dsorx

I live in South Florida where it has been over 85 degrees with high humidity since I got my AT RX-8 back in March. This week we got our 1st "cold" front of the fall and the temperatures have been 75-80 degrees with low humidity.

Since the weather changed, my RX-8 seems much quicker. I was more than happy with the original performance, but the new acceleration and speed is very noticable. The slight hesitation at low rpm seems to be gone. I also have only about 3300 miles on the car (in 7 months), so maybe it is attributable to engine break-in.

Is there anything to this theory, or is it all in my head?

Thanks!

Abracadabra

I have an AT as well(Palm Beach county, FL...got it late August 2004...build date June 04. I too have noticed that yesterday the car was more responsive with the lower humidity...48 percent, than in the past humid days. Interesting that you mentioned it as well.

cgrx

Its cold and rainy in NorCal and the car feels better then average today...

irish8

Increased cold air = faster 8.....I'm moving to Alaska!!

- Irish

FSUBeachDude

Hey, I'm in North Florida in Pensacola. There is something to an increase in engine performance with your recent weather. Engines perform better when in an environment with lower humitity, cold temperatures, with increased mass airflow. The same applies for jet engines which is where I first learned about performance modifiers.

FSUBeachDude

I used to live in Adak, Alaska but I wouldn't go so far as to say I'd go back there for a little performance. At least not for an extended time. Maybe the mainland for a little longer since it is so clean and beautiful up there.

speedweasel

You guys have noticed what all pilots live by.
Higher air density improves the power on all normally aspirated engines.
Water vapor and high temperatures both reduce air density.
Any NASCAR guy can tell you the best day to race is a crisp, cool, dry day.

Anybody have some bootleg performance charts from Mazda showing Performance vs Air Densty?

FSUBeachDude

You're right, fly cold, fly fast (ram effect), and below isothermal.

StewC625

One of the car mags I subscribe to just had a thing in it regarding performance at different temperatures (can't remember if it was Road & Track, Car & Driver or Autoweek), but here's the jist of what I remember:

1) In their testing, the "ideal" weather for extracting maximum performance from a car was when the ambient temperature was in the 60's with as low humidity as possible. At that temperature, everything seemed to be optimized.

2) At higher temps, performance suffers for several reasons - engine is spending more of its energy on cooling itself (running the cooling fan for example), the air is less dense, meaning there are fewer oxygen molecules per given volume of air, higher humidities make the air harder to "burn" (the moisture "puts out the fire"), and the drive train is looser due to thermal expansioin, making more "slop", the fuel is more apt to vaporize rather than atomize, resulitng in less-than-perfect combustion, and other effects.

3) At lower temps (as the temp descends into the 40's, 30's, 20's) performance falls again as well because a) the engine is burning energy HEATING itself rather than moving the car; b) Fuel atomization suffers, affecting combustion and increasing unburned hydrocarbons in the exhaust, c) (and I found this to be most amazing) - tire resistance rises exponentially - the stat the article cited said that at 20 degrees F, tire resistance is 2x to 4x greater than it is at 65 degrees - the example they gave was that if the car spent 1.5 horsepower on a 65 degree day overcoming the rolling resistance of the tires, that it could spend up to 6 or 8 horsepower on a 20 degree day doing the same thing! c) The air is more dense, increasing wind resistance or aerodynamic drag.

So, the ultimate place to live apparently is in Chicago during the months of October and of April - highs in the mid 60's, low humidities. Too bad we have no "sports car twisties" and stifling traffic ....

OH, and I found the article - the November Car & Driver - and the article is about fuel economy and "why what you get isn't what the EPA gets." A topic of interest to many of us ...

foxman

Sounds like the ultimate place to live is San Diego, with generally mild weather, low humidity and lots of twisties...unfortunately also lots of traffic!

StewC625

Yup, I'd agree there - the ultimate place to live, IS San Diego. I love it there! Other than that whole "the earth shakes and knocks your house down" thing you Californians have to contend with ...

REX888

Anybody heard of Durban, South Africa (Major Port and Tourist Destination) (Hot, Mildly Humid). I live in Johannesburg which is +- 5250 Feet above sea level (Med Temps, Dry,). I got my RX8 3 weeks ago and went down to Durban for the weekend. The car is a monster at the coast, felt like a turbocharged car. We use 95 Octane Unleaded Fuel in JHB, where as at the coast they use 97 Octane Unleaded Fuel so that must have an impact as well.

What Octane Fuel do you guys use in the States?

foxman

Shell VPower 93 octane, currently around $2.25 per gallon (ouch!).

StewC625

Have been using either VPower or BP/Amoco Ultimate - both 93 Octane but the US method of octane measurement is different than in SA - we use the "average" Octane, which is the Research number (method used in your area - highest measurement) averaged against the "Motor" Octanme number which is the lowest measurement method - averaged (formula is R+M/2).

I'd guess your fuel is similar to our 93/94 Octane ...

Although I'm guessing you pay a LOT more than $2.25/gallon.

Actually right now experimenting wiith my first tank of "regular" 97 Octane fuel from Shell. If there's no performance difference, then I'm not paying the juice ...

G8rboy

I lived in West Palm for many years, and I can tell you the best performance mod I ever did to my Miata was moving to Illinois. The cooler, denser air provided a very noticeable power increase. I'm noticing the same right now with the RX8- temps in the 40's and 50's for the last couple of weeks and no humidity are making for some great driving.

scratchjunkie

cold air = dense = more oxygen = more zoom

StewC625

To a point.

Really cold air = less fuel atomization, more fuel consumption, more rolling resistance = less zoom

HeelnToe

It's an effect that's *very* apparent with turbocharged cars. Ask Ike

My Eclipse was a completely different car on cold days vs. hot days. I used to call the nice, cool fall mornings "turbo weather."

I haven't noticed a difference with the 8, but I'm sure it's there... just less dramatic.

REX888

If my calcs are right we pay around $2.70 USD per gallon.

RXE16T

On a somewhat related subject, maybe a forum member based in Alaska should do a 1/4 mile run and post his timeslip.

Should be interesting to see the time.

DAC17

Echo the "turbo" comment. Days in the 40's and 50's in CT have really brought out the juice in my Cobb Stage 2 WRX. Between that and the 8, I'm amazed the law hasn't gotten me yet....