Don’t Trust Strangers! How Dynos Lie and When to Believe One


Well, what if you have a Dynapack that bolts to the hubs?  That at least eliminates the variables attributed to the wheels, tires, and alignment.  #4, fluid temperatures have an effect.  Warmer engine oil and tranny fluid have lower viscosity resulting in lower power loss; so dynoing a car that is cold will have a different result compared to a car that’s already warmed up.  I think it’s good practice to do numerous dyno pulls until the results become consistent.  This should be a sign that all the fluids are up to operating temperature.

A STI bolted up to a Dynapack.  No worries about straps, wheels, tires, or alignment here. 

#5, the weather.  You say there’s a weather correction factor though.  Not so fast my friend!  It works okay for a naturally aspirated engine in a relatively narrow temperature range, but throw in a turbo engine and larger temperature and elevation differences and it can be way off.  Let’s have a closer look at the SAE J1349 correction factor.

This is the SAE J1349 AUG2004 (August 2004) revision that was adjusted from the previous JUN90 (June 1990) revision to better take into account frictional losses.  Notice that the ambient pressure is for dry air.  So if it’s humid out, it needs to corrected to a dry air value.

The SAE correction factor is only meant to be relatively accurate in the range of 15C to 35C temperature and 900 to 1050 millibar with the standard values being 25C and 990 millibar.  The two variables taken into account are temperature and ambient air pressure.  These two variables affect air density which therefore alters power output.  Using some good ole math, we can calculate air density as a function of temperature and pressure.  I did each individually while keeping the other variable constant to make it simple.

As you can probably guess, the SAE J1349 correction factor has standard values of 25C for temperature and 990 millibar (99000 Pascals) for pressure.

For a naturally aspirated engine, the SAE correction looks to work pretty decently within the specified range that the correction factor is supposed to be good for.  At 15C, the relative air density difference is about 3.5% and the correction factor is 2%.  The SAE correction factor takes into account some frictional losses and there are probably some differences in pumping losses due to air density, so the correction factor seems about right.  Looking at 0C temp though, there’s a 9.2% difference in density and only a 5% correction.  Air pressure outside of the range of the correction factor similarly shows a big discrepancy; ambient pressure of 84556.29 Pa (1500m elevation) has about a 14.5% density difference but a 20% correction factor.  So if you stay within the defined limits of the SAE J1349 correction factor, it works pretty well for a naturally aspirated engine.  What happens with a forced induction engine though?

This subject of dyno correction factor and forced induction cars at altitude came up when I as helping DG-Spec and Project Scion tC.  The series they were racing in had a horsepower limit and all the cars were to be dyno tested up in the high altitudes of Salt Lake City for the round at Miller Motorsports Park.  At the time, Project Scion tC had a TRD supercharger setup which runs a fixed boost pressure.  The great thing about forced induction cars is that they basically compensate for the reduction in air density at altitude; that’s why forced induction cars lose less horsepower at altitude as compared to naturally aspirated cars.  However, the SAE correction factor does not take this into account.

Absolute air pressure in the intake manifold based on elevation and boost pressure.

Salt Lake City is at an average elevation of 1320 meters, so let’s look at the 1500m level in my chart.  For a naturally aspirated engine, the intake charge density is now about 83.5% compared to sea level.  For a forced induction car running 1.5 bar of boost (21.75 psi), it still has about 93.4% of the charge density.  However, the SAE correction factor applies the same 1.201 to both.  If both NA and FI cars put down 300whp at sea level (my hypothetical limit for the racing series), then I’ll approximate the NA car having 250.35whp (300whp * 0.8345) and the FI car having 280.14whp respectively at elevation.  Now you apply the 1.201 correction factor to both.  The NA car is up to a corrected 300.67whp and the FI car is way over the limit at a corrected 336.45whp.

To illustrate this problem, here’s a hypothetical situation.  So forum member #1 dyno’d his turbocharged STI resulting in 300whp at sea level and forum member #2 dyno’d his STI up in Denver making 335whp.  Same mods, same type of dyno, same air temps, same boost pressure, and SAE corrected numbers used.  Forum member #1 calls member #2 a liar and #2 says the tuner for #1 sucks.  Fun isn’t it?  Beyond the ambient pressure issues with forced induction cars and the SAE correction factor, there are temperature issues too.

The thing with forced induction engines is that their power output is highly dependent on the temperature of the air going into the intake manifold.  They are much more susceptible to knock if the charge temp going into the cylinder is hot.  This is due to the fact that the temperature of the denser charge from FI in the cylinder will have a greater temperature increase on the compression stroke as compared to a less dense charge for an NA engine.  The end result is much higher in-cylinder temps leading to detonation (for a refresher, read this).  If you take a naturally aspirated engine and compare the power outputs at 70F and 110F temps, the power reduction will probably be relatively close to the change in air density.  If you do the same with a turbocharged car though, it will be knocking all over the place in the 110F temps and pulling a lot of timing which will significantly cut power, much more than just the percentage change in air density.  Try tuning a turbocharged car running 20psi of boost and see how much timing you can run in 110F temps versus 70F temps.  It’s a big difference in timing resulting in a big difference in power.

There’s another significant difference between dyno testing a NA car versus a FI car.  FI cars use intercoolers to cool down the charge air before it goes into the intake manifold.  For the intercooler to do its job, it needs airflow.  A lot of airflow at a high air speed (typical driving speed would be good).  How many dynos do you see with a proper fan setup to blow the required air over an intercooler?  It gets even more complicated with cars such as STIs and Porsche 911 turbos that don’t have their intercoolers mounted in the front.  To dyno a STI, there really needs to be a fan blowing directly into the hood scoop (assuming the owner hasn’t gone front mounted).  For a 911, a pair of fans should be used blowing into the intercooler intakes on each of the rear fenders of the car.  Improper airflow through the intercoolers of FI cars will result in elevated intake charge temps in the intake manifold and a correspondingly large power loss.

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