cliffyk

Here's another look at the audio stream during the 2700 rpm/60 mph run (i got bored with TV).



This is a FFT (Fast Fourier Transform) spectrum analysis of the audio. The single cursor in the blue circle at the top is set the the peak of the most predominate frequency in the audio stream; at this point that was the exhaust pulses. In the red elipse at the bottom you can see that the predominate frequency was 154.0Hz.

Doing the same math as before this is 154.0 * 60 / 4 = 2310 rpm.

Zooming in on the "How fast..." graph in my previous post you can see that this nails it down solid--60 is just a bit north of 2250 rpm...



I didn't do this originally because the exhaust pulses were right there "in-my-face" in the time domain..

huey_33

Well Cliffy, as usual you were right. My tach needle is off. Just finished installing a window switch in my car for my nitrous system I have coming and noticed my RPM's are off by 400. Turned my car on, tach read 1100RPM's and the window switch read 700RPM's. Good Call!!

cliffyk

Way cool,

There's that same 400 rpm difference that the spectrum analysis revealed--this means the indicator needle in the tach is misplaced.

Years ago we used that sort of spectral analysis on manufacturing lines to pinpoint worn bearings, imbalances caused by worn shafts, etc.