A friend and I were talking about our TTs and we both commented on how our cars seemed to respond much better on cooler days-no surprise there. So it lead us to wonder: does anyone know of a correlation/"rule of thumb" between the air intake temperature and horsepower, specifically with the 996TT? Obviously the cooler the air the more dense and intuition (as well as my butt dyno) tells me that there is a gain-but how much?

I'm sure the correlation is not linear. In other words the answer is not as simple as 10 hp for every 5 degree change in temp. Also, I think temperature differences will change HP differently depending on where they occur-the 10 degree difference between 90 and 80 should generate a greater HP gain/loss than the one between 60 and 50-or is this not correct? Is another consideration the HP rating-will higher HP cars see greater swings in their HP than those experienced by lower HP cars due to similar temperature drops?

Not looking for a big scientific explanation rooted in thermodynamic theory, but rather a simple rule of thumb as it related to our cars. But if someone takes the time to explain it scientifically I will surely read it.

Thanks in advance...

 

There are to many variables for there to be a simple answer that you are looking for. Ambient temperature is a factor in the IAT, but humidity will also effect the IAT. Its my understanding that IAT deltas can fluctuate depending on temperature and humidity.

I would also say that cooler temps do not add HP, but rather, warmer temps reduce HP...

 

Sure density will make a difference which should be linear since power is made by fuel and air. All you need is the formula for air density vs temp. As far as I know, our ECUs will adjust timing depending on air intake temps to keep our engines unharmed. This one is much harder to estimate but some tuners might know the formula.

CW

 

I doubt any one can account for all the variables-was hoping for a rule of thumb that isolated temp.

That would be great. Hopefully some tuners will chime in with their experiences, even if anecdotal.

 

Very good question..... subscribed.

 

Here is an interesting theoretical link:

Engine Tuning Calculator

 

I see what you guys are saying but the numbers and link seem low. Maybe "feel" reads high, but if the temp goes from 90 to 50, I am supposed to make but 10 more hp given a 500hp starting point?

Maybe those numbers apply more directly to na motors?

Not arguing....just "seems" like a low number.....

 

Is it true? For a 700hp car, a 40F drops only got 14hp gain! It seems not that much as I expected.

 

I can tell you that here in Phoenix when it is 115 Degrees, my car is much slower than in the winter. I think it is mostly due to the ECU pulling timing. If I fill up with 100 octane, the car is violently fast again. The car is smart and just wants to protect itself.

 

Way to many variables. The density of the air increases as the temperature drops, so the more air that can be crammed into the engine the better the effects. A 408ci with 2 88mm turbos is going to notice a much more drastic difference in air temps, than a NA Honda 1.8l, etc.

 

Thanks for the link!

Now according to the calculator, if I input the following values:
60 Air Temp
29.235 Hg Altimeter
57 Dew Point
0 feet Altitude

The above parameters will yield 100% relative horsepower.

So keeping all other variables constant, if we increase the Air Temp from 60 to 80 we get 97.8 Relative HP. According to this result, a 700HP car would see a 15.4HP decrease (700HP X .978 = 684.6HP), correct or am I missing something?

If I'm correct I must say that I expected much greater differences than what this calculator is producing-I must be missing something becuase my butt dyno is not that sensitive. Perhaps there are greater temperature differences going on internally...

Assuming the 996TT has reached normal operating temperature and that the ambient temperature is 80F. Anybody know (or estimate) what temperature is expected at the compressor's outlet? What temperatures are expected post intercooler (stock) or intake? The answer to the second question has to be above 80F becuase that's the best an air-to-air intercooler can provide.

 

Not really. Cooler air adds HP AND Warmer air reduces HP. They mean the same thing, just going the other way.

Think on a hot day, you lose a lot of intercooler efficiency. PLUS the air right off the pavement, is usually a lot hotter than the air you feel. Finally, a cool 40-50 degree California day is literally 60 degrees cooler than a hot day 100-110. The car really feels sluggish at those temperatures.

 

ah, it's getting technical, that's fine!

you need to differ between the physical and SAE/ISO/DIN correction factor of temperature and the actual result that is due to ECUs interaction.

First of all the physical dense of air correlates nearly with Square root of the actual temp related to the absolute temp in terms of degrees kelvin (-> 90 degrees is not "twice as hot" as 45).

if you omit the air pressure the equation for the SAE/ISO correction factor/coefficient is equal to (actual temp in kelvin / 298)^0,6
If you set it into relation with the temp before in winter or summer you have the difference in %.
the 97,8% already mentioned are probably calculated in the same way.
this numers would be very true for an old school non aspirated engine with no electronic.

but these numbers lack alot of circumstances we also face!

We have turbines, aircooled I/C, knock sensors, different fuel and a ECU that trys to get our engine safe.

simply by generating boost to above 1 bar we increase the temperatur of the air a lot (>120 degree celsius) and try to get it cold again with the I/C, which has usually an effinciency of ~80%, which roughly means the intake temp at WOT is always many degrees above environmental air.
If you have an OBD2 monitor you can see that very nicely.

in summer conditions you reach a certain trigger temp of the (stock) ECU very fast. it then will go back with ignition and even boost later on. Also knocking probability increases and the ECU reacts with lowering power.

A stock ECU normaly trys to compensate air temp and pressure diffs, but only to a certain amount. I know many Turbo drivers who claim that their max boost went down and they were afraid it could be a failure, but it is the ECU that decides it wont need any more boost if the air is already dense enough.

Any of you having a good data logger can check it, if 60-130 times are really better than in summer. there is no big difference. If you drive in the desert of nevada you will always suffer from ECU safety mappings but if I compare with zero and 25 degrees celsius, I cannot see anything significant.

 

Excellent! Using turbos at 1 bar increases the air temp to 120C, that's almost 250F! No wonder ICs are so critical.

So if our ICs are 80% efficient, what would the intake temp be if ambient air was 80F? I understand GT2 ICs are a bit more efficient than stock (by 15%?) and that other after market ICs are even better than that. What kind of reductions in the temperature would be expected by using those, all other things being equal?

 

80% means roughly said: (250-80)*(100%-80%)=34F over ambient. therefore a 90% A/C would decrease the intake air for another 17 degrees. this is not a big deal but can be important if you are close to some temp where the ECU wants to throttle down.