'm well acquainted with the reasons why cold air allows for greater air-mass, and the functioning of intercoolers - I just really struggled to follow your logic.

The problem with this theory lies in that first sentence - this is not how the Subaru knock strategy works. It's a more reactive process - see here. It's also a curious question as to whether the BRZ is knock sensor controlled, since I suspect every ODB2 compliant vehicle since 1996 is likely to have knock sensors (I stand to be corrected).

I agree that direct injection reduces in chamber temperatures, that's the whole advantage, but DI is not used at all load ranges.

I read the link. Not sure what you mean. Knock corrected engines run maximum ignition advance possible (I'm generalizing a bit) and then retard ignition as needed when knock is detected. Pre knock sensor engines couldn't do this so maximum ignition advance was always set at less than the maximum possible. The other link in that tuner thread expands a little:

http://www.romraider.com/RomRaider/H...ndKnockControl

Just because the software only corrects by retarding the ignition doesn't make it "one way" which is what I think you think. If you start from maximum possible advance under ideal conditions then of course the correction will be one way. But that is an illusion since the system could as easily work in "both" directions. Ignition timing is no longer linear but 3D mapped. So it is an oversimplification to say the ignition is always retarded by the knock sensor. Really the knock sensor just modifies the ignition timing map in a designated way. It will always retard the ignition when knock is detected and advance it when knocking ceases, in a continuous feedback loop. There is no room for cold intake air to affect bmep in this system.

Similar systems controlling turbo boost can appear to be two way but isn't really, it's one way in the other direction. Base boost is fixed at a safe level and then the wastegate controller is "tricked" into "seeing" less boost pressure than is actually present in the intake. The boost pressure is permitted to rise until either the engine detonates or a pre-set maximum boost pressure is reached. Then boost is reduced progressively until detonation ceases and is permitted to build up again. This is a feedback loop.

Both systems operate on the same principle: feedback loop.

Cold air cools an intercooler very effectively permitting additional boost observable even on a non calibrated boost gauge. Also, in very cold conditions the over boost cut out is more readily reached.

For our highly tuned engines cold intake air can make no discernible difference in power output. I am happy for anyone to show this to be wrong by dyno or other means: increase in too speed observed or quicker acceleration times.