Category Archives: Misc.

Is it Real or is it DynoJet?

Recently it has seemed to me that a large percentage of dyno results that have been made public are developed on a DynoJet brand dynamometer.   I like the consistency that a DynoJet has with the measured wheel horsepower and this is the brand of dyno I seek out when I look for a place to have my S4 dyno’d.

The frequency that I have put my car on a DynoJet, along with the abundance of street logs that I have recorded, provides a good collection of results to use to compare the two methods for estimating vehicle wheel horsepower.

The summary below shows the setup that my S4 had and the DynoJet generated whp and the road acceleration derived whp.  The last column to the right shows the amount of change between the DynoJet and road measurement.

DynoJet vs Street

Comparison of Peak Wheel Horsepower Measurement by Method
SetupDynoJetRoad% Change
FT21-274394165.2%
FT21-254183984.8%
K044053854.9%
K04/AWE3993736.5%
K03 Stk2372168.9%
K03 Nef2832588.8%
FT F4H3843635.5%

From these seven data points the average change from DynoJet to street is about 6%.

Another source of wheel horsepower also derived from street data is the calculation made by the ECUxPlot program.  The application appears to take into account similar variables, but I suspect something may have been left out, or is assumed to have less influence, as compared to the assumptions I make in my calculations.

Shown below is a series of pulls made by my car with the calculated wheel horsepower displayed:

ECUxPlot Calculated Wheel Horsepower
ECUxPlot Calculated Wheel Horsepower

These same logs when evaluated using my calculations produce the following:

ECUx Calculated Wheel Horsepower
ECUx Calculated Wheel Horsepower

It may not be immediately clear, but the calculations I have made are greater than the results calculated by ECUxPlot.  Out of the four data points, ECUxPlot reports a peak wheel horsepower value about 4.5% less than my calculation.

So which is more accurate?  I’m not sure, I have found that my calculations more closely match the results that were recorded on a Mustang Dynamometer but who’s to say how accurate that dyno was.

TiAL 605 Dyno Race Gas
26 PSI – 100 Octane – No WMI

An outcome of this comparison is, if it is assumed that the commonly used ECUxPlot is more accurate, the difference between DynoJet recorded wheel horsepower and that from ECUxPlot would be even greater than what I have calculated.

Using ECUxPlot’s calculations, the results are on average 9.2% less than that reported by the DynoJet.

Conclusion:

I am reasonably confident that the street measured results are more accurate than the DynoJet values.  The DynoJet would seem to read above truth by somewhere from 6% to 9.2%.

Crank up the preload?

Observing the intermediate results of an in progress tune of an SRM RS6/K24 equipped S4 it was clear the car was developing boost much sooner than the typical S4 with these turbochargers.

In the chart below the lines are of the subject car and the colored dots are data from other S4’s.  It’s clear that the subject S4 is generating boost on average about 250 rpm sooner than past averages.

SRM RS6/K24 with 18 psi WG preload

During discussion to try to discover what differences this car may have that could led to such a result, one possibility proposed linked the outcome to a higher than normal preload on the turbocharger wastegates.  The normal preload setting for these turbochargers is in the 14-16 psi range, in the case of the subject car the setting is 18 psi.

If this cause is correct it has some noteworthy ramifications.  To try and determine if raising preload more generally has a similar effect I looked to some data I collected with my S4 showing the boost onset with FrankenTurbo F4H turbochargers before and after the addition of helper springs to the BorgWarner K03 wastegates that I was using.

FrankenTurbo F4H Helper Spring Effect
FrankenTurbo F4H Helper Spring Effect

The solid lines of the chart above are for the unaided K03 wastegates and the dashed lines are with high spring rate helper springs.

It appears that the addition of the springs slightly improved the boost onset when boost levels reached double digits, but when accounting for the Intake Air Temperature, it is clear that the IAT’s were higher in the unaided case.  Higher intake air temperatures result in reduced spool up, thus the slight difference in boost onset can be explained by the difference in IAT.

In the case of my S4 with the F4H turbochargers, increasing the wastegate preload did not appreciably change the rate of boost onset.