Yup. Another stupid basic engine question.

Does a modern fuel injection system attempt to correct for variations in temp and/or pressure to get the mix right?

 

Yes, which is why they are able in maintaining fuel efficiency over a wide rpm range.

 

OK, so would it therefore also limit the variance in HP over a temp/pressure range?

 

Yes, but a microcontroller does not change the laws of physics or chemistry. Higher elevation results in thinner air resulting in less power.

 

Very interesting topic guys! I can't make up my mind which approach is right and which one isn't. Both approaches would make sense. But if cold or hot air doesn't make a difference in terms of power then I'm wondering :

- why are we then all so worried of overheating the engine box which is heating up the air boxes and consequently the air going through the intake. If hot air delivers more power, it would be a good thing no? If it doesn't matter, why do we worry about this?

- in dynos, the CF is related to air pressure and temperature what would means that it DOES make a difference in the way HP is calculated.

Just wondering

 

Don't know about air pressure but when I used to drive small capacity crappy cars in my youth and tended to screw every last rev out of each gear I was most certainly of the view that cooler weather yielded better performance.

Best performance of all was found in cool temperatures mixed with fog or mist but slightly more difficult to use it since one couldn't see where one was going!

Now that I'm all grown up and mature I can't say I tend to notice the difference anymore. Or is that because I'm now driving a car the limits of which can't realistically be utilised to the full on public roads............?

To the original poster - drive that car just let the fluids get up to temperature before pushing on.

 

I fully agree that according to the universal gas laws the temperature difference should be more effective than the atmospheric difference when you have extreme temperatures such as in forced induction, but in the Uk the difference between a "hot" day and a "cold" day is only between 9 celcius and 20 celcius. The atmospheric pressure changes quite considerabley between those temps and if you think of the engine as a fixed volume compressor, then the induced mass will be higher with a higher base pressure. For example, if the rp was 10:1 and you started with 1 bar you would compress to 10 bar, but if you started with 2 bar you would compress to 20 bar. The mass of the air is measured at a known flow rate which is volumetric, therefore the quantity and therefore weight of the air must be higher for a given volume if it's at a higher pressure.

I think.

Aaron

 

Modern fuel injection systems measure the exhaust gasses with an oxygen sensor and correct the fuel/air ratio so that if you put in crappy fuel (e15) the car will still stay within the narrow window where catalitic converters work. This feedback system finds changes in atmospheric conditions to be easy to correct compared to good-gas versus bad-gas.

I might note, that the only atmospheric condition the F355 system measures is air temperature (as part of the MAF). Although it does amalgamate the manifold vacuum to regulate fuel pressure to the injectors.

-----------------

Air density can change over a 33% range from -40dF (North Dakota winter) through 110dF (Phoenix summer), while the greatest depression ever measured at sea level is only 13% (870 mBar) below standard atmospheic pressure (see:: hurricane Tip).

Now if you look at:

http://wahiduddin.net/calc/cf.htm

You will see a formula relating dyno corrections terms that uses pressure directly and takes the square root of the temperature. To a first order the square root of 33% {SQRT(1.33)} is 15% (1.15) which is still greater than the greatest ever measure pressure drop at sea level. At best, and away from hurriane and tornado activities, pressure is relegated to 5%-ish effects.

You really DO have greater HP in colder conditions--you also have LESS traction as keeping the tires warm is difficult at best.

 

Oh geez, now you've done it.
Blampee's next forum is going to be 'Tire Warmers - who makes the good ones?' LOL

 

oy vey

 

I have observed the following phenomena. The car seems to “sing’ or hit a sweet spot in the cool evening following a warm or hot day. This is particularly evident the greater the temperature differential between day and evening. I have noticed this over a wide range of cars, carbed and injected, over many years. I had always assumed it was due to either air temperature or volume. Of course temp, volume and pressure are symbiotic so I guess it could be all 3 influencing. Or just one. From this thread I’m still not sure which. Either way that sweet spot sure is nice when you hit it.