Density Altitude, and it's impact on your engine's power output

[FNBADAZ06]

Before I begin, let me just say that the driver's ability to control a car's driveline when being piloted down an 1/8 or 1/4 dragstrip is paramount to running a stellar ET, period. Power without control means nothing (hhhmm...sounds like a commercial punchline ), and the more efficient you are as a driver in controlling and wielding that power, the faster you'll be.

I've seen a few threads recently on CF that question what benefits Density Altitude (DA) has on the performance envelope of our car's, or the "corrected" dyno outputs we see on dyno graphs. Since the car's performance include's the drivers ability to control and apply the power to the ground, along with precision in shifting thru the gears, DA numbers don't tell the full story. On a dyno run, we can turn off any corrections being applied to see what the "true" horsepower and torque figures were during the run.

With that said, what we can talk about is what impact density altitude has on an engines specific horsepower output rating if we know the corrected SAE power output, and what advantages (or disadvantages) that DA has in generating the horsepower we all crave.

I'm going to concentrate on the LS7 engine, as that's what I drive, but obviously this applies to any engine.

Let's recap some information, and I'll start with the "official" rated horsepower of the LS7, pulled straight from the Interweb :rofl:.....

"Society of Automotive Engineers (SAE) released a new testing procedure, J2723, to determine horsepower. The new 7.0 liter LS7 engine for the 2006 Z06 Corvette, is the first engine certified under the new procedure. The output of the LS7 is certified at 505 horsepower at 6300 rpm and 470 lbs.-ft. of torque at 4800 rpm."

And, what values does SAE J2723 use in this certification ?

Wiki says " The new SAE J2723 is actually not a new set of correction factors, it is simply a new procedure for using the existing factors (J1349 and J1995) used by automotive manufacturers"

OK....So what is J1349 ?

" The newer SAE J1349 standard is a 77°F (25°C) day with 0% humidity and a barometric pressure of 29.234 in-Hg (99 KPa)."

So, at 77°F (25°C) with 0% humidity and a barometric pressure of 29.234 in-Hg (99 KPa) the LS7 puts out a certified 505 horsepower at 6300 rpm and 470 lbs.-ft. of torque at 4800 rpm.

Sounds pretty simple so far.

But what happens when any of those values change....what happens to the power output ? We know it can be a benifit or a handicap on rated horsepower production, depending on which way those numbers move. And the dynometer software we use only corrects the horsepower and torque readings one way....to a known set of correction factors like SAE, DIN or Standard.

So....is there an available calculator to apply changes to the values that SAE J1349 uses, to determine a new correction factor to see what approximate horsepower an engine makes when temps, barometric, or humidity, change outside of 77*, 0% humidity, and a barometric pressure of 29.234 in-Hg (99 KPa), if we have a known, valid SAE HP figure ?

The only one's I've seen previously were using ET, trap speed, and vehicle weight, to calculate an approximate HP number...but they seem somewhat fallable.

If we had one that used local weather conditions, this would then give us an appropriate number to use as a correction factor to adjust a known SAE horsepower figure up, or down....and see what kind of performance boost (if any) the "great air DA's" are giving to our engines. :

And lo and behold, I found one

Engine Tuning Calculator

Using this calculator you can apply a correction factor to any horsepower rating, be it SAE crank horsepower or a chassis dyno horsepower, based on local meteorlogical conditions.

Here's an example of an LS7 that is used in a -DA of -2194'.....the conditions reported by Dragtimes.com for Maryland International Raceway (MIR), a track that spits out more ET and trap records for stock Corvette's than any other track. Plug in the reported meteorlogical conditions for 12/10/2011 at 11:00 a.m. ....

A stock SAE Certified 505 HP LS7 would make 110.1% of rated power.......or an additional 51 HP at the flywheel :D

You now have approximately SAE 556 HP at the crank, or using a 15% driveline loss calculation....an additional 43.35 RWHP over stock :

p.s. - for the math wiz's amongst us, I found the correction formula's in it's raw state on the Land and Sea dyno web site http://www.land-and-sea.com/dyno-tech-talk/corrected-horsepower.htm

[FNBADAZ06]

By the way, reversing the process , the formula shows the DA of the SAE correction factor by placing the 77* ambient temps, 0% humidity, and 29.234 inHg into the equation :

The DA equates to +1947' ASL :

At this DA, with 0% humidity (an unrealistic situation in our atmosphere, by the way ) a factory stock LS7 should make SAE 505 HP at the crank on an engine dyno

To get a more realistic meteorlogical condition, let's use Mean Sea Level Pressure at 29.92 in Hg (101.325 kPa), 0' above sea level for altitude, 77*F for ambient temp, and a more realistic 73.8% relative humidity (68*F Dew Point). Using these values, we also get our proper 1.0 correction factor, and DA is +1459' ASL

[FNBADAZ06]

And finally, a nice sunny day in Phoenix at 113* , 22% humidity, and a 29.00 baro

Say goodbye to 7% of SAE power...before you even take into account the high IAT's and ECT's which will sap even more power because of timing reduction.

The 505 crank HP that the LS7 puts out is NOW probably only 430 HP...if I'm lucky

The 430/436 crank HP that the LS3's put down is NOW probaly only 364/370 HP.

The LS2 400 crank HP is NOW 336 HP.

The LS1 350 crank HP ? Try 289 HP

If your a Crossfire car, don't even think of running the correction factor as you'll instantly become suicidal once you see you barely make enough power to run the accessories bolted on the front of the engine

[Two Fangs]

:bannana4: :bannana4:

[LastC3]

Good info .... way

(( and thx for the honorable CFI mention ))