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Old 10-31-2005, 07:26 PM   #18
Scooby Specialist
Member#: 14141
Join Date: Jan 2002
Location: [email protected] @ 5800 ft on 13T
2002 Impreza WRX


Very good post!

You are correct that altitude correction is difficult under the best of conditions, and for turbocharged cars doubly so. Here in Coloroado it is common for SAE altitude correction factors to compute out in the 1.25 - 1.27 range because of the way the SAE standard is setup. (most dynos do the calculation automatically so the user has no direct control over the calculation only the data inputs like air temp).

In practice we find that the proper altituded correction factor is typically about 1/2 the SAE value. One shop here in the Denver area has recognized this and uses a fixed 1.15 correction value that gives more reasonable results when compared to sea level dyno numbers. Unfortunately by doing so, they lose all day to day correction for different air temp/ barometer readings so to make comparable tests on the same car you would need to make those corrections by hand calculation.

The altitude correction factor for turbocharged cars can never be a simple mulitiple of the NA correction factor because the change in altitude directly effects the effeciency of the turbocharger itself. By the same token how much boost you are running also changes the proper correction factor.

Below is a discussion thread I posted over on awdpirates on this same topic with a few minor editorial notes.


There have been many long and bloody discussions on several forums regarding a fair and meaningful correction factor for Dyno runs at high altitude. The current attempts to compare high altitude dyno numbers for supercharged and turbocharged cars to near sea level runs indicates there is a problem.

It is clear to most of the performance community that the current dyno correction formula based on the SAE J1349 ( Jun 90 ) procedure is broken for high altitude turbocharged cars. It causes a huge over correction for sea level conditions.

Over the last few years, my own research and many on-line dialogues have led me to the following analysis of why the current formula is broken.

I propose a simple modification that appears to fix the problem and give more reasonable corrections.

Any comments or discussion please post to the motor forum!


Discussion :

The SAE J1349 (Jun 90) power correction formula is:

Metric units millibar / deg C
CF = 1.180 [ ( 990/Pd) x (( Tc + 273)/(298)) ^ 0.5 ] - 0.18

in hg / deg F units
CF = 1.180 [ ( 29.23/Pd) x (( Tc + 460)/(505)) ^ 0.5 ] - 0.18

Pd = dry air pressure ( ie absolute air pressure minus the contribution of the partial pressure of the water vapor in the air)

Tc = Ambient air temp ( ie temp outside the car, not the intake air temp)

The formula is supposed to give you the hp the car would make if it was at an absolute air pressure of 990 mb (29.92 in hg) 0% humidity and 25 deg Centigrade (77 deg F) outside air temp.

My understanding is that the ambient air temp measurement is supposed (according to the dyno manufacturers as they specify that their temperature sensor be placed some distance from the car) be the free air temp well away from the car, not the air temp under the hood where the intake is.

If you measure it under the hood, at the air filter, a car that was dyno tested with the hood closed, and has crappy heat management and pulls in very hot air would be corrected to a higher hp number than a identical car with the hood open and cool air flooding the engine compartment from the dyno cooling fans.

One way to cook the dyno numbers produced by a chassis dyno is to artificially increase the dyno correction number by giving it false air temp data. If the air temp sensor is positioned so it reads heated air, like one dyno tune plot I saw, where the intake air temp was listed as 101 deg F.

This is likely the actual intake air temp during the test, not the ambient dyno cell air temperature.

For accurate adjustment for air temp, it should be measured at the air filter!!
Not at some fixed location in the dyno cell.

If you make x hp at an outside air temp of 101 deg F, then you would make significantly more if the outside air temp was 77 deg F. This is what the correction factor formula attempts to do.

By telling the dyno the outside air temp is that hot, it makes a bigger correction to get back to what the hp would be under standard conditions for the formula which are, 990 mb dry air pressure and 25 deg C air temp.

Basically the formula uses the long standing principal that an engines power output varies at the inverse of the square root of the intake air temp, and directly with the local air pressure.

If your dyno conditions are exactly 990 mb dry air pressure and 25 deg C ambient air temp, the correction goes to zero or 1.00.

SAE appears to have tried to come up with a correction formula that comes out to a very user friendly correction factor of the form 1.xx , and attempts to correct for an assumed drive line power loss of 18%.
(I'm useing the term drive line loss to account for all drive train related power losses including rolling friction of the tires)

The way they structured the formula when they did that, broke the formula for test conditons that fall well outside their "norm".

The formula in essence assumes all cars are NA, and have an 18% driveline power loss to the wheels. This causes it to underestimate drive line losses on AWD cars which appear to be in the mid to high 20% range.

If you discard the first and last terms of the formula, which appear to be a drive line loss correction, and add a factor for absolute manifold pressure, you still get a correction in the form of a decimal number near 1.00 as your correction of power compared to their standard conditions.

You would need to compensate for drive train loss independently if you wanted to correct back to flywheel hp.

For example if you throw out the first and last terms, you have an equation of:

(sorry I like english units -- to convert to metric simply replace the 460 value with 273, and the 29.23 value with 990 and so on for the other inputs)

in hg / deg F units
Larry'sCF = ( 29.23/Pd) x (( Tc + 460)/(505)) ^ 0.5

For the test conditions of this dyno report which was taken at an altitude of about 5500 ft, of 101.41 deg F, and 24.49 in/hg ( we will ignore the humidity to simplify things)

Larry'sCF = ( 29.23/24.49) x ((101.41 +460)/505) ^ 0.5

Larry'sCF = (1.19396) x (1.05437) = 1.25887 <---- this would be the correction factor appropriate for a NA engine. For a turbo charged or supercharged engine you would have to also include a correction for the boost pressure, because the difference between sea level absolute manifold pressure and high altitude manifold pressure when expressed as a ratio would change the higher your boost pressure became.

For example = 20 psi boost at 29.24 in-hg, vs 20 psi boost at 24.41 in-hg air pressure

would be 40.7 in-hg boost, plus 29.24 in-hg atmospheric pressure = total manifold absolute pressure of 69.947 in-hg.

20 psi boost at 24.41 in hg air pressure = 65.11 in-hg abs

the ratio is 69.95/65.11 = 1.074 correcton at 20 psig manifold pressure

if the boost is only 10 psi, the ratio becomes 49.59/44.76 = 1.108 correcton at 10 psig manifold pressure

So the higher the manifold boost, the lower the effect of altitude. Compare the two boosted values above to the 1.194 correction factor due to air pressure in the NA case.

This all assumes there are no effeciency changes in the turbocharger or suprecharger due to altitude which we all know is not the case, but much better than the standard SAE formula.

So my final modified correcton formula for the example should be:

Larry'sCF = ( [(boost)+29.23]/[(boost)+24.49]) x ((101.41 +460)/505) ^ 0.5

Larry'sCF = ( [(boost)+990]/[(boost)+829]) x ((38.56 +273)/298) ^ 0.5

boost = manifold gauge pressure in millibars.
millibar = 0.1 kpa
in-hg= 3.386389 kilopascal (kPa)
1 atmosphere= 101.325 kilopascal (kPa)
1 psi= 6.894757 kilopascal (kPa) = 68.94757 mb

Boost expressed as in-hg = 2.036 x psi
Boost expressed in mb = 68.9116 x psi


boost = manifold gauge pressure in millibars.
millibar = 0.1 kpa
in-hg= 3.386389 kilopascal (kPa)
1 atmosphere= 101.325 kilopascal (kPa)
1 psi= 6.894757 kilopascal (kPa) = 68.94757 mb

Boost expressed as in-hg = 2.036 x psi
Boost expressed in mb = 68.9116 x psi


In general form the modified factor would be:

Larry'sCF = ( [(boost)+990]/[(boost)+Pd]) x ((Tc +273)/298) ^ 0.5

Larry'sCF = ( [(boost)+29.23]/[(boost)+Pd]) x ((Tc +460)/505) ^ 0.5

Boost= manifold gauge pressure
Pd = local dry absolute air pressure
Tc = local ambient air temp


Now the above also assumes (unspoken) that all other variables are controlled, such as tire pressure, tie-down tension, oil temp and weight, coolant temp etc. etc.

Fact is, very few dyno operators even have the means,(to measure oil temps, and coolant temps in real time) let alone the time and economic interest in controlling all those variables. Transmission gear lube temps (for example) can make a difference of over 20 ft/lb of torque between a warm gear box and a gear box at full operating temperature. The same sort of effect is seen in turbo spool up with header temps. An engine that is started and run on the dyno with relatively cool exhaust headers will spool noticably slower than the same car allowed to stabilize at normal operational temps before the actual pull is made.

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