996 turbo difference between hot and cold air ???

[Glenn McMenamin]

I have noticed a big difference in performance in my Turbo Cab, for driving it on a hot day, and then late at night when the air is cooler.

I know that cold air has more Oxygen (or so i was told), but didn't expect the difference to be so noticable.

Taking it to the French Alps this week, so wondering how much it will be effected by the altitude ??


Any views on this guys ??


G

[GetCarter]

Completely unscientific - but I've noticed about 15 mph top speed difference since it's been hot. (Could also be rubbish Highlands fuel kicking in).

Steve

[turbobloke]

It's density, air has same oxygen % but you get more going into the engine when the air is cold (and more dense). Noticeable on most turbo cars.

[TKD]

The greater the density of air, the greater the explosion when mixed with fuel, which results in greater forward propultion (you go faster)

A cold damp night/morning (esp foggy) will see a much improved increase in performance (doesn't take as long to crash into a tree)

In hot humid conditions, a turbo will often feel a little flat. I've heard of some people re-routing the air con into the air box for increased performance.

[dcw@pr]

Surely the effects would be less noticeable in a turbo than an NA car? The air in the cylinders of the NA car is at atmospheric pressure, ie. denser when cold, but the air in the turboed cylinder has been increased to a set level (controlled by the wastegate), so the difference between hot and cold air will be nullified?

[rico]

This is why people like GruppeM make carbon inlets. These shield the intake of air into the engine from the heat of the engine.

They keep the air colder, thus denser, thus giving more oxygen into the engine.

[toppstuff]

Every car I have ever owned always went much better with cool, crisp morning air in its lungs.

It will be an issue in hot climates. The car will go fine, but taking it out at night or in high mountain air will always surprise you with the difference it makes.

[TKD]

dcw@pr said:

Surely the effects would be less noticeable in a turbo than an NA car? The air in the cylinders of the NA car is at atmospheric pressure, ie. denser when cold, but the air in the turboed cylinder has been increased to a set level (controlled by the wastegate), so the difference between hot and cold air will be nullified?

Air plays a greater role in a turbo car, this is why it is more noticeable.

[turbobloke]

dcw@pr said:

Surely the effects would be less noticeable in a turbo than an NA car? The air in the cylinders of the NA car is at atmospheric pressure, ie. denser when cold, but the air in the turboed cylinder has been increased to a set level (controlled by the wastegate), so the difference between hot and cold air will be nullified?

It's more complex in turbocharged engines as there's both temperature and pressure (pre-compression) to take into account, as well as any intercooling that may be used. The temperature of the gases emerging from a turbo operating at 1 bar can be surprisingly high, after all you see them glowing red hot from time to time in race settings.

If the engine is being fed hot, high pressure air, the maximum power available is significantly lower than if it’s inhaling cold, high pressure air. So temperature still matters even when you're squashing the air up to get more in.

When the engine is being fed cooler air, through lower ambient temperatures or efficient intercooling or water injection or any combination, you can run more boost, or whatever, and increase output without detonation. However, go too far in this direction and the fuel will form micro-droplets before ignition and the result is the same as detonation, melted piston crowns.

Mountain air will often be cooler but if you go too high and the air is thinner (lower density again) the gains can be offset.

[dazren]

This could be one of the contributing factors to my not been able to match my 192 top speed at Brunters on subsequent visits. The 192 day was notably colder than the other days. Till now I had been imagining there was a tailwind.

DAZ

>> Edited by dazren on Monday 16th August 22:07

[dogsharks]

colder air has a more dense charge and therefore makes more power, and that's why I laugh every time I see a cone type filter inside an engine compartment (most of which are located right behind the radiator or over an exhaust header).

It would be interesting to see if cold humid air is better than cold dry air, which I believe it is.

Dogsharks

[diver944]

I believe the reason that a cold AND humid intake creates more power is similar to the cooling effect of a water injection system. The humid air has a built in water injection affect, and it's free

[cyrus1971]

Yup altitude in my recent Alpine hike made slight a difference, it was also 35 degree heat and not humid – just hot dry air. All these combined to make the car feel noticeably slower.

Well known about colder air being more dense, hence more Oxygen, hence better burn, hence power.

Contradictory as it may sound damp air is better still for Turbos. The water in the air makes for reduced “knock” and a smother feel to your driving. My old SAAB 900 8 valve turbo was at its best driving on a foggy / misty winter night. Cold and Damp is best, hot dry Alps are at the opposite end and I guess this is why I felt about 20bhp less power in the Alps.


Have a fun trip - those roads are great.

[RoShambo]

Some of the higher performance Subaru's have a water spray that sprays a mist of water into the air coming into the car via the bonnet scoop.

My mate designed one himself for his car that didn't have one.

Would be easy to fit one each side that sprays a mist of water into the air coming into each side scoop on the Turbo.......

[TKD]

My Impreza STI Type R had this, I'm surprised makers of other high performance turbo cars haven't thought of doing something similar.