Great little mathematical exercise done by Arin @ APR.
First off for those who don’t know, dynos read torque and plot a torque curve, then apply a formula to the torque number at each rpm point and create a plot of an hp curve. The formula is:
torque x RPM / 5252 = HP
i.e. a car that dynos a flat 300 torque right across the curve (for argument’s sake) would produce the following HP readings
at 2000 rpm HP = 300 (tq) x 2000 / 5252 = 114 hp
at 5252 rpm HP = 300 (tq) x 5252 / 5252 = 300 hp
at 7000 rpm HP = 300 (tq) x 7000 / 5252 = 399 hp
Since you’re dividing by 5252…and multiplying by RPM, at 5252rpm the torque number always equals the hp number (provided we’re talking lbs-ft and hp of course)
. That’s just an interesting little math thing for the people who didn’t know this. I found it neat when I learned that a few years back.
Anyway, someone noticed that the GIAC dyno plots for their tune are VERY steep up to redline, while the APR (APR inhouse dyno) hp plots recently released looked kinda flat towards redline.
Arin explained that because of the multiplication by RPM and division by 5252, higher RPM readings will appear different (the slope of the hp line) depending on the numbers in the torque curve.
He took a B8 S4 dyno. Then he added 10 wtq across the range, and calculated the corresponding hp curve, and plotted it. Then he took the standard dyno, and subtracted 10 wtq across the range, and plotted the corresponding hp curve. He did this for 10 variations. Here’s what it looked like…
Even though the torque curve is the exact same shape, just moved up or down changes the steepness or flatness of the hp curve in the higher RPMs. So, if you have a modest reading dyno, your hp line is going to look a little flat overtowards redline. If you have a happy reading dyno, the hp curve is going to look much more steep over towards redline.
I just thought this was neat. He posted it as an explanation of his firm’s dyno numbers, but it shone a light on why the GIAC cars are reading such MONSTROUS peak HP numbers…and this is evident in the steepness of the plot. This also exacerbates the difference between peak torque and peak hp.
