Speed Density

iTrader: (38)

Just wondering why in higher DAs you have to run less compression, wereas in lower DA you can run much more.

LC

I am on the opposite wagon... Who's right?

Lower DA less compression... Higher DA more compression?

gun5l1ng3r

I would think that a higher DA (density altitude) there is more air/oxygen per volume of space and thus, the air is thicker. With thicker air, compression is already done a little bit for you, so you don't need to run as high of a CR to make the same HP at a lower DA.

LC

Thats backwards.. Higher = thinner air...

anyway.. depends on other weather conditions as humidity and pressure.. but higher = thinner air, for sure...

believe me.. i race @ 8000DAs... running a full second slower and more!

AOC

Wow, 8,000 DA??? And I thought we had it bad when we reach 3000 DA. lol.

SuperSporttruck

iTrader: (2)

Ubergeist

iTrader: (6)

Correct down to the t.

gun5l1ng3r

Yeah, I had it backwards. oopps!

SStrokerAce

iTrader: (2)

Yeah you want smaller ports and more compression and usually a tad more cam in higher altitude/DA.

Bret

gun5l1ng3r

When you raise the compression on a motor that is at say 8000 ft DA, do you have to raise the octane accordingly? Or does the less dense air allow more compression before detonation?

For example a motor at sea level with 10:1 CR needs about 91 octane fuel. If you have a motor at 8,000 DA, could you raise the compression to 11.5:1 and still run 91?

98 Raptor TA

iTrader: (9)

how do you figure out the DA of a certain area? I'm wondering what the DA is here in Dallas, TX

~RedLineLs1~

iTrader: (1)

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

I like to use www.accuweather.com to get all of the information I need. You can check the last 24 hours of weather by the hour.

BlackHawk T/A

iTrader: (3)

I know in airplanes / jets the higher you fly the slower you go because of the thinner air netting you less power, however you use less fuel at the same time.

98 Raptor TA

iTrader: (9)

Looks like Texas is 2248 DA

MadBill

All else being equal, you need less octane at higher altitude, and that's exactly what the oil companies give you!
Example # 1: I've fueled up in Flagstaff AZ at 7,000 + ft., and by the time I got to Phoenix (altitude ~1,500?) the engine was pinging like crazy...
Example # 2: When I was handling the technical aspects of the Players' Ltd./GM Motorsports Showroom Stock series for Camaros and Firebirds in the late eighties, about 10 cars from Calgary (altitude ~ 3,500 ft.) trashed pistons due to detonation at a race in Vancouver (altitude ~ 300 ft.). It was really hot, and it turned out they had all brought their own fuel from home. None of the local racers had any trouble..
An oil patch dictum is: "Don't ship fuel downhill."

~RedLineLs1~

iTrader: (1)

Must be nice. The DA here in Colorado is always around 7000+ft. Last time I went to the track it was 7900ft.

MadBill

Yeah, but that means you can impress the Flatlanders by casually mentioning the 13.5:1 CR in your tow truck...

BlackHawk T/A

iTrader: (3)

Actually Houston is under 2000, even being as insanely humid as it is right now, if the numbers are correct. Looks like elevation plays a much bigger role than humidity. In comparison i'm sitting at about 3000 DA.

stroked30th

iTrader: (2)

Yep, it sucks!

Old SStroker

As you increase altitude your Indicated Airspeed drops, but your True Airspeed stays the same or increases. Indicated Airspeed can be thought of as the number of molecules you plow thru in a period of time, while True Airspeed is how fast you pass each molecule.

A jet transport that cruises at 650 mph (true A/S) at altitude would have trouble going that fast near sea level.

A higher DA, or less dense air causes less drag on the airplane or the car for the same True A/S, so you get some gains from high DA, especially at higher speeds on the strip. Pro Stocks aren't that much slower in Denver.