Density altitude (Tulsa ok.)

RCJ · Post by **RCJ** » Sat Jul 19, 2008 7:39 pm

I ran my car on june 20, 12.65et.Temp 71, hum.55, BP 29.85 I was given a program to find the density alt. 1166ft is what I got.Made some changes.Ran july,19 12.67et ,temp 80, hum72, BP 29.89 Density alt.1841.Did the changes help and the air slowed the car down?

Post by **Stan Weiss** » Sat Jul 19, 2008 9:33 pm

You would have lost 2.5-2.75% engine power from the increase in Density Altitude. I would say the changes helped.

David Redszus · Post by **David Redszus** » Sat Jul 19, 2008 10:56 pm

While the changes in conditions would have produced a change in air density of -2.4%, that does not mean that power will be reduced by 2.4%.

Air density and power do not correlate exactly. Neither does air density and mixture. In your case the change in air would have reduced power by .67%.

In addition, air density has an effect on vehicle speed due to change in aerodynamic drag forces.

Post by **Stan Weiss** » Sat Jul 19, 2008 11:57 pm

I used his weather data and SAE J816 Dyno Correction for calculations to get the differnence in power.

David Redszus · Post by **David Redszus** » Sun Jul 20, 2008 11:02 pm

My calculations were based on SAE J1349 which is the same as ISO 1585 and JIS D1001.

J1349 calculates as follows: A^1.2 * B^0.5

What this illustrates is that dynos may or may not be very accuate depending on which correction factor is being used.

Post by **Stan Weiss** » Mon Jul 21, 2008 12:47 am

Based on the 2006 FORD Performance Parts catalog that would be SAE J607. Original run correction 1.032908 (VP 0.420066) later run correction 1.05549 (VP 0.741798) difference 0.022582 or 2.2582 %. The only thing that I can think of is that one of us coming up with the wrong corrected Barometric Pressure / Vapor Pressure.

David Redszus · Post by **David Redszus** » Thu Jul 24, 2008 11:22 pm

Stan
Are you assuming that a change in air density corresponds to the change in power?

Post by **Stan Weiss** » Thu Jul 24, 2008 11:48 pm

Dave,
I am using this formula A^1.2 * B^0.5. I just noted that the 2006 FORD Performance Parts catalog calls that SAE J607. For the Original run I get a correction of 1.032908 (my Vapor Pressure 0.420066). For the later run I get a correction of 1.05549 (my Vapor Pressure 0.741798).
Stan

RCJ · Post by **RCJ** » Fri Jul 25, 2008 8:16 am

The change I made was a performer intake with a 1'' 4 hole spacer to a RPM air gap with 1'' open spacer.

Post by **Stan Weiss** » Fri Jul 25, 2008 1:21 pm

Stan Weiss wrote:Dave,
I am using this formula A^1.2 * B^0.5. I just noted that the 2006 FORD Performance Parts catalog calls that SAE J607. For the Original run I get a correction of 1.032908 (my Vapor Pressure 0.420066). For the later run I get a correction of 1.05549 (my Vapor Pressure 0.741798).
Stan

Sometimes I just what is not there. I just check the formula for the forth time A^1.2 * B^0.6

hotrod · Post by **hotrod** » Mon Jul 28, 2008 8:49 pm

You might want to browse around and look at the calculators this guy has for figuring air density and relative power output. He has detailed explanations for how the math is done as well.

He is a professional engineer and knows his stuff on this subject.

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

Larry