loonyjuice

Just a thought, but I'm sure many of you have noticed how much perkier a car is when running in colder conditions. Does anyone have any idea how much extra power is generated between say a difference of 30 degrees or so? It's currently 6 degrees here in Manchester today.

MAGK944

Don't have a chart but as I simply understand it, cooler air is more dense. Higher air density feeds more oxygen into the engine.

That is why some turbo's/supercharger systems use intercoolers (charge air coolers) to increase air density by substantially reducing the temperature of the pressurized air from the turbo/super to the intake manifold. By lowering boost air temperature, the denser air is more oxygenated and allows complete fuel combustion.

There is also an excellent byproduct of lower intake air temperatures; they produce lower EGTs. If EGTs never build to the temperature where the computer tuner limits fuel delivery, all of the additional power is always available. Lower EGTs also help to reduce the thermal load on the engine and its cooling system, which keeps them running cooler as well.

M

quinnfiske

Is EGT monitored by the system? I thought it just monitored coolant temp.

Around here the effect is really noticable. At 0 degrees F, my 951 is a rocket. I can notice it on the boost gauge as well as subjectively.

ekeeton

What Mike said. There's a lot of very ugly math behind it but the four factors are air temperature, pressure, humidity, and altitude. As temperature, humidity, and altitude increase, density decreases.

loonyjuice

I was thinking, in rough terms, that since it's a noticable increase when colder, that it must be in the order of say 5 extra bhp. Maybe more for a turbo.

MAGK944

I was speaking generically here, not model specific. We are of course resitricted my our rather dated engine management.

Tell me about it, living in South Florida, 80+deg, 80+humidity, it's a good job we have zero altitude here otherwise we would just never move.

Very difficult to guesstimate, as ekeeton says, too many factors to take into account. Just enjoy the extra power you are getting on the cooler days.

M

Pcar944

Wouldnt it make no diff for a turbo?

Mike C.

It certainly feels a bit more powerful on my (early) 85 NA when it's cold. However it may only be while the engine is not up to operating temp and therefore not in closed loop mode (i.e. the DME is not adjusting air-fuel ratio according to oxygen sensor output). As I understand it, when in open loop mode, the FA defaults to a richer value and with the cooler, more dense air you get a little boost in power. But I'm not sure if/what Euro models used oxygen sensors. If your car has no O2 sensor, well then my theory would not apply...

mike661zero

I have heard a few times that for every 11 degrees F that you change you will gain/lose 1% horsepower. I have no idea how true it is but I seem to remember hearing it from sources that knew what they were talking about.

bad_monkey

Another reason that boosted cars run an intercooler is that it cools the intake charge, and cooler charge means less total energy. Detonation can occur due to the increase in reaction potential as the charge is compressed (think less space, more fuel and air colliding) and the initial fuel and air temps play a big factor in this.

It's possible to run more boost without detonation as long as you keep the temperatures down.

Some racers I know use direct water/alcohol or water/methanol injection to cool the combustion chambers and increase the duration of combustion leading to more efficient use of the fuel. In effect it also allows you to run leaner at max power without the normal risks.

I've heard Nitrous Oxide also cools the intake charge, as well as bringing more oxygen to the mix - but I've not got much data on how efficient this is with regards to avoiding detonation.

I'm currently looking into water injection, hoping that perhaps I can run some decent boost with a supercharger on my NA, while retaining the Euro-spec high compression pistons. (yes, I know, I'm gonna break it)

In short, cold air good.

whalebird

Around here where I drive constantly between 3000 and 6000 ft. in altitude I notice a nice boost on cool mornings. When its foggy and cool I really notice. Turbo and supercharged cars really like this. I don't know much about the weather end of this discussion but throttle responce changes with altitude and temp. variations. This brings me to something I've been wanting to ask. Has anybody ever diagnosed a faulty altitude sensor. On my 924S its the round thing next to the DME that has a Denso(Japanese) decal on it. Its real expensive to replace but I've never seen or heard of problems with them and one might be able to play with it to slightly effect mixture and make more power. Love to know what effect it plays on the performance of the car. Sorry if this is OT.