pzull

Just wondering how the cars are running in Europe with the heatwave. Here in the tropics, I always feel that my car runs alot better late at night or really early in the morning.

Mid day gets to 33 celsius, just wondering what the car feels like at 40 degrees that's present in Europe now.

Christer

no difference, but then it might be because I don't run with an undertray. Sorry, I'll get me coat.

DaveK

I haven't noticed that with mine - but logically, I think they should feel slower. When I got my car dyno'd I was told air temp made a big difference - a few degrees can lose you 10bhp.

Jim Howley

Air temperature makes a massive difference to the performance of your car which is why dry winter days are so much fun

Apparently colder air is better for the combustion process and more efficient so the colder the air going in the better, hence I assume the purpose of having an intercooler.

IanR

When it cooled down over here, I noticed quite an improvement in performance, particularly in throttle response/pick-up - this was with temps dropping from 35C to 25C.

Apparently its the air density that matters - as cold air is denser than warm, effectively you get more oxygen into the engine the colder it is.

Would this mean that an engine with MAF would be less susceptible to temp variations? Presumably a MAF measures the mass of air rather than the volume - the denser the air, the more mass for a given volume, so the DME should be able to correct the mixture more accurately.

Gavin

My car never feels as sharp in the hot weather.....Does humidity have an adverse effect as well ?

Dave R.

Gavin,

Yup, humidity generally reduces engine performance.

Water vapor displaces some of the oxygen so there is less oxygen (imagine mixing one glass of lemonade with one glass of milk, now one glass of the mixture has half as much milk as the original glass of milk... proportions are not the same with air & water vapor but you get the idea). Also, water vapor is less dense than air, so the overall mixture is less dense than dry air at the same temperature and pressure.

Here is a short article on the USA Today website that explains nicely, scroll down a bit... hyperlink

"High, hot & humid" is worst combination for engine (and airplane wing) performance.

However a couple links that came up when I googled "air density humidity" indicated that octane requirements decrease as humidity increases, relating for example to the practice of water injection in supercharged engines.


-Dave
'90 C2

andrew964

Mine doesnt feel too sluggish, just stalls alot with the Air-Con on (Rs Flywheel and buggered up strosek ECU!)

pzull

Bugger that explains it. I get hot (33 degrees ave day time) and humid (90%) all days and most nights of the year.

That water injection thingy, does it work on naturally aspirated cars? My friend uses it on his supercharged car with great results (without intercooler) as he fills it with ice, water and methanol. Its something called aquamist.

scupper

Yep! That's why we living in the NE pity the folks in Southern Cal, Texas, etc. You all never get to experience the thrill of driving in 25 degree weather when the engines pulls like a champ!! Right, guys?

Chuck
93 C2

Drew_K

Yup. From my engineering days, I remember that the engine efficiency equation is a function of intake and combustion temperature. The lower the intake temperature (or higher combustion temperature), the higher the potential efficiency.

914und993

At lower speeds your car will accelerate more slowly when the air temperature is higher, but at higher speeds it may accelerate faster and reach a higher maximum speed because that hot ambient air also has less density. The reduction in wind resistance at high speeds more than makes up for the loss of horsepower at high ambient temperatures - on a flat or downhill road, anyway.

Chip

Adrian

The optimum ambient operating temperature for the 911 engine is 15 to 25C at sea level. As ambient temperature rises above 25C the air density drops and the resulting fuel air mixture is incorrect but the DME cannot adjust for it because it doesn't know about it . The air temp sensor in the air flow assembly helps but its calculations are fairly rough. As you increase in altitude and the ambient air temp continues to rise power output is severely reduced.
Here in Switzerland during this heat wave a definate loss of performance was noted in all vehicles not just Porsches. Even the turbo boys suffered because they got so hot. Extremely humid and hot weather will cause hunting idles especially on a hot start. We have had quite a few of these days as well. Been like living back in Darwin Australia.
Ciao,
Adrian
964C4

914und993

The O2 sensors in the 964 exhaust stream will allow the DME to keep the fuel/air mixture correct, that is their entire reason for being. Of course, as air density falls due to heat and altitude, no changes in fuel/air mixture are going to recover the lost power.

Even a carburetor will keep the fuel/air mixture in the ballpark with altitude and air density changes, though certainly not as well as a closed-loop fuel injection system.

Chip

pzull

I think I might have read somewhere or somehow got the impression that when amibient temperatures go above a certain level (28 degrees?) the DME would automatically retard the ignition even if the knock sensor doesn't say so. Anyone heard/read this?